Heat Transfer Review

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Slide 1

Heat Transfer

March 20, 2007

Slide 2

Warm-up

Wrap a fur coat around a thermometer. Will its temperature rise? Explain why or why not.

If you hold one end of a metal nail against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.

Slide 3

Answer

No, the thermometer will not heat up because it is the same temperature as the coat. There has to be a difference in temperature in order for heat transfer to take place

No, cold does not flow. Heat flows from your hand to the nail to the ice. Since your hand is losing heat, it feels cold. HEAT is the only thing that flows.

Slide 4

Practice Question

Explain why “firewalkers” can walk safely across a bed of red-hot coals in bare feet.

Slide 5

Answer

Because the rate of heat transfer is slow, because coals are not very good conductors. If they walk fast enough, there is only a small amount of heat transferred to their feet, and they do not burn.

Slide 6

Practice Question

If heat transfers from hot to cold, and stops when the two objects reach the same temperature, explain why it is possible for the inside of your car to be hotter than the air outside on a hot summer day.

Slide 7

Answer

The greenhouse effect. Electromagnetic waves from the sun enter your car through the windows and are absorbed by the seats and other objects in the car. They then re-emit electromagnetic radiation of a different wavelength that cannot pass through glass. All of that energy is therefore trapped inside the car, causing the temperature to rise.

Slide 8

Practice Question

Since a hot cup of tea cools more rapidly than a lukewarm cup of tea, would it be correct to say that a hot cup of tea will cool to room temperature before a lukewarm cup of tea will?

Slide 9

Answer

No, because as it cools, the rate of heat transfer slows down until it is just as slow as the lukewarm cup of tea. The only way to make it cool faster is by introducing convection. (mpemba effect)

Slide 10