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Check my answer for power output question

  1. Jun 23, 2015 #1
    1. The problem statement, all variables and given/known data

    a 0.25 kg piece of ice at -30°C is warmed by an electric heater and the following graph of temperature is produced.
    -use the information on the graph to determine the power output of the heater

    the graph shows that the ice went from -30degC to -10 degC in 150s
    2. Relevant equations
    Q=mcΔt
    p=w/Δt

    3. The attempt at a solution
    Q=mcΔt
    =(0.25)(3.3*105)((-30)-(-20))
    =82500*(-20)
    1.65*106

    should this be negative?

    p=w/Δt
    =1.65*106/150
    =11000
     
  2. jcsd
  3. Jun 23, 2015 #2

    SteamKing

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    You don't use units in your calculations, so they're hard to check.

    When doing calculations on the rise in temperature of a substance, you want to subtract the colder temp. from the hotter temp.

    Which is colder, -30 °C or -10 °C ?
    (also, you have a typo in the temperatures used in your calculation of Q.

    What is 3.3×105 ? This doesn't seem to be the heat capacity of ice.
     
  4. Jun 23, 2015 #3
    3.3*105 is the latent heat of fusion for water that ive been given, now i see where i may have messed up.
     
  5. Jun 23, 2015 #4

    gneill

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    Staff: Mentor

    The value you've used for the specific heat of ice looks odd to me. Where did it come from and what are the units?
    Note that the temperature change was positive, rising from -30C up to -20C. You've got the order of the temperatures reversed. You should have ΔT = Tfinal - Tinitial
    You might want to use different variable names for the change in time and change in temperature. It is common to use upper case T for temperatures and lower case t for times.

    [Edit: I see that Steam King beat me to it. I'll leave it to him]
     
  6. Jun 23, 2015 #5
    im going to retry my answer. will post shortly
     
  7. Jun 23, 2015 #6
    =0.25kg*2100J/(kg °c)*((-10°C) - (-30) °C)
    =525*20°C
    =10500J

    P=10500J/150s
    =70w

    is this ok?
     
    Last edited: Jun 23, 2015
  8. Jun 23, 2015 #7

    SteamKing

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    -10 - (-30) is how much? This is a simple problem in arithmetic.
     
  9. Jun 23, 2015 #8
    its 20, but my answer has a negative. i was going to edit it but don't know how to
     
  10. Jun 23, 2015 #9

    SteamKing

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    Just hit the edit button in the lower left hand corner.
     
  11. Jun 23, 2015 #10
    I see now. Thanks!
     
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