Basic Physics Part IIBasic Physics Part II

Work Energy and PowerWork Energy and Power

Spring 2005

UCSD Physics 8 2005

2

Energy the capacity to do workEnergy the capacity to do work

bull This notion makes sense even in a colloquial contextThis notion makes sense even in a colloquial contextndash hard to get work done when yoursquore wiped out (low on

energy)ndash work makes you tired yoursquove used up energy

bull But we can make this definition of energy much more But we can make this definition of energy much more precise by specifying exactly what we mean by precise by specifying exactly what we mean by workwork

Spring 2005

UCSD Physics 8 2005

3

Work more than just unpleasant tasksWork more than just unpleasant tasks

bull In physics the definition of work is the application of In physics the definition of work is the application of a a force through a distanceforce through a distance

W = FW = Fdd

bull WW is the is the work work donedonebull FF is the is the forceforce applied appliedbull dd is the is the distancedistance through which the force acts through which the force actsbull Only the force that acts in the direction of motion Only the force that acts in the direction of motion

counts towards workcounts towards work

Spring 2005

UCSD Physics 8 2005

4

Units of EnergyUnits of Energy

bull Force is a mass times an accelerationForce is a mass times an accelerationndash mass has units of kilogramsndash acceleration is ms2

ndash force is then kgms2 which we call Newtons (N)

bull Work is a force times a distanceWork is a force times a distancendash units are then (kgms2)m = kg m2s2 = Nm = Joules (J)ndash One joule is one Newton of force acting through one meterndash Imperial units of force and distance are pounds and feet so

unit of energy is foot-pound which equals 136 J

bull Energy has the same units as work JoulesEnergy has the same units as work Joules

Spring 2005

UCSD Physics 8 2005

5

A note on arithmetic of unitsA note on arithmetic of units

bull You should carry units in your calculations and You should carry units in your calculations and multiply and divide them as if they were numbersmultiply and divide them as if they were numbers

bull Example the force of air drag is given byExample the force of air drag is given by

FFdragdrag = = frac12cfrac12cDDAvAv22

bull cD is a dimensionless drag coefficient

is the density of air 13 kgm3

bull A is the cross-sectional area of the body in m2

bull v is the velocity in ms

units (kgm3)(m2)(ms)2 = (kgm2m3) (m2s2) =kgm2m2

m3s2

kgm4

m3s2= = kgms2 = Newtons

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

2

Energy the capacity to do workEnergy the capacity to do work

bull This notion makes sense even in a colloquial contextThis notion makes sense even in a colloquial contextndash hard to get work done when yoursquore wiped out (low on

energy)ndash work makes you tired yoursquove used up energy

bull But we can make this definition of energy much more But we can make this definition of energy much more precise by specifying exactly what we mean by precise by specifying exactly what we mean by workwork

Spring 2005

UCSD Physics 8 2005

3

Work more than just unpleasant tasksWork more than just unpleasant tasks

bull In physics the definition of work is the application of In physics the definition of work is the application of a a force through a distanceforce through a distance

W = FW = Fdd

bull WW is the is the work work donedonebull FF is the is the forceforce applied appliedbull dd is the is the distancedistance through which the force acts through which the force actsbull Only the force that acts in the direction of motion Only the force that acts in the direction of motion

counts towards workcounts towards work

Spring 2005

UCSD Physics 8 2005

4

Units of EnergyUnits of Energy

bull Force is a mass times an accelerationForce is a mass times an accelerationndash mass has units of kilogramsndash acceleration is ms2

ndash force is then kgms2 which we call Newtons (N)

bull Work is a force times a distanceWork is a force times a distancendash units are then (kgms2)m = kg m2s2 = Nm = Joules (J)ndash One joule is one Newton of force acting through one meterndash Imperial units of force and distance are pounds and feet so

unit of energy is foot-pound which equals 136 J

bull Energy has the same units as work JoulesEnergy has the same units as work Joules

