Conduction, Conduction, Convection+RadiationConvection+Radiation

QuestionQuestionTouch 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 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 = A (TH-TC)/L

» Q/t = A T/x

= “thermal conductivity”» Units: J/s-m-C

» good thermal conductors…high » good thermal insulators … low

R = L/(A = thermal resistance

demos

TH

Hot

TC

Cold

L = x

Area A

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Question 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 waterExample: 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 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?How does an oven work?

Heat Transfer Convection 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 floorA/C ducts on ceilingstove heats water from bottom“riding the thermals”

heater

demos

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Heat Transfer: RadiationHeat Transfer: Radiation All things radiate and absorb electromagnetic

energy

Iemit = eAT4

Iabsorb = eAT04

Inet = Iemit - Iabsorb = eA(T4 - T04)

» if T > T0, object cools down

» if T < T0, object heats up

T

Surroundings at T0

Hot stove

HW

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SummarySummary

Conduction - contact

Convection - fluid motion

Radiation – Electromagnetic waves

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