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# If a material is heated from the one side (100C), does it require the same amount of cooling (-100C) to get it cold? Answered

If a material is heated from the one side and it takes 10min to transfer the energy to the other side, how long will it take to get the material cool to it's original temperature? (10min?) Does it depend on heating versus cooling. in other words, if it is heated at 100degrees, do you need to cool it at -100degrees to equal the time it takes to enter versus the time it takes to exit?

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## Discussions

This is difficult because the simple equations assume that the temperature is the same throughout the material.

If that is the case and there are no other losses the cooling time will be the same as the heating time for the same temperature difference. It's a function of the specific heat capacityof the material, which is constant for a given material.

If you were to apply -100C as you suggest (rather than 0C), the cooling time would be half.

You're correct, but see my note above. You'll only get equal times for the same delta-T if you're using direct conduction (heat source/heat sink). Most people think of "cooling" as leaving out in the air. In that case the extremely low specific heat of air makes the cooling time substantially longer than the corresponding heating time.

You have a misconception in your question. The rate of heating or cooling of an object depends on the temperature difference, not the temperature value (and things which do depend on absolute temperature use kelvins, where boiling is 373K, not celsius).

If you are heating an object from room temperature (20C) to 100C, that's a temperature difference of about 80C. If it takes 10 minutes using a direct contact heat source, then you have a heat transfer rate of 8K/min (kelvin and celsius intervals are the same).

If you want to cool the object from 100C back to room temperature (20C), then if you use a direct contact heat sink, it will take the same ten minutes, because the heat transfer rate for the object is known (8K/min).

What is a "direct contact heat sink"? Think of something like a metal plate with water channels drilled through it. You circulate room temperature water through the channels, so the plate is "always" at room temperature.

If you just let the object sit in air, the transfer of heat to air is very poor, and the cooling will take much longer in air (the overall heat transfer rate is combined from the object's intrinsic behaviour plus the low rate of transfer to air).

. Heat energy flow is determined mainly by ΔT (delta T), the difference in temperature between the object and its environment.

Q. Does it depend on heating versus cooling? A. Yes. It depends on the ambient temperature.