Spring 2005

UCSD Physics 8 2005

5

A note on arithmetic of unitsA note on arithmetic of units

bull You should carry units in your calculations and You should carry units in your calculations and multiply and divide them as if they were numbersmultiply and divide them as if they were numbers

bull Example the force of air drag is given byExample the force of air drag is given by

FFdragdrag = = frac12cfrac12cDDAvAv22

bull cD is a dimensionless drag coefficient

is the density of air 13 kgm3

bull A is the cross-sectional area of the body in m2

bull v is the velocity in ms

units (kgm3)(m2)(ms)2 = (kgm2m3) (m2s2) =kgm2m2

m3s2

kgm4

m3s2= = kgms2 = Newtons

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

3

Work more than just unpleasant tasksWork more than just unpleasant tasks

bull In physics the definition of work is the application of In physics the definition of work is the application of a a force through a distanceforce through a distance

W = FW = Fdd

bull WW is the is the work work donedonebull FF is the is the forceforce applied appliedbull dd is the is the distancedistance through which the force acts through which the force actsbull Only the force that acts in the direction of motion Only the force that acts in the direction of motion

counts towards workcounts towards work

Spring 2005

UCSD Physics 8 2005

4

Units of EnergyUnits of Energy

bull Force is a mass times an accelerationForce is a mass times an accelerationndash mass has units of kilogramsndash acceleration is ms2

ndash force is then kgms2 which we call Newtons (N)

bull Work is a force times a distanceWork is a force times a distancendash units are then (kgms2)m = kg m2s2 = Nm = Joules (J)ndash One joule is one Newton of force acting through one meterndash Imperial units of force and distance are pounds and feet so

unit of energy is foot-pound which equals 136 J

bull Energy has the same units as work JoulesEnergy has the same units as work Joules

Spring 2005

UCSD Physics 8 2005

5

A note on arithmetic of unitsA note on arithmetic of units

bull You should carry units in your calculations and You should carry units in your calculations and multiply and divide them as if they were numbersmultiply and divide them as if they were numbers

bull Example the force of air drag is given byExample the force of air drag is given by

FFdragdrag = = frac12cfrac12cDDAvAv22

bull cD is a dimensionless drag coefficient

is the density of air 13 kgm3

bull A is the cross-sectional area of the body in m2

bull v is the velocity in ms

units (kgm3)(m2)(ms)2 = (kgm2m3) (m2s2) =kgm2m2

m3s2

kgm4

m3s2= = kgms2 = Newtons

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

4

Units of EnergyUnits of Energy

bull Force is a mass times an accelerationForce is a mass times an accelerationndash mass has units of kilogramsndash acceleration is ms2

ndash force is then kgms2 which we call Newtons (N)

bull Work is a force times a distanceWork is a force times a distancendash units are then (kgms2)m = kg m2s2 = Nm = Joules (J)ndash One joule is one Newton of force acting through one meterndash Imperial units of force and distance are pounds and feet so

unit of energy is foot-pound which equals 136 J

bull Energy has the same units as work JoulesEnergy has the same units as work Joules

Spring 2005

UCSD Physics 8 2005

5

A note on arithmetic of unitsA note on arithmetic of units

bull You should carry units in your calculations and You should carry units in your calculations and multiply and divide them as if they were numbersmultiply and divide them as if they were numbers

bull Example the force of air drag is given byExample the force of air drag is given by

FFdragdrag = = frac12cfrac12cDDAvAv22

bull cD is a dimensionless drag coefficient

is the density of air 13 kgm3

bull A is the cross-sectional area of the body in m2

bull v is the velocity in ms

units (kgm3)(m2)(ms)2 = (kgm2m3) (m2s2) =kgm2m2

m3s2

kgm4

m3s2= = kgms2 = Newtons

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

5

A note on arithmetic of unitsA note on arithmetic of units

bull You should carry units in your calculations and You should carry units in your calculations and multiply and divide them as if they were numbersmultiply and divide them as if they were numbers

bull Example the force of air drag is given byExample the force of air drag is given by

