# If you stop cooling a resistor and stop current simultaneously?

## Main Question or Discussion Point

If you stop cooling a resistor and stop current simultaneously then what happens to the temperature of the resistor in the next few moments?

My question is actually about whether a computer CPU will continue to heat up at all when the computer is switched off (Stopping any power to the CPU, so no resistance (and heat) being generated but also no cooling from the fans) or whether it will only get as high as the temperature of the CPU at the instant the computer was switched off ?

I guess this is more or less the same as this simpler question:

Imagine you have current flowing through a resistor which is capable of generating large amounts of heat but you are also cooling the resistor in some way so that it is at a stable temperature at say 20 degrees above room temperature.
How would the temperature of the resistor change if you were to simultaneously stop the flow of current by breaking the circuit and stop cooling the resistor?

Would it continue to increase for a moment or a while?
Would it immediately start to drop?

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Drakkith
Staff Emeritus
Since heat is being produced by current flowing through the resistor, stopping the current also stops heat production. So even if you stop cooling, you aren't producing any more heat and the resistor will immediately begin to cool.

256bits
Gold Member
... so that it is at a stable temperature at say 20 degrees above room temperature.
What part of the resistor would be at 20 degrees?

Heat flows from a hotter temperature to a cooler temperature. The interior of the resistor would have to be at a higher temperature than the surface, which being in contact with the air, would be at a higher temperature than the air itself.

Perhaps your question is more to the line of whether or not the surface temperature of the resistor rises after both the current and the blowing air across it has stopped.

With no more current flowing, there is no more heat production within the resistor so therefore the interior temperature does not rise.

The surface, on the other hand, can rise to a higher temperature, as heat moves from the interior to the exterior, simply due to the reason that natural convection, in most cases, is not as efficient as forced convection in removing heat.