conduction, convection and radiation trivia

Hi

I have a few conduction/convection/radiation trivia questions, some of which i think i know the answer to and a couple i don't. Please say if the ones i have answered are right/wrong and answer the ones i don't know :) Any elaboration or comments also welcomed

1 Why is the element at the bottom on an electric kettle
Convection currents means cold water at the bottom of kettle

2. Why is freezer box at top of fridge
Don't know. I would have thought it would be warmest at the top due to convection

3. Why is it more likely to be fristy on a clear night than cloudy night
Clouds stop heat escaping

4. Why is back of refrigerator painted black
I don;t know, do they mean the inside or outside of a fridge? Black is a better radiator and absorber of radiation. If the outside of the fridge was black would radiate more heat but also absorb more too. If inside of fridge was black this would absorb more heat out of the fridge i guess. In either case, why just the back? Why not sides too?

5. What would happen if you sprinked black soot on ice in the antartic
Don;t understand the relvance of the antartic but presumbaly the ice would be more likely to melt

6. Why are heat sinks black, with large aluminium fins
Fins gives large surface area; black will absorb the heat better, aluminium will conduct heat away

Many thanks
andiejeAsked:
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lee555J5Connect With a Mentor Commented:
1 Heat from the element will rise through the fluid in the kettle
2 Cool air is denser than warm air and therefore goes down through the refrigerator below
3 correct AND clouds keep moisture in air and not on ground
4 black is best color for radiating (and absorbing) heat
5 Black would absorb more heat from sun and transfer it to the ice directly below it and melt it
6 a correct, b radiate, not absorb, c correct
Lee
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andiejeAuthor Commented:
Thanks for your answer Lee

I still dont understand 2 and 4.

Cool air is denser so falls to the bottom of the fridge i get that. This the coldest part of the fridge is at the bottom so wouldn't it be easier then to have the freezer box at the bottom as it would take less energy to keep it cold?

Regarding the fridge again, do you know if it is black on the inside or outside of the fridge?
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lee555J5Commented:
2 You have to work to cool the freezer, so you might as well use its escaping cool air to cool the fridge below.
4 outside, to radiate
Lee
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Photon0137Connect With a Mentor Commented:
Hi, Andieje

Just to expand and possibly clarify answers 2 and 4.

1) That thing in your electric kettle is known as an immersion heater. As it is turned on, it heats water. The heated water expands and becomes less dense and rises. This replaces the cooler water on top and starts a convection current. This allows heat to become distributed throughout the fluid.

If it was at the top, it will just heat the water on top and the hot water will stay there. the only way through which heat ca propagate is through conduction and fluid are generally poor conductors. It's really about efficiency.

2) The freezer is at the top for the same reason the heater is at the bottom. As it cools air, the cooler air contracts, becomes more dense and sinks. This forces the warmer air to the top to be cooled. The same logic applies if the freezer was placed on the bottom.

3) Clouds are made of water vapor and are opaque to IR radiation, especially in the longer wavelength. SO if there were clouds above, they will trap heat by not letting the radiation escape. By now you have heard about green house gases. This is the principle on which a greenhouse works; the glass of the greenhouse is opaque to certain wavelengths and traps heat inside.

If there are no clouds, then the heat escapes and the temperature drops. As heat is lost, temperature drops and the air can hold less water vapor, so it condenses and if it's cold enough it freezes.

4) Black is a good radiator and absorber. But as the black fins are at a higher temperature than the surroundings, heat will flow from hot to cold. How effectively this takes place depends on many conditions, the color of the fins being one of them. If they were white, then it won't emit heat as effectively.

If the fridge were black, the inside is cooler, so the tendency is for heat to go from the warmer (outside) surroundings to the inside of the refrigerator. Painting it white reduces the rate at which heat is transferred.

5) If you sprinkled black soot, it would absorb more thermal radiation and its temperature would increase. It may be enough to melt the ice.

6) Aluminum is a good conductor and has a higher thermal capacity than air. So it can absorb more heat per degree rise than air. It's really more an issue of it being better at conducting heat away.

Placing fins does increase more of the surface area and hence the heat sink has more area to radiate heat away. By coloring it black, will make it a better radiator.

Hope that helps.

David
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Paul SauvéRetiredCommented:
2. Why is freezer box at top of fridge
Don't know. I would have thought it would be warmest at the top due to convection
 
It's more convenient - they make models with the freezer below. (2 doors)
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BigRatCommented:
>>2. Why is freezer box at top of fridge

The normal construction places the motor compressor in the base of the "box", so that the bottom is heavier than the top and it won't fall over when knocked.

This means that the compressor forces the coolant into the radiator at the bottom. The top of the radiator is where the coolant then goes into the nozzle which reduces the pressure and hence cools the coolant down even further. It then makes sense to feed the cold coolant directly into the freezer part since the coolant is at it's lowest temperature. After going into the freezer, where it will warm up, it is then passed into the refigerator and finally back down into the compressor. Thereofre the freezer box is at the top of the fridge box.
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BigRatCommented:
>>5. What would happen if you sprinked black soot on ice in the antartic

You'd be arrested, since there is an international agreement not to pollute the antartic. Everybody must take their rubbish home with them.
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BigRatCommented:
>>6. Why are heat sinks black, with large aluminium fins

because they are cheaper than heat sinks with copper fins.
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lee555J5Commented:
@BigRat
Re: 5. Who would arrest you? The penguins? :-)
Lee
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BigRatCommented:
@Lee

Probably the US military.
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andiejeAuthor Commented:
many thanks
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