Heat Physics 102 Professor Lee Carkner Lecture 2

PAL #2 Galileo Thermometer  How does it work?  Water heats up and expands, becomes less dense, as the density of the water decreases it can’t support heavier balls which drop, balls have different densities (lower ones marked with lower T are denser)  Limitations  Not very accurate, limited range, needs to be kept upright, won’t work in free fall, delicate, can freeze solid

Heat  What is heat?   Same temperature, no heat  Heat is not a “thing”, it is a transfer of energy  Units:  Joules  Kilojoules (kJ) =  calories (cal) =  Calorie (Cal) = 1 kilocal = 1000 cal =  For rates of heat transfer (Q/t), unit is the Watt (W) = J/s

Specific Heat   The specific heat (c): c = Q/m  T   It is the amount of heat need to change 1 kg of stuff 1 degree C   Can rewrite as: Q =mc  T

Calorimetry  To do experiments with heat we use a calorimeter   The total heat exchange is the sum of the heat from all processes   Q 1 + Q 2 + Q 3 … = 0  Always write  T = T f -T i   Use consistent units  Make sure units for T and m match units for c

How Does Heat Move?  Heat (like information) is transferred in different ways  Conduction   Convection   Radiation  Photons are emitted and absorbed

Conductive Heat Transfer  The rate of heat transfer via conduction is: Q/t = kA(T 1 -T 2 )/L  where:  T 1 is the temperature of the hot side and T 2 is the temperature of the cold side   L is the thickness   k is in units of W/ m K  High k =  Low k = L A T1T1 T2T2 Q

Conduction Rate Factors  Free electrons   Density  Low density materials (like gases) don’t have a lot of collisions  Cross sectional area   Temperature difference   Thickness  Heat takes less time to move through thinner material

Convection Rate Factors  Fluidity   Energy exchange with environment   How rapidly will the material lose heat?   Small temperature difference, not enough density difference to move

Radiative Heat Transfer  All objects emit photons   The amount of heat radiated out from an object is called the power (P):  Q/  t = P r =  AeT 4  where   =  X W/m 2 K 4   e is the emissivity (number between 0 and 1)  0 = no energy absorbed or emitted   1 = max energy absorbed or emitted 

Radiation Rate Factors  Surface area   Emissivity   Temperature  Radiation is strongly dependant on T

Radiation Exchange  All objects emit and absorb radiation  P net =  Ae  T 4 -T 4 2 )  Where T 2 is the temperature of the surroundings  Note that T must be in Kelvin 

Next Time  Read:  Homework: CH 14, P: 13, 37, CH 13, P: 29, 48

If two objects are in thermal equilibrium with each other 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