you did work against friction, where did the energy go?

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

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Page 1: You did work against friction, where did the energy go?

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

Page 2: You did work against friction, where did the energy go?

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.

Page 3: You did work against friction, where did the energy go?

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

Page 4: You did work against friction, where did the energy go?

Change of Phase of State• States of matter

– Solid– Liquid– Gas

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

Page 5: You did work against friction, where did the energy go?

Substances in Phase Transfer

40

32

212

40

0

100

FC

Page 6: You did work against friction, where did the energy go?

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

Page 7: You did work against friction, where did the energy go?

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

Page 8: You did work against friction, where did the energy go?

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

Page 9: You did work against friction, where did the energy go?

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 ?

Page 10: You did work against friction, where did the energy go?

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

Page 11: You did work against friction, where did the energy go?

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 involve

multiple heat transfer

types like this.

Page 12: You did work against friction, where did the energy go?

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)

Page 13: You did work against friction, where did the energy go?

Heat gained

Q: Heat

m: mass

ΔT: Change in Temperature

Q mC T

Page 14: You did work against friction, where did the energy go?

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

Page 15: You did work against friction, where did the energy go?

Calorimerty

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

Calorimerty depends on the conservation of energy.

Qgained + Qlost = Zero

Page 16: You did work against friction, where did the energy go?

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 CT

kg J kg C kg J kg C

15.7ofT C

a a a b bm c T m c T

0gained lostQ Q

Page 17: You did work against friction, where did the energy go?

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/kg

Water Hv=2.26x106J/kg

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

.015m kgSpecific Heat Phase Change

Page 18: You did work against friction, where did the energy go?

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 CT

kg J kg C kg J kg C

23.0ofT C

Page 19: You did work against friction, where did the energy go?

Heat of fusion example #2

How 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 / )20

8360

heat waterQ mC T

Q kg J kg C C

Q J

43.34 10 8360

41760total

total

Q x J J

Q J

Page 20: You did work against friction, where did the energy go?

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