FFdragdrag = = frac12cfrac12cDDAvAv22

bull cD is a dimensionless drag coefficient

is the density of air 13 kgm3

bull A is the cross-sectional area of the body in m2

bull v is the velocity in ms

units (kgm3)(m2)(ms)2 = (kgm2m3) (m2s2) =kgm2m2

m3s2

kgm4

m3s2= = kgms2 = Newtons

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

6

Kinetic EnergyKinetic Energy

bull Kinetic Energy the energy of motionKinetic Energy the energy of motionbull Moving things carry energy in the amountMoving things carry energy in the amount

KEKE = = frac12frac12mvmv22

bull Note the Note the vv22 dependence dependencemdashmdashthis is whythis is whyndash a car at 60 mph is 4 times more dangerous than a car at 30

mphndash hurricane-force winds at 100 mph are much more

destructive (4 times) than 50 mph gale-force windsndash a bullet shot from a gun is at least 100 times as destructive

as a thrown bullet even if you can throw it a tenth as fast as you could shoot it

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

7

Numerical examples of kinetic energyNumerical examples of kinetic energy

bull A baseball (mass is 0145 kg = 145 g) moving at 30 A baseball (mass is 0145 kg = 145 g) moving at 30 ms (67 mph) has kinetic energyms (67 mph) has kinetic energy

KE = frac12(0145 kg)(30 ms)2

= 6525 kgm2s2 65 J

bull A quarter (mass = 000567 kg = 567 g) flipped about A quarter (mass = 000567 kg = 567 g) flipped about four feet into the air has a speed on reaching your four feet into the air has a speed on reaching your hand of about 5 ms The kinetic energy ishand of about 5 ms The kinetic energy is

KE = frac12(000567 kg)(5 ms)2

= 007 kgm2s2 = 007 J

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

8

More numerical examplesMore numerical examples

bull A 1500 kg car moves down the freeway at 30 ms (67 A 1500 kg car moves down the freeway at 30 ms (67 mph)mph)

KE = frac12(1500 kg)(30 ms)2

= 675000 kgm2s2 = 675 kJ

bull A 2 kg (~44 lb) fish jumps out of the water with a A 2 kg (~44 lb) fish jumps out of the water with a speed of 1 ms (22 mph)speed of 1 ms (22 mph)

KE = KE = frac12frac12(2 kg)(2 kg)(1 ms)(1 ms)22

= 1 kgm2s2 = 1 J

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

9

Gravitational Potential EnergyGravitational Potential Energy

bull It takes It takes workwork to lift a mass against the pull (force) of gravity to lift a mass against the pull (force) of gravitybull The force of gravity is The force of gravity is mmgg where where mm is the mass and is the mass and gg is the is the

gravitational accelerationgravitational accelerationF = mg (note similarity to F = ma)

ndash g = 98 ms2 on the surface of the earthndash g 10 ms2 works well enough for this class

bull Lifting a height Lifting a height hh against the gravitational force requires an against the gravitational force requires an energy input (work) ofenergy input (work) of

E = W = F E = W = F h = mghh = mghbull Rolling a boulder up a hill and perching it on the edge of a cliff Rolling a boulder up a hill and perching it on the edge of a cliff

gives it gravitational gives it gravitational potentialpotential energy that can be later released energy that can be later released when the roadrunner is down belowwhen the roadrunner is down below

