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Neeed help am stuck with these

  1. Mar 23, 2009 #1
    1. The problem statement, all variables and given/known data

    Define specific heat capacity and latent heat of fusion.
    Copper melts at a temperature of 1356 K with a latent heat of fusion
    of 210,000 J/kg (Joules per kilogram). Calculate the thermal energy required
    to convert a copper bar with a mass of 40 kg at a temperature of 280 K into
    molten metal at 1360 K if the average specific heat capacity of copper (liquid
    and solid) is 385 J/kg/K (Joules per kilogram per Kelvin) in the temperature
    range considered here

    2. Relevant equations

    energy SCH(specific heat capacity) * mass * change in temp

    3. The attempt at a solution


    i tried to use this formula but the numbers am gettin isnt write , if possible can someone guide me on how to do this or what am i doing wrong, am good with biology but am am also learning physics which is very difficult for me .
     
  2. jcsd
  3. Mar 23, 2009 #2

    LowlyPion

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    Welcome to PF.

    Maybe if you show your calculation it will be easier to figure what may be going awry?
     
  4. Mar 23, 2009 #3
    Thanks for the welcome .

    SHC heat change is 1360 k - 280 k = 1080K

    1080 k * 40kg (mass) = 43200


    43200 * 385 j/kg/k = 16632000

    to calculate quantity of thermal energy, then multiply mass, the specific heat capacity (for liquid water this is 4.19J/g degrees Celsius), and the change in temperature. and this is what i used but the number is tooo high and loooks wrong , i dont know what i am doing wrong
     
  5. Mar 23, 2009 #4
    Thanks for the welcome .

    SHC heat change is 1360 k - 280 k = 1080K

    1080 k * 40kg (mass) = 43200


    43200 * 385 j/kg/k = 16632000

    to calculate quantity of thermal energy, then multiply mass, the specific heat capacity (for liquid water this is 4.19J/g degrees Celsius), and the change in temperature. and this is what i used but the number is tooo high and loooks wrong , i dont know what i am doing wrong
     
  6. Mar 23, 2009 #5

    LowlyPion

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    What about the 210,000 j/kg to melt it to liquid?
     
  7. Mar 23, 2009 #6
    do i need to divide 210,000 by 40kg mass = 5250 OR??
     
  8. Mar 23, 2009 #7

    LowlyPion

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    Didn't it say it was J/kg?

    Is divide the right thing to do?
     
  9. Mar 23, 2009 #8
    not sure, is there a formula which you can telll me , sorry i dont want to sound stupid, but maths and physics are not my thing but i have tooo to this its one of my modules please help me further and next time i would be able to this myself
     
  10. Mar 23, 2009 #9

    LowlyPion

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    You figured the energy needed to raise the temp of the 40 kg.

    But now you still need to consider the heat of fusion - the energy that will make it liquefy. If you need 210,000J to liquefy 1 kg of cooper already at 1360 K, then how much more to liquefy 40 kg?

    Add the 2 together and you have your total thermal energy.
     
  11. Mar 23, 2009 #10

    so 210,000 j/kg * 40kg = 8400000

    but what do i need to add it too in order to get the thremal energy
     
  12. Mar 23, 2009 #11

    LowlyPion

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    What about the energy to get if from 280K to 1360 K?
     
  13. Mar 23, 2009 #12
    so 1350k - 280k = 1070k

    so 1070k needed to get it to 1360k

    then add 1360k to 8400000 but most likely not OR???
     
  14. Mar 23, 2009 #13

    LowlyPion

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  15. Mar 23, 2009 #14
  16. Mar 23, 2009 #15

    LowlyPion

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    You figured out in post 3 how much energy it took to raise the copper to the melting point of 1360K.

    Then you figured how much to melt the block, once it got to 1360K

    Total energy required then is the sum of those 2 energy requirements.
     
  17. Mar 23, 2009 #16
    and what was The i had figured how much to melt the block, once it got to 1360K
     
  18. Mar 23, 2009 #17

    LowlyPion

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    Are you asking about what you found in post 10?

    The results of post 3 and post 10 added together should be your answer.
     
  19. Mar 24, 2009 #18
    16632000 + 8400000 = 33432000 j/kg

    Thats the answer and thats the right sysmbol i put at the end?
     
  20. Mar 24, 2009 #19

    LowlyPion

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    No. They asked for energy.

    That would just be in Joules.
     
  21. Mar 24, 2009 #20

    16632000 + 8400000 = 33432000Q

    Q = m x T x Cp

    Q = change in thermal energy
    m= mass of substance
    T = change in temperature (Tf – Ti)
    Cp = specific heat of substance

    i got this from a website

    so is it 33432000 Q OR whats the symbol for the thermal energy
     
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