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Homework Help: Specific Heat Capacity Question - Unknown Mass

  1. Feb 26, 2010 #1
    Q. Thermal energy is supplied to a melting solid at a constant rate of 2000 W. Calculate the mass of the solid changed to liquid in 2.0 minutes. Assume that the specific latent heat of fusion of the solid is 95 000 J kg-1 and that heat exchange with the surroundings may be neglected.

    I was doing my physics homework just fine until I came to this question. I have NO idea how to do this :s Please help!

    Note: I'm really bad physics. Like REALLY bad and I've just started to get the hang of it a little.
  2. jcsd
  3. Feb 26, 2010 #2
    What is the total heat energy supplied to the melting solid over the time interval?
  4. Feb 26, 2010 #3
    I believe it'll be 2000 W x 2.0 minutes = 4000 J
    Do I need to convert it into seconds first?
  5. Feb 26, 2010 #4
    Yes, you do need to convert it into seconds because one watt of power is equivalent to one joule of energy input/output per second. W = J/s
  6. Feb 26, 2010 #5
    So 2 minutes = 120 s
    2000 x 120 = 240000 J

    How do I find the mass now?
  7. Feb 26, 2010 #6
    Good, so we've established that 240 kJ of thermal energy was supplied to melt the solid. From your inability to carry on, I gather that you have no idea what the "specific latent heat of fusion" means? The specific latent heat of fusion of a substance is defined as the energy per unit mass required to change the substance from the solid to liquid phase (ie melt it) without any change in temperature. In this case, this implies that 95 kJ of energy is required to just melt 1 kg of the solid. So, if we had to use 240 kJ of energy to melt the amount of solid that we have, how many kilograms would that be?
  8. Feb 26, 2010 #7
    That makes a lot more sense now :)
    240 รท 95 = 2.526315789
    Not quite sure how to round this...
  9. Feb 26, 2010 #8


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    The convention is to round it so that it has the same number of significant digits as the least accurate input number. Here, you can just round to 2.5, because 95 has two significant digits.
  10. Feb 26, 2010 #9
    Thank you so much both of you. This was really helpful for such a non-physics person like me.
    God bless you two!
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