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Homework Help: Microwave energy problem (please check my answers)

  1. Jan 17, 2010 #1
    1. The problem statement, all variables and given/known data

    Power microwave problem?
    A 1000W microwave oven heats 500 g of water from 10.0 degrees Celsius to 55.0 degrees C in a time of 2 min. the heat capacity of water is 4200 J/kgdegrees C



    2. Relevant equations
    have i done this properly? there are 4 other questions with this


    3. The attempt at a solution

    P=1000W
    m=0.5kg
    T1=10.0C
    T2=55.0C
    change in T=45C
    t=120s
    c=4200J/kg degress C

    a) how much thermal energy did the water gain?

    have i done this correctly

    Q=mcT
    =(.5kg)(4200 J/kgC)(45C)
    =94500 J
    Q=9.5 x 10^4 J

    b)How much power was associated with this thermal energy?

    have i done this correctly

    P=E/t
    =9.5x10^4 J / (120s)
    P=787.5 W

    c) What percentage of the electricity power that went into the microwave actually went into heating the water?

    Did i do this correctly

    787.5 W / 1000 W x 100
    = 79%

    d) Why didn't all the electricity energy of the microwave get converted to thermal energy of the water?

    Because loss of energy was also due to opperating the microwave and making cup of water spin

    What are other reasons?

    Thanks!
     
  2. jcsd
  3. Jan 17, 2010 #2

    sylas

    User Avatar
    Science Advisor

    It all looks about right to me. There's a bit of energy going into the heating the air and the cup, but I that would be small. The spinning of the cup would be even smaller. Mostly losses of some kind in the microwave operation sounds about right to me. A microwave oven usually has vents around the back to help get rid of waste heat, which would be most of the losses.

    Cheers -- sylas
     
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