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Homework Help: Entropy - physical system

  1. Jan 29, 2007 #1
    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. jcsd
  3. Jan 29, 2007 #2
    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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