Transfer of heat by radiation and Second Law of Thermodynamics

Click For Summary

Discussion Overview

The discussion revolves around the transfer of heat by radiation in the context of the Second Law of Thermodynamics. Participants explore scenarios where a cooler fluorescent bulb appears to heat a warmer object, questioning whether this contradicts established thermodynamic principles. The conversation includes theoretical implications, practical examples, and clarifications regarding energy transfer mechanisms.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the Second Law of Thermodynamics states heat does not flow spontaneously from cold to hot bodies, questioning if the behavior of a fluorescent bulb contradicts this law.
  • Others propose that the energy emitted by the bulb is not purely thermal energy, suggesting that the bulb's operation involves other energy transfer mechanisms.
  • A participant introduces the example of a laser cutting steel, arguing that energy transfer in this case does not violate the Second Law since it is not thermal energy transfer.
  • Clarifications are made regarding the term "spontaneously," with some participants emphasizing that energy transfer can occur with external assistance, such as power sources or work done on the system.
  • Concerns are raised about the consistency of arguments, particularly regarding the presence of a power source and the role of components like a ballast in the fluorescent bulb's operation.
  • Some participants suggest that the observed heating might be due to waste heat from other sources rather than the bulb itself.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the behavior of the fluorescent bulb contradicts the Second Law of Thermodynamics. Multiple competing views remain regarding the nature of energy transfer and the implications of the law.

Contextual Notes

Participants express uncertainty about the specific conditions under which the bulb heats a warmer object, and there are unresolved discussions about the definitions of thermal energy versus other forms of energy transfer.

Phynite
Messages
6
Reaction score
0
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 ? :confused:
 
Science news on Phys.org
Phynite said:
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).
 
Phynite said:
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 ? :confused:
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
 
Nugatory said:
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.

Andrew Mason said:
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 ?
 
Phynite said:
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.
 
DaleSpam said:
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.
 
Phynite said:
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.
 
Phynite said:
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.
 
Phynite said:
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
 

Similar threads

High School How are thermodynamic laws related to one another?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad The Second Law of Thermodynamics and the Stefan-Boltzmann Law
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad An experiment against the second law of thermodynamics
  • · Replies 37 ·
2
Replies
37
Views
5K
Undergrad Correct statement of 2nd law of thermodynamics?
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Entropy change due to heat transfer between sources
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Theoretical maximum efficiency of a heat engine without Carnot
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad A paradox about energy balance and steady state
  • · Replies 2 ·
Replies
2
Views
3K
Graduate Machine in Clausius' 2nd law of thermodynamics?
  • · Replies 7 ·
Replies
7
Views
16K
Undergrad Relating Entropy and the 2nd law
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Definition of Heat and the First Law of Thermodynamics (discrepancy?)
  • · Replies 5 ·
Replies
5
Views
2K