Change of entropy in a resistor

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Discussion Overview

The discussion revolves around the change of entropy in a resistor when a current passes through it. Participants explore the implications of entropy change in the context of thermodynamics, particularly focusing on the behavior of the resistor at a constant temperature while current flows through it.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant cites a textbook example stating that the change in entropy of a resistor held at a fixed temperature of 300K with a current of 10 amps for 300 seconds is zero, arguing that since the temperature does not change, the state does not change, and thus entropy remains unchanged.
  • Another participant questions the meaning of 'change of state' in the context of a resistor, suggesting that it may refer to a change in energy due to heat absorption from the current, proposing that the entropy should be calculated as ΔQ/T, where ΔQ is the heat released.
  • Some participants inquire about the physical differences in the resistor after current has flowed through it, emphasizing the release of heat and work done during the process.
  • One participant explains that while the resistor facilitates heat transfer, it is the electrons in the circuit that perform work and produce heat, suggesting that the entropy of the electrons decreases while the environment's entropy increases due to heat flow.
  • Another participant draws an analogy to a hot rock dropped in water, indicating that while the water initially gains heat, the overall entropy change may balance out when the water cools back down.

Areas of Agreement / Disagreement

Participants express differing views on the change of entropy in the resistor, with some asserting that it remains unchanged while others argue that heat transfer and work done imply a change in entropy. The discussion remains unresolved regarding the interpretation of entropy change in this context.

Contextual Notes

There are assumptions regarding the definitions of state and energy changes, as well as the conditions under which entropy is considered. The discussion does not resolve the mathematical implications of heat transfer and entropy calculations.

KFC
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There is an example in the textbook to show the change of entropy. A resistor being held at fixed temperature of 300K. 10 amp current passes through the resistor for 300 seconds. The change in entropy of the resistor is ZERO. The reason written in the book reads since the temperature doesn't change, the state doesn't change, so the entropy doesn't change.

I am very confuse about this statement ... if it is asking the change of entropy of gas, it is easy to understand, but now it is asking something like resistor, what does it mean by 'change of state'? Does it mean change of energy? If so, the resistor will absorb energy from the current and change some of them to heat, so the entropy of the resistor so be [tex]\Delta Q/T[/tex], [tex]\Delta Q[/tex] is heat released and T is temperature.
 
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In what way is the resistor different after the current has flowed through it?
 
Vanadium 50 said:
In what way is the resistor different after the current has flowed through it?

But it release heat and work done.
 
heat flows into the environment, the entropy of the environment increases by δQ/T, T being the temperature of the environment.

The resistor doesn't really do anything, it is the electrons in the circuit that is doing all the work and producing heat. The resistor is merely a medium that does the heat transfer from electrons to the environment. Of course, the entropy of the electrons changes as well (it decreases).

This is the same thing as dropping a hot rock in water, the water becomes hot initially and does some work. However, at the end, when the water cools back down, there is no entropy change. It is the rock that ends up losing entropy and the environment ends up gaining it.
 
Last edited:
KFC said:
But it release heat and work done.

Let me ask again. In what way is the resistor itself different after the current has flowed through it?

If I gave you two resistors, and I told you that one of them had current flowing through it yesterday and the other didn't, how could you tell them apart?
 

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