Heat Physics 102 Professor Lee Carkner Lecture 3 “If you can’t stand the heat, get out of the kitchen.” -Harry S. Truman

PAL #2 Galileo Thermometer  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

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

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

Specific Heat   The specific heat (c): c = Q/m  T  c has units of J/kg C   Need to know the mass of the stuff (m) and the change in temperature (  T)  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

PAL: Quenching a Dagger  Suppose a silver dagger of mass m s at T s is immersed in a mass m w of water at T w. Derive an expression for the final temperature of the water when thermal equilibrium is reached.

How Does Heat Move?  Heat (like information) is transferred in different ways  Conduction   Convection   Radiation 

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   Low k = small heat transfer L A T1T1 T2T2 Q

Conduction Rate Factors  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  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  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  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  Surface area   Emissivity  Dark objects emit and absorb more than light ones  Temperature 

Radiation Exchange   Total power transferred is power absorbed minus power emitted P net =  Ae  T 4 -T 4 2 )   Note that T must be in Kelvin  No radiation at 0 K

Next Time  Read:  Homework: CH 14, P: 13, 37, CH 13, P: 29, 48  Help sessions start next week  Tuesday and Thursday 6-7pm Science 120