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Simple thermodynamics

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data

    6. Cameron burns a 0.5g peanut beneath 75g of water, which increases in temperature from 22 ºC to 40 ºC
    a. Assuming 40% efficiency, what is the food value, in J, of the peanut?
    b. What is the food value in J per gram?

    2. Relevant equations
    q=mCt


    3. The attempt at a solution

    i just wanted to know does question a mean i take 40% from the calculated joules???
     
  2. jcsd
  3. May 12, 2009 #2
    No, it's not, what it's saying to you is that the energy given off by the peanut to heat up the water is only 40% of the total energy given off in combustion, you need to work out the 100%. (was confused there for a sec :D)
     
  4. May 12, 2009 #3
    No. It means only 40% of the peanut's energy goes into the water
     
  5. May 13, 2009 #4
    what so how am i meant to calulate the other 60% thats not possible???
     
  6. May 13, 2009 #5
    Of course it's possible, that's like me saying to you;

    You have 4 apples, which is 40% of the total number of apples from the bag, how many apples were in the bag?
     
  7. May 13, 2009 #6
    er could you put that to me algerbarically please... i mean it makes sence if it was 50% all you do is double it
     
  8. May 13, 2009 #7
    could it be .4x=(Joules calulated)???
     
  9. May 13, 2009 #8
    0.0

    From 50% to 100% you times by 2....

    From 40% to 100% you times by...

    And ^ yes you're right.
     
  10. May 13, 2009 #9
    :smile:
    Beat me to it again.
    Although I considered replying earlier
    in similar vein I just couldn't bear to do it.

    David
     
  11. May 13, 2009 #10
    lawl =D
     
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