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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

HeatPAL #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 (DT)
Q =mcDT

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

csmsDT + cwmwDT = 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

Convection After cooling the air may fall back down Examples: baseboard heating, boiling water, Earth’s atmosphere

- Hot air is less dense than the cooler air above it

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?

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
- s = 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

- 0 =

- s = the Stefan-Boltzmann constant

Radiation Exchange

- All objects emit and absorb radiation
Pnet = sAe(T4-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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