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Find the mass of a metal block with heat capacity 60JK

  1. Oct 5, 2016 #1
    1. The problem statement, all variables and given/known data
    A metal block of heat capacity 60 J K -1 loses 1MJ of heat energy to its surroundings. Find the decrease in its temperature.

    2. Relevant equations
    Q = m c Δ θ
    Heat energy added = mass x specific heat capacity x rise in temperature
    heat energy lost = blah blah blah x drop in temperature

    3. The attempt at a solution

    c = 60
    m = ? How do I find this out so I can do the equation ?
    Δ θ = This is what I need to find out for the overall answer.
     
  2. jcsd
  3. Oct 5, 2016 #2

    billy_joule

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    Science Advisor

    You have one equation with two unknowns so you don't have enough information to find change in temperature.
    Expressing change in temp. in terms of m is the best you can do with the info you have.
     
  4. Oct 5, 2016 #3

    gneill

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    Staff: Mentor

    Note that the given constant is not a specific heat capacity constant, it's the heat capacity of the block in Joules per degree Kelvin. As such you do not need the mass as it's already taken into account (think of it as being the specific heat of the substance multiplied by the mass of the block). The constant would apply to this block and this block only.
     
  5. Oct 6, 2016 #4
    OK so 600JK -1 (600 joules per kilo per Kelvin) is necessary to change the temperature by `
    1 MJ of heat is lost...
    Is that 1 megajoules ? so that is 1 million joules?
    The answer at the back says that the change in temperature is 1667 degrees Celsius.. I am sorry I can't see how to get that answer
     
  6. Oct 6, 2016 #5

    gneill

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    Staff: Mentor

    Is the given constant 60 or 600 J/K ? You have 60 in the problem statement.... Ah, I can see that if the answer is 1667 C that "600" is the correct value.

    Note that there's no "kilo" in the definition of the given constant. It's 600 Joules per degree kelvin. If the block temperature changes by one degree (C or K) then its heat content has changed by 600 Joules.

    Yes, MJ is shorthand for megajoule, 106 Joules.
     
  7. Oct 6, 2016 #6
    ah I get it! I have to divide a million by 600, and I understand why, too. Thank you! :)
     
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