Course Webpage: http://panda.unm.edu/Courses/Thomas/Phys161fa11/P161Syllabus.htm

Course Grading Policy

Online homework due every class. Late HW 1/3 off per day! You will want to get Mastering Physics and an iClickerRegistration info on the webpage

How do most humans learn physics?“Assisted Struggling”…. you must try until your brain hurts, and then our “Specialiste Intellectual” (SI) Matt Curry (or yrs trly) can (we hope) help.

Why is so much pain involved?Answer: Pain wires the neurons.

Wednesday: Bring clickers,Know your schedule,Vote for SI scheduling.

Let’s Begin! Chapter 17

Pedagogical Goals:Understand “linear” thermal expansionMemorize & Apply conversions between K, °C, °FUnderstand that heat is energy transferred owing to TCompute calorimetric equilibrium for ice & water mixesUnderstand conduction, convection, radiation

Written HW Due Friday Aug 26 17-44,17-104

Most objects expand on heating. Demonstration: On heating the ring, the hole

A] will get bigger, as it expandsB] will get smaller, as the metal around expands inward

ANY smooth function, over a small range, “looks like” a line

The length of a rod, when heated, MUST BE

L = L0 + (T-T0)L0

is the “coefficient of linear expansion”.

What is the coefficient of volume expansion ?

If you want to directly measure the expansion of the temperature sensing liquid, use a thermometer: big bulb, very narrow tube

Daniel Gabriel Fahrenheit (1686–1736)

The zero point was determined by placing the thermometer in brine: a mixture of ice, water, and ammonium chloride, a salt. This is a frigorific mixture.

96 degrees, was the level of the liquid in the thermometer when held in the mouth or under the armpit of his wife.

Celsius used ice/water and water/steam for 0 and 100.

If you put evenly spaced marks on the thermometer, what assumption are you making???

Glass balls of different density nearly float in oilEach ball is labeled with the temperature at which it just floats.

Consider a ball that is just barely floating at 70°F.If you heat the GT to 72°C, what happens?

A] the ball floats up (more)B] the ball sinks

Galilean Thermometer Most objects expand on heating. Since the mass stays the same, when an object expands, what happens to its density?

A] goes upB] goes downC] unchanged

Gases also expand on heating.

Demo

There appears to be an “ideal” behavior of gases, approached as the density is low and for certain compositions (e.g. neon)

Graph: Volume vs T for different samples

Kelvin temperature

€

TF =9

5TC + 32°

TK = TC + 273.15 Units: K

Memorize the above.The ‘size’ of 1K = 1°CNote: hip scientists say “Kelvin”, not “Degrees Kelvin”

What is the approximate boiling temperature of water, in K?

A] 100 K

B] 212 K

C] 273 K

D] 373 K

E] 485 K

Heat = Energy transferred because of a difference in temperature. Heat flows spontaneously from a hotter object to a colder object. It never flows spontaneously from a colder object to a hotter one. It does not flow between objects at the same temperature.

Water in a hot pan never spontaneously freezes, even though it could do that without violating conservation of energy!(if it made the pan even hotter!)

“Specific Heat”, c, is the amount of heat needed to raise the temperature of a gram of material a degree (or a Kelvin)

Q=mc differential form: dQ = mcdT)

“Latent heat” (of vaporization, or of fusion) is heat per gram of material needed to boil (vaporize) or freeze (fuse). Q=mL

Adding heat does not always increase temperature!

Conduction = heat transfer by contact w/o bulk motion

Convection = heat transfer by heat-induced fluid flow around a hot object (recall, air is a fluid.)

Radiation : all objects above 0K radiate electromagnetic waves… “Black body” radiation. If you put a rock in a vacuum box (in vacuum), the rock will radiate and the box will radiate. If the rock is hot, it will radiate more, and that radiation will be absorbed by the walls of the box, making the box hotter.

Color & intensity depend on T

• Radiated energy increases with T(T4)

• Cooler things are redder (humans are infrared, redder than red), hotter things are bluer.

All things radiate a little less than this theoretical limit. (To keep balanced, they also absorb a little less!)

Memorize.

Mixing Ice & Water

You must be able to solve these problems:

X g of ice at -20°C + Y g of water at 0° are mixed in a thermos.What is the equilibrium composition and temperature?

X g of ice at 0°C + Y g of water at 60°C are mixed in a thermos. What is the equilibrium composition and temperature?

An “easy” one to start with:

10 g of ice at 0°C is mixed with 30 g of water at 0°C in a thermos.What is the equilibrium temperature and composition?

