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# Heat - PowerPoint PPT Presentation

Physics 102 Professor Lee Carkner Lecture 3. “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman. Heat. PAL #2 Galileo Thermometer. How does it work? Limitations

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Presentation Transcript

Professor Lee Carkner

Lecture 3

“If you can’t stand the heat, get out of the kitchen.”

-Harry S. Truman

Heat

• How does it work?

• Limitations

• Not very accurate, limited range, needs to be kept upright, won’t work in free fall, delicate, can freeze solid

• a) They are at the same pressure

• b) They have the same volume

• c) They are not in contact with each other

• d) They are not exchanging heat

• e) Their temperatures cannot be measured

Which of the following places the temperatures in the correct rank, highest to lowest?

• a) 50 X, 50 W, 50 Y

• b) 50 X, 50 Y, 50 W

• c) 50 W, 50 X, 50 Y

• d) 50 Y, 50 W, 50 X

• e) 50 Y, 50 X, 50 W

Four metal plates all made of the same material are increased in temperature by the same amount. Rank the height increase from most to least.

• a) 1, 2, 3, 4

• b) 1 and 4 tie, 2 and 3 tie

• c) 1, 4, 2 and 3 tie

• d) 2 and 3 tie, 1, 4

• e) All tie

Heat increased in temperature by the same amount. Rank the height increase from most to least.

• What is heat?

• Same temperature, no heat

• Heat is not a “thing”, it is a transfer of energy

• Units:

• Joules

• calories (cal) = 4.186 J

• For rates of heat transfer (Q/t), unit is the Watt (W) = J/s

Specific Heat increased in temperature by the same amount. Rank the height increase from most to least.

• The specific heat (c):

c = Q/mDT

• c has units of J/kg C

• Need to know the mass of the stuff (m) and the change in temperature (DT)

• Can rewrite as:

Q =mcDT

Calorimetry increased in temperature by the same amount. Rank the height increase from most to least.

• To do experiments with heat we use a calorimeter

• The total heat exchange is the sum of the heat from all processes

• Q1 + Q2 + Q3 … = 0

• Always write DT = Tf-Ti

• Use consistent units

• Make sure units for T and m match units for c

• PAL: Quenching a Dagger increased in temperature by the same amount. Rank the height increase from most to least.

• Suppose a silver dagger of mass ms at Ts is immersed in a mass mw of water at Tw. Derive an expression for the final temperature of the water when thermal equilibrium is reached.

How Does Heat Move? increased in temperature by the same amount. Rank the height increase from most to least.

• Heat (like information) is transferred in different ways

• Conduction

• Convection

Conductive Heat Transfer increased in temperature by the same amount. Rank the height increase from most to least.

• The rate of heat transfer via conduction is:

Q/t = kA(T1-T2)/L

• where:

• T1 is the temperature of the hot side and T2 is the temperature of the cold side

• L is the thickness

• k is in units of W/ m K

• Low k = small heat transfer

A

T2

T1

Q

L

Conduction Rate Factors increased in temperature by the same amount. Rank the height increase from most to least.

• Free electrons

• Density

• Cross sectional area

• Large window loses more heat than small

• Temperature difference

• Thickness

• Heat takes less time to move through thinner material

Convection Rate Factors increased in temperature by the same amount. Rank the height increase from most to least.

• Fluidity

• Energy exchange with environment

• How easy is it to heat (by conduction or radiation) the material in the first place?

• Temperature difference

Radiative Heat Transfer increased in temperature by the same amount. Rank the height increase from most to least.

• All objects emit photons

• The amount of heat radiated out from an object is called the power (P):

DQ/Dt = Pr = sAeT4

• where

• s =

• 5.6696 X 10-8 W/m2 K4

• A is the surface area

• e is the emissivity (number between 0 and 1)

• 0 =

• perfect reflector

• 1 =

• perfect absorber or black body

Radiation Rate Factors increased in temperature by the same amount. Rank the height increase from most to least.

• Surface area

• Emissivity

• Dark objects emit and absorb more than light ones

• Temperature

Radiation Exchange increased in temperature by the same amount. Rank the height increase from most to least.

• Total power transferred is power absorbed minus power emitted

Pnet = sAe(T4-T42)

• Note that T must be in Kelvin

• No radiation at 0 K

Next Time increased in temperature by the same amount. Rank the height increase from most to least.