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

10

First Example of Energy ExchangeFirst Example of Energy Exchange

bull When the boulder falls off the cliff it picks up speed When the boulder falls off the cliff it picks up speed and therefore gains kinetic energyand therefore gains kinetic energy

bull Where does this energy come fromWhere does this energy come from

from the from the gravitational potential energygravitational potential energybull The higher the cliff the more kinetic energy the The higher the cliff the more kinetic energy the

boulder will have when it reaches the groundboulder will have when it reaches the ground

mgh

becomes

frac12mv2

Energy is conserved sofrac12mv2 = mgh

Can even figure out v since v2 = 2gh

h

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

11

Examples of Gravitational Potential EnergyExamples of Gravitational Potential Energy

bull How much gravitational potential energy does a 70 kg How much gravitational potential energy does a 70 kg high-diver have on the 10 meter platformhigh-diver have on the 10 meter platform

mgh = (70 kg)(10 ms2)(10 m)

= 7000 kgm2s2 = 7 kJ

bull How massive would a book have to be to have a How massive would a book have to be to have a potential energy of 40 J sitting on a shelf two meters potential energy of 40 J sitting on a shelf two meters off the flooroff the floor

mgh = m(10 ms2)(2 m) = 40 J

so m must be 2 kg

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

12

Ramps Make Life EasyRamps Make Life Easy

bull To get the same amount of work done you can To get the same amount of work done you can eithereitherndash apply a LARGE force over a small distancendash OR apply a small force over a large distancendash as long as W = Fmiddotd is the same

bull Ramp with 101 ratio for instance requires one tenth Ramp with 101 ratio for instance requires one tenth the force to push a crate up it (disregarding friction) the force to push a crate up it (disregarding friction) as compared to lifting it straight upas compared to lifting it straight upndash total work done to raise crate is still the same mghndash but if the work is performed over a longer distance F is

smaller mg10

hmg

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

13

The Energy of HeatThe Energy of Heat

bull Hot thingsHot things have more energy than their have more energy than their coldcold counterpartscounterparts

bull Heat is really just kinetic energy on microscopic scales Heat is really just kinetic energy on microscopic scales the vibration or otherwise fast motion of individual the vibration or otherwise fast motion of individual atomsmoleculesatomsmoleculesndash Even though itrsquos kinetic energy itrsquos hard to derive the same

useful work out of it because the motions are random

bull Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu)ndash One calorie (4184 J) raises one gram of H2O 1ordmCndash One Calorie (4184 J) raises one kilogram of H2O 1ordmCndash One Btu (1055 J) raises one pound of H2O 1ordmF

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

14

Energy of Heat continuedEnergy of Heat continued

bull Food Calories are with the ldquobigrdquo C or kilocalories Food Calories are with the ldquobigrdquo C or kilocalories (kcal)(kcal)

bull Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter 1 cal heats 1 cc of water 1centimeter 1 cal heats 1 cc of water 1ordmC and ordmC and likewise 1 kcal (Calorie) heats one liter of water 1ordmClikewise 1 kcal (Calorie) heats one liter of water 1ordmCndash these are useful numbers to hang onto

bull Example to heat a 2-liter bottle of Coke from the 5Example to heat a 2-liter bottle of Coke from the 5ordmC ordmC refrigerator temperature to 20ordmC room temperature refrigerator temperature to 20ordmC room temperature requires 30 Calories or 1225 kJrequires 30 Calories or 1225 kJ

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

15

Heat CapacityHeat Capacity

bull Different materials have different Different materials have different capacitiescapacities for heat for heatndash Add the same energy to different materials and yoursquoll get

different temperature risesndash Quantified as heat capacityndash Water is exceptional with 4184 JkgordmCndash Most materials are about 1000 JkgordmC (including wood air

metals)

bull Example to add 10Example to add 10ordmC to a room 3 meters on a side ordmC to a room 3 meters on a side (cubic) how much energy do we need(cubic) how much energy do we need

air density is 13 kgm3 and we have 27 m3 so 35 kg of air and we need 1000 J per kg per ordmC so we end up needing 350000 J (= 836 Cal)