A] 0°C, 10 g ice, 30 g water

B] 0°C, 20 g ice, 20 g water

C] 0°C, no ice, all water

D] 0°C, all ice, no water

E] 0°F, all ice, no water

• Heat flows from hot to cold (energy is conserved)• Decision points!

20 g ice at 0°C is mixed with 40 g water at 80°C in a thermos.

Without knowing the specific heat or latent heat (except both are positive, of course):What is the composition at equilibrium?

A] It has to be all water

B] it might be all water, OR ice & water (but cannot be all ice)

C] it might be all ice, OR ice & water (but cannot be all water)

D] it has to be all ice

E] we can conclude nothing

20 g ice at 0°C is mixed with 40 g water at 80°C in a thermos.

The hot water will give up heat to the cold ice.The water will get colder, and the ice will get melted.

Eventually the process described will END.(It may be followed by a different process!)The process described HERE will END because

All the ice melts (in which case, we are no longer taking heat from water and putting it into melting ice)

OR

The water reaches 0°C (in which case, heat transfer stops.)

Either we end up with a mix of water & ice, or all water. We cannot decide YET.

20 g ice at 0°C is mixed with 14.2 g water at 80°C in a thermos. Qmelt = 6680 J (heat req’d to melt ice)Q->0°C = 4768 J (heat req’d to cool the hot water to 0°C)

How much heat is actually transferred from the hot water to the ice?A] 4768 JB] 6680 JC] 5724 J (average of A&B)

ANNOUNCEMENT: THE OFFICE OF ACCESSIBILITY RESOURCE CENTER IS LOOKING FOR A STUDENT IN THIS CLASS TO VOLUNTEER TO PROVIDE NOTES FOR THIS CLASS. THE STUDENT WILL BE PAID A STIPEND FOR THE SEMESTER.  INTERESTED STUDENT SHOULD COME BY OUR OFFICE AT 2021 MESA VISTA HALL TO COMPLETE THE REQUIRED HIRING PAPERWORK. 

Announcement 2: HW due next Friday Chapter 18: exercises 10, 14, 42

ANNOUNCEMENT 3. YOUR PROFESSOR DOES NOT SEEM TO MULTIPLY WELL.

LET’S REVISIT 20g ice at 0°C + 40 g water at 80°C

With the correct multiplication, what is the final composition?

A] all water

B] a mix of water & ice

SI Problem SessionsT 3-4W 12-2Th 5-6:30Regener 111

For the next three weeks, we will study gases.

We will figure out what happens when you heat them, and what happens when you compress them.

Why?

Here’s a question, true or false:

Heat never flows from a cold object to a hot one.

A] True

B] False

While you think about this question, don’t mind me… I’m just going to have a popsicle!

Here’s a question, true or false:

Heat never flows from a cold object to a hot one.

A] True

B] False

The popsicle was made by extracting heat from cold (flavored) water and adding it into my warmer kitchen.

Heat never spontaneously flows from a cold object to a hot one. But we can use a freezer (or refrigerator) to move heat from cold objects to warm.

We will study gases to understand how to use them as a “working substance” in a refrigerator.

Along the way, perhaps we can gain a better understanding of the limits of refrigerator performance, and the limits of engine performance.

We will discover that there is a limit to the efficiency of each; that limit results from the fact that

Heat never spontaneously flows from a cold object to a hot one.

P = F/A

Gauge pressure = pressure above ambient

The ideal gas law works only and exclusively with T in Kelvin.

Remember the IJK rule, the Ideal gas law inJim’s class requiresKelvin temperature

Pressure can be measured in N/m2, or in atm. (1 atm ~ 105 N/m2)

Volume can be measured in m3, or in liters. (1000 liters = 1 m3)

n is number of moles

R is the gas constant, = 8.3 J/(mol•K) = 0.082 L•atm/(mol•K)

An ideal gas occupies 1 liter at 32°F and 1 atm pressure. If the volume is held fixed, at about what temperature will the pressure be 2 atm?

A] 64°F

B] 100°C

C] 273°C

An ideal gas occupies 1 liter at 32°F and 1 atm pressure. If the volume is doubled and the temperature raised to 273°C, what happens to the pressure?

A] it is reduced to 1/4 atm

B] it is reduced to 1/2 atm

C] it stays the same

D] it increases to 2 atm

E] it increases to 4 atm

For a monatomic ideal gas, we can show that

the product pV

= 2/3 of the translational kinetic energy of the molecules, Ktr!

So nRT = (2/3)Ktr

Temperature is proportional to the kinetic energy per molecule.

SI Problem SessionsT 3-4W 12-2Th 5-6:30Regener 111

Correction to SISchedule: