Physics 106P: Lecture 1 Notes
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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 = eAT4
Iabsorb = eAT04
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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