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Entropy - physical system

  • Thread starter Brewer
  • Start date
  • #1
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1. Homework Statement
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. Homework 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?
 

Answers and Replies

  • #2
212
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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 [tex]dS = \int \frac{I^2R}{T}dt[/tex] 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?
 

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