# Entropy - physical system

1. Jan 29, 2007

### Brewer

1. The problem statement, all variables and given/known data
A 50 ohm resistor carries a current of 250 mA and is maintained at a constant temperature of 303K by thermal contact with a heat sink.

Calculate the entopy change of

i) the resistor, and
ii) the universe,

if the current flows for 30 seconds.

2. Relevant equations
dS >= 0?

3. The attempt at a solution
I'm not sure how to attempt this question for a physical problem. Up until now I've been restricted to doing questions of this type based upon ideal gases, so I'm not sure how I would apply that to a physical system like this. Any hints anyone?

2. Jan 29, 2007

### Brewer

Right, I've had another look at this.

I've said that dQ=dW and that work = power * time, so dW = Pdt = I*I*R*dt

From here I've substituted this into the equation $$dS = \int \frac{I^2R}{T}dt$$ with limits 0 and 3.

Following this through I get an answer of 0.031 J/kg. Does this sound about right? Then for the entropy change of the universe it'll be just the negative of this answer?