Transcript Physics 106P: Lecture 1 Notes

Conduction,
Convection+Radiation
Question
Touch base of chair, and desk top, which
feels colder?
A) Base
B) Same
C) Desk
Both must be the same temperature (room temperature), but
metal feels colder because it conducts heat better/faster.
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Heat Transfer: Conduction

Hot molecules have more KE than cold molecules

High-speed molecules on left collide with low-speed
molecules on right
 energy transferred to lower-speed molecules
 heat transfers from hot to cold

I = rate of heat transfer = Q/t [J/s]
I = k A (TH-TC)/L
» Q/t = k A T/x
k = “thermal conductivity”
» Units: J/s-m-C
» good thermal conductors…high k
» good thermal insulators … low k
TH
L = x
TC
Cold
Hot
Area A
demos
R = L/(Ak) = thermal resistance
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Question
If you want to maximize the
amount of Heat flow which
substance should you choose?
1. Aluminum
2. Copper
3. Steel
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Example: Boiling water

A steel pot with an area of 0.05m2 and thickness of
0.006m is placed on top of a burner so that its
bottom surface remains at 1500C. How much
water boils away each minute?
Question
If you want to limit the amount
of Heat flow (Insulate) which
substance should you choose?
1. Air
2. Wood
3. Glass
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How does an oven work?
Heat Transfer Convection
Air heats at bottom
 Thermal expansion…density gets smaller
 Lower density air rises

Archimedes: low density floats on high density
Cooler air pushed down
 Cycle continues with net result of circulation
of air
 Practical aspects

heater ducts on floor
A/C ducts on ceiling
stove heats water from bottom
“riding the thermals”
heater
demos
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Heat Transfer: Radiation
 All
things radiate and absorb electromagnetic
energy
Iemit = eAT4
Iabsorb = eAT04
Surroundings at T0
Hot stove
T
Inet = Iemit - Iabsorb = eA(T4 - T04)
» if T > T0, object cools down
» if T < T0, object heats up
HW
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Summary

Conduction - contact

Convection - fluid motion

Radiation – Electromagnetic waves
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