Thermal Energy

• Thermal Energy: The total internal Energy

• Internal Energy: The sum of the kinetic and potential energies of the internal motion of particles that make up an object.

• You did work against friction, where did the energy go?

Temperature• Molecules move at different speeds.• Temperature relates to the AVERAGE kinetic

energy of the molecules in a substance.• The amount of KE is also related to the phase of

the material

Solid

Temperature• When heat is added the molecules move

faster.The higher the temperature, the faster the molecules move and the less they are bound to each other.

What is Heat?• Heat is energy in transfer from an object of

higher temperature to one of lower temperature.

• The quantity of energy transfer from one object to another is because of a difference in temperature.

Cold Hot

Heat Flow

Warm Warm

Change of Phase of State• States of matter

– Solid– Liquid– Gas

• What happens to the temperature when matter is changing states?

Substances in Phase Transfer

40

32

212

40

0

100

FC

Phase Change• Between A-B: The ice is warming to 0oC• Between B-C: Thermal energy melts the ice at 0oC• Between C-D: The water is warm to 100oC• Between D-E: The water boils and changes to vapor at

100oC• After E: The temperature of the vapor increases

Latent Heat

-50

0

50

100

150

Time

Tem

pera

ture

(C)

A

B

C ED

Thermal Energy Transfer• Conduction:

– Transfer of Kinetic Energy by contact

• Convection:– Heat transfer by the motion of a fluid (e.g. air)

• Radiation:– Electromagnetic waves carry energy

Note: Conduction and Convection require matter

Conduction• Conduction is the transfer of heat through molecular

collisions.• This form of heat transfer best occurs in solids where

molecules are closely packed.• Materials that conduct heat well are called conductors.

(Eg. metals such as copper and iron)

Magnification

Convection• Convection is the transfer of heat though moving fluids.• A fluid is any substance that flows, which includes all

liquids and gases.• Examples include convection ovens and cloud formation.

Convection ?

Radiation (not radioactivity)• Radiation is the transfer of heat via electromagnetic waves.• These waves include visible light, but are mostly infrared.• No matter is required for this type of heat transfer.• Examples include the sun’s heat and warmth felt from a

flame.

Open Space

?Radiation

Heat Transfer Question• Consider a camp fire burning vigorously.

– How is heat normally transferred while warming its viewers?• Radiation

– How is heat transferred when you put a hand in the smoke?• Convection

– How is heat transferred to a stick when it is placed in the hot coals?

• Conduction

• Some situations involvemultiple heat transfer types like this.

Specific HeatHeat: The energy that flows as a result of a difference in temperature

Q: The symbol for heat. Measured in Joules (J)

C: the symbol for Specific Heat.The amount of energy needed to raise a unit of mass one temperature unit. (J/kg K)

Heat gained

Q: Heatm: massΔT: Change in Temperature

Q mC T

Sample Heat Problem• How much heat is required to raise the temperature of a

2.4kg gold ingot (c = 129J/kgK) from 23°C to 45°C?

2.4kgQ mc T

2.4 129 22JkgKQ kg C

6811.2Q J

F IT T T 45 23T C C 22T C

Light

Heat TransferExample 1

A 0.40kg block of iron is heated from 295K to 325K. How much heat is absorbed by the iron?

C=450J/kg K

Q mC T

(.4 )(450 / )(325 295 )Q kg J kg K K K 5400Q J

Calorimerty

Calorimeter: A device used to measure changes in thermal energy.

Calorimerty depends on the conservation of energy.

Qgained + Qlost = Zero

Calorimerty Example #1A .4kg block of zinc @115°C is placed in .5kg of water @15°C. Find the final temperature.

( ) ( ) 0a a f ai b b f bim C T T m C T T

a a ai b b bif

a a b b

m C T m C TT

m C m C

.4 (388 / )(115 ) .5 (4180 / )(15 ).4 (388 / ) .5 (4180 / )

o o o o

f o o

kg j kg C C kg j kg C CTkg J kg C kg J kg C

15.7ofT C

a a a b bm C T m C T

0gained lostQ Q

Calorimerty Example example #2A .1kg block of brass @90°C is placed in .2kg of water @20°C. Find the final temperature.

a a ai b b bif

a a b b

m C T m C TT

m C m C

.1 (376 / )(90 ) .2 (4180 / )(20 ).1 (376 / ) .2 (4180 / )

o o o o

f o o

kg j kg C C kg j kg C CTkg J kg C kg J kg C

23.0ofT C

Heat of fusion Example #1If 5,000J is added to ice at 0oC, how much ice is melted?

Q=mHf Q=mHv

Water Hf=3.34x105J/kgWater Hv=2.26x106J/kg

/ fm Q H55000 / 3.34 10 /m J x J kg

.015m kgSpecific Heat Phase Change

Heat of fusion example #2How much heat must be transferred to 100g of ice at 00C unit the ice melts and the temperature of the resulting water rises to 200C?

Water Hf=3.34x105J/kg

5

4

.1 (3.34 10 / )

3.34 10

melt ice fQ mH

Q kg x J kg

Q x J

0 0.1 (4180 / )208360

heat waterQ mC T

Q kg J kg C CQ J

43.34 10 836041760

total

total

Q x J JQ J