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Homework Help: First law of thermodynamics rate problem

  1. Sep 22, 2008 #1
    1. The problem statement, all variables and given/known data
    consider a room that is initially at the outdoor temperature of 20 degrees C. The room contains a 100-W lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy content of the room when all of these electric devices are on.

    2. Relevant equations
    dE/dt (of the control mass)= -Q(convection)+W(electrical)-Q(conduction)

    3. The attempt at a solution
    I tried to take into account that the electrical work is given, but im unsure on how this correlates to the rate of increase of energy. am i solving the first equation i wrote for dE/dt? and if thats the case, how would i find the average heat transfer coefficiant for Q(conv) since i can cancel out Q(conduction)?

    thanks a lot
  2. jcsd
  3. Sep 23, 2008 #2


    User Avatar

    Staff: Mentor

    You are told to assume no heat is lost.

    Honestly, I have no idea what to make out of your analysis of the problem, it seems to be completely off.
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