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Heat
Physics 102Professor Lee
CarknerLecture 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
Heat What is heat?
Same temperature, no heat
Heat used to be thought of a fluid (caloric) that could flow to change temperature
Heat is represented by the letter Q
Measuring Heat
Common unit of heat is the calorie: Amount of heat necessary to increase the temperature
of 1 gram of water by 1 C
In nutrition the Calorie is used Case sensitive!
For rates of heat transfer (Q/t), unit is the Watt (W)
Heat and Temperature
If you heat a metal spoon and a
wooden spoon for the same time, which will have a higher T?
The specific heat
Specific Heat
The specific heat is defined as:
c has units of J/kg C
Need to know the mass of the stuff (m) and the change in temperature (T)
Q =mcT
Today’s PAL
A certain amount of heat Q will warm 1 g of material A by 3 degrees C and 1 g of material B by 4 degrees C. Which material has the greater specific heat? Explain.
Calorimetry
Insulated container that prevents heat transfer from outside
Since calorimeter is insulated, negative heat lost cancels out positive heat gained Q1 + Q2 + Q3 … = 0
Heat gained always positive, heat lost always negative
Make sure units for T and m match units for c
Example: Quenching a Dagger
Suppose a silver dagger of mass ms at Ts is immersed in a mass mw of water at Tw. What is the final temperature of the water?
Qsilver + Qwater = 0
csmsT + cwmwT = 0
csms(Tf - Ts) + cwmw(Tf- Tw) = 0
csmsTf -csms Ts + cwmwTf - cwmw Tw = 0
csmsTf + cwmwTf = csms Ts + cwmw Tw
Tf = (csms Ts + cwmwTw)/(csms+ cwmw)
How Does Heat Move?
Heat (like information) is transferred in different ways
Conduction
Radiation
Convection
Conduction
Why?
They interact and collide with other atoms and electrons and pass the energy on
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
Radiation How does the energy from the Sun get
to Earth? How can energy be transported with no
physical contact?
Photons are emitted by the Sun and absorbed by you
All objects emit photons
Radiation Rate Factors
Surface area
Emissivity
Radiation is strongly dependant on T
The Surface of the Sun
Convection
Hot air is less dense than the cooler air above it
After cooling the air may fall back down
Examples: baseboard heating, boiling water, Earth’s atmosphere
Convection Rate Factors
Fluidity
Energy exchange with environment ? How rapidly will the material lose heat?
Small temperature difference, not enough
density difference to move
Today’s PAL A hot piece of metal is at the bottom of a
canister that can be completely filled with: solid iron liquid water air a vacuum
Consider the heat flow from the bottom to the top. In which situation(s) would there be no conduction? In which situation(s) would there be no convection? In which situation(s) would there be no radiation?
Conduction Diagram
L
AT1
T2
Q
Conductive Heat Transfer The rate of heat transfer via conduction is:
where:
T1 is the temperature of the hot side and T2 is the temperature of the cold side
A is the cross sectional area L is the thickness k is the thermal conductivity
High k = large heat transfer Low k = small heat transfer
Radiative Heat Transfer The amount of heat radiated out from an
object is called the power (P):
where = the Stefan-Boltzmann constant
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 Exchange
All objects emit and absorb radiation
Pnet = AeT4-T42)
Where T2 is the temperature of the surroundings
Note that T must be in Kelvin
Next Time
Read: 13.6-13.11 Homework: CH 14, P: 13, 47, CH
13, P: 29, 48 Help sessions start next week
Tuesday and Thursday 6-8pm Science 304
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