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

16

Chemical EnergyChemical Energy

bull Electrostatic energy (associated with charged Electrostatic energy (associated with charged particles like electrons) is stored in the chemical particles like electrons) is stored in the chemical bonds of substances bonds of substances

bull Rearranging these bonds can release energy (some Rearranging these bonds can release energy (some reactions reactions requirerequire energy to be put in) energy to be put in)

bull Typical numbers are 100Typical numbers are 100ndash200 kJ per molendash200 kJ per molendash a mole is 60221023 moleculesparticlesndash works out to typical numbers like several thousand Joules

per gram or a few Calories per gram (remember 1 Cal = 1 kcal = 4184 J)

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

17

Chemical Energy ExamplesChemical Energy Examples

bull Burning a wooden match releases about one Btu or Burning a wooden match releases about one Btu or 1055 Joules (a match is about 03 grams) so this is 1055 Joules (a match is about 03 grams) so this is gt3000 Jg nearly 1 Calggt3000 Jg nearly 1 Calg

bull Burning coal releases about 20 kJ per gram of Burning coal releases about 20 kJ per gram of chemical energy or roughly 5 Calgchemical energy or roughly 5 Calg

bull Burning gasoline yields about 39 kJ per gram or just Burning gasoline yields about 39 kJ per gram or just over 9 Calgover 9 Calg

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

18

PowerPower

bull Power is simply energy exchanged Power is simply energy exchanged per unit time or how fast you get work per unit time or how fast you get work done (Watts = Joulessec) done (Watts = Joulessec)

bull One horsepower = 745 WOne horsepower = 745 Wbull Perform 100 J of work in 1 s and call Perform 100 J of work in 1 s and call

it 100 Wit 100 Wbull Run upstairs raising your 70 kg (700 Run upstairs raising your 70 kg (700

N) mass 3 m (2100 J) in 3 seconds N) mass 3 m (2100 J) in 3 seconds 700 W output 700 W output

bull Shuttle puts out a few GW (gigawatts Shuttle puts out a few GW (gigawatts or 10or 1099 W) of power W) of power

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

19

Power ExamplesPower Examples

bull How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 secondsmeters in 2 secondsmgh = (10 kg)(10 ms2)(2 m) = 200J

200 J in 2 seconds 100 Watts

bull If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ordmC with a ordmC with a 1000 W space heater how long will it take1000 W space heater how long will it take

We know from before that the room needs to have 360000 J added to it so at 1000 W = 1000 Js this will take 360 seconds or six minutes

But the walls need to be warmed up too so it will actually take longer (and depends on quality of insulation etc)

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments

Spring 2005

UCSD Physics 8 2005

20

AnnouncementsAssignmentsAnnouncementsAssignments

bull Next upNext upndash flow of energy and human energyexercise

ndash a simple model for moleculeslattices

ndash electrons charge current electric fields

bull AssignmentsAssignmentsndash Transmitters start counting for participation credit Tuesday 411

ndash HW1 Chapter 1 in Bloomfield 1E4 1E7 1E8 1E20 1E25 1E34 1P1 1P8 1P9 1P10 1P14 1P16 1P18 1P22 Chapter 2 2E28 2P10 2P11

bull E Exercise P Problembull due Thursday 413 in class (or in box outside 336 SERF by 330PM

Thursday)

ndash First QO due Friday 414 by 6PM via WebCT

ndash read chapter 2 pp 54ndash59 61ndash62 71ndash72 chapter 7 pp 206ndash207

• Basic Physics Part II
• Energy the capacity to do work
• Work more than just unpleasant tasks
• Units of Energy
• A note on arithmetic of units
• Kinetic Energy
• Numerical examples of kinetic energy
• More numerical examples
• Gravitational Potential Energy
• First Example of Energy Exchange
• Examples of Gravitational Potential Energy
• Ramps Make Life Easy
• The Energy of Heat
• Energy of Heat continued
• Heat Capacity
• Chemical Energy
• Chemical Energy Examples
• Power
• Power Examples
• AnnouncementsAssignments