# Transfer of heat by radiation and Second Law of Thermodynamics

Second law of thermodynamics states that heat does not flow spontaneously from cold to hot bodies. But a cool fluorescent bulb is perfectly capable of heating something that had already started out being warmer than the bulb itself. Is this not a contradiction to the law ?

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Nugatory
Mentor
But a cool fluorescent bulb is perfectly capable of heating something that had already started out being warmer than the bulb itself.
Can you describe the conditions under which this happens more clearly? I suspect that you're seeing the effects of heat from another source (for example, waste heat from a faulty ballast).

Andrew Mason
Homework Helper
Second law of thermodynamics states that heat does not flow spontaneously from cold to hot bodies. But a cool fluorescent bulb is perfectly capable of heating something that had already started out being warmer than the bulb itself. Is this not a contradiction to the law ?
The fluorescent bulb is not a purely thermal source. So the flow of energy from the bulb is not heat flow.

An even better example would be a laser. The temperature of the laser device used to cut steel is much less than temperature of the steel that the laser strikes. But this does not violate the second law since the energy flow from the laser to the steel is not a transfer of thermal energy.

AM

Can you describe the conditions under which this happens more clearly? I suspect that you're seeing the effects of heat from another source (for example, waste heat from a faulty ballast).
No, there is no other heat source. The system contains only the bulb and an object that is hotter than the bulb.

The fluorescent bulb is not a purely thermal source. So the flow of energy from the bulb is not heat flow.

An even better example would be a laser. The temperature of the laser device used to cut steel is much less than temperature of the steel that the laser strikes. But this does not violate the second law since the energy flow from the laser to the steel is not a transfer of thermal energy.

AM
Then how about a system with two identical bodies of temperatures T and T/2 ?

Dale
Mentor
No, there is no other heat source. The system contains only the bulb and an object that is hotter than the bulb.
Then the bulb cannot be lit. Bulbs don't light if they are isolated like that.

Then the bulb cannot be lit. Bulbs don't light if they are isolated like that.
I didn't mean that there is no power source.

D H
Staff Emeritus
I didn't mean that there is no power source.
You forgot that key word "spontaneously" in this interpretation of the second law of thermodynamics. Think of "spontaneously" as meaning "without outside help". An example: In a couple of weeks my AC will be running 24/7, transferring heat from my cool house to the hot and muggy outdoors. This isn't a violation of the second law because energy is being used to facilitate that reverse heat transfer. The same applies to Andrew's laser and to your fluorescent bulb.

Dale
Mentor
I didn't mean that there is no power source.
Your posts are self-contradictory. The faulty ballast that Nugatory mentioned is part of the power source for a flourescent light. In response to that you said no there is nothing but the bulb. And now you mention that there is also a power source, which includes the ballast than Nugatory mentioned. Please post one scenario and don't keep changing things!

If you have a power source then work is being done on the system and heat can be moved up a temperature gradient. However, I think that Nugatory's original suggestion is more likely. The ballast is probably faulty and heating up the surrounding structures.

Andrew Mason
Homework Helper
I didn't mean that there is no power source.
Phynite, it seemed rather obvious to me that you were speaking about a working fluorescent bulb. Your question was a good one. The key to the answer is recognizing that the second law of thermodynamics prohibits spontaneous flow of a certain form of energy: thermal energy - from cold to hot. It does not prohibit the flow of energy in general.

AM