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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. Heat. What is heat? Same temperature, no heat

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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

• 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

• 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)

• If you heat a metal spoon and a wooden spoon for the same time, which will have a higher T?

• The 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

• 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.

• 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

• 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)

• Heat (like information) is transferred in different ways

• Conduction

• Convection

• Why?

• They interact and collide with other atoms and electrons and pass the energy on

• 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

• 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

• Surface area

• Emissivity

• Radiation is strongly dependant on T

• 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

• Fluidity

• Energy exchange with environment

• ?

• How rapidly will the material lose heat?

• Small temperature difference, not enough density difference to move

• 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?

A

T2

T1

Q

L

• 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

• 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

• 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