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Calculate heat required to melt ingot

  1. May 30, 2016 #1
    1. The problem statement, all variables and given/known data
    Given that aluminium has a specific heat of 0.9KJ/KgK, a melting point of 660 degrees Celsius and a latent heat of fusion of 390KJ/Kg, calculate the heat energy required to melt a 10Kg ingot, starting from a room temperature of 20 degrees Celsius.

    2. Relevant equations
    dQ= m c dt
    q=Lm m

    3. The attempt at a solution
    (10)(0.9)(640)=5760KJ

    I get a different answer using the latent heat equation, so which should be used for working out the heat energy for melting and why are both specific and latent heat given, if not as a redherring?
     
  2. jcsd
  3. May 30, 2016 #2
    If you heat the ingot to 660C using 5760 kJ, has it melted yet?
     
  4. May 30, 2016 #3

    DrClaude

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    Is heating the ingot to 660 °C sufficient to melt it?

    Please note that the prefix kilo is written with a lowercase k.
     
  5. May 30, 2016 #4
    660 is the melting point of aluminium. If it is the case that the first equation is sufficient then what is with the fact that they mention latent heat aswell?
     
  6. May 30, 2016 #5

    DrClaude

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    Is it sufficient to bring a substance to its melting point for it to melt? What is the definition of latent heat?
     
  7. May 30, 2016 #6

    SteamKing

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    Do you understand what latent heat of fusion means?
     
  8. May 30, 2016 #7
    'The specific latent heat of fusion, l, of a substance is the heat needed to change a mass of 1 kg the substance from a solid at its melting point into liquid at the same temperature.'

    So I need to add the heat to raise it to boiling point to the heat to melt it?

    5760kJ+(390000*10)=3905760000J
     
  9. May 30, 2016 #8

    DrClaude

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    Yes, but check those units.
     
  10. May 30, 2016 #9

    SteamKing

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    Yes.

    But you've made some kind of silly mistake in the calculation above.

    The heat required to raise the aluminum to the melting point (not the boiling point) is 5760 kJ. Once the aluminum is at the melting point, it takes 390 kJ/kg to turn solid aluminum into molten aluminum.
     
  11. May 30, 2016 #10
    5760000+(360000*10)=9660000J
     
  12. May 30, 2016 #11

    SteamKing

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    That looks much better, except it's 390000 * 10, not 360000.

    Always proofread your work to avoid silly mistakes like this.
     
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