Heat and Temperature

PHYS 1090 Unit 4

Put Your Hands Together!

(Activity 1)

• Doing work on your hands made them warmer.

• Adding energy raised the temperature.

• Work had the same effect as heat.

Mechanical Equivalent of Heat

James Joule’s life-long obsession

No difference between adding heat to a system and doing work on it.

Source: Griffith, The Physics of Everyday Phenomena

Heat Units

• Joule

• Calorie (cal): amount of heat needed to raise 1 gram of water 1 degree C (or K) = 4.184 J.

• British Thermal Unit (BTU): amount of heat needed to raise 1 pound of water 1 degree F = 1054.35 J

Specific Heat

• The amount of heat required to change the temperature of a unit mass of substance.

– C = specific heat– q = heat added– m = mass of sample– T = temperature change

Cp =q

mT

Another Heat Unit

• U.S. Food Calorie: Cal = 1000 cal

• Food energy values are often presented in kJ in other countries

First Law of Thermodynamics

E = q + w

whereE = change in internal energy

q = heat input

w = work input

Kinetic-Molecular Theory

• Everything is made of molecules.

• The molecules are constantly moving in random directions.

• (Absolute) temperature is proportional to molecular translational kinetic energy.

• Molecules colliding with objects they contact causes pressure.

Absolute Zero

• When molecules have zero kinetic energy

• Absolute zero = −273.15 °C

• Kelvin temperature = °C + 273.15

• Absolute zero = 0 K

• 0 °C = 273.15 K

Liquid N2 Boil

(Activity 2)

•More boiled away with cold water

•More heat transferred from cold water

•Because there was more cold than hot water

Warm and Cool Colors

(Activity 3)

• Dye dispersed faster in warm water.

• Why?

• Water molecules were moving faster in the warm water.

Phase Change

(Activity 4)

• Melting ice temperature was constant even though heat was being added– Specific heat is infinite?

• Boiling water temperature was also constant– Boiling temperature 100°C?

Sensing Latent Heats

(Activity 5)

• Evaporation is a cooling process.

• Condensation is a warming process.

Phase Changes

• Potential energies:

• During a phase change, potential energy, not kinetic energy (temperature) changes.

• Heating or cooling a changing phase does not change its temperature!

Solid

Liquid

Gascondense

freeze

evaporate

melt

depositsublime

Phase Changes

• Melting, boiling, freezing, condensing…

• Added or removed heat changes the substance’s potential rather than kinetic energy

• Water freezes at 0 °C, boils 100 °C

• Not all heat transfer is expressed as a temperature change.

Heats of Phase Changes

• To melt 1 kg water at 0 °C: 335 kJ

• To boil 1 kg water at 100 °C: 2,255 kJ

ice

Liquid water

steam

Water temperature with heating

-100

-50

0

50

100

150

200

0.0E+00 1.0E+06 2.0E+06 3.0E+06 4.0E+06

heat input (J/kg)

tem

pera

ture

(C

)

Heating Curve for Water

Water boils

Ice melts

335 kJ/kg

2,255 kJ/kg

Heat Transfer Mechanisms

• Heat transfers between objects by:

• Conduction: collisional transfer of kinetic energy

• Convection: buoyancy-driven fluid circulation

• Radiation and absorption of electromagnetic waves

Conduction

(Activity 6)

• Thermal energy moves through different materials at different speeds.

• Conductivity varies with material; solids > liquids > gases

Convection

• Hot water stayed on top, cold stayed on the bottom

• Hot water moved to the top, cold to the bottom (with mixing)

Radiation

(Activity 8)

• Coil heated your hand from afar.

• Heating less intense farther away.

• Aluminum blocked heat.

Radiation

• Power output increases as T4.

• Objects are heated by absorbing radiation.

• Objects are cooled by emitting radiation.

Entropy

(Activity 9)

• Dice moved in either direction with equal probability

• Began highly localized

Localized Dispersed

• Inevitable and irreversible

Model of Diffusion

• Particles randomly move from high concentration to low concentration– Individual flow either way– Net flow high → low– Equilibrium: uniformity

• Energy flows in the same way– Transfer in collisions– Heat flow: high temp → low temp

Entropy

• A measure of “disorder”• Related to the number of equivalent ways to

arrange a system

Low entropy High entropy

Overall Summary

• Particles and energy tend to become spread out uniformly.

• Total entropy increases in all processes that actually occur.

Drinking Bird

• Evaporation from the bird’s head cooled the vapor inside.

• This reduced the head pressure to less than in the bottom bulb.

• The higher pressure in the bottom pushed the fluid uphill, making the bird top-heavy.