# Change of entropy

1. Dec 26, 2008

### KFC

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 $$\Delta Q/T$$, $$\Delta Q$$ is heat released and T is temperature.

2. Dec 27, 2008

Staff Emeritus
In what way is the resistor different after the current has flowed through it?

3. Dec 27, 2008

### KFC

But it release heat and work done.

4. Dec 27, 2008

### tim_lou

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: Dec 27, 2008
5. Dec 27, 2008