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Homework Help: Heat that are needed

  1. Nov 7, 2007 #1
    How many calories of heat are required to raise the temperature of 5 kg of water from 60° to the boiling point? Select the correct answer.

    a. 4.2 x 10^5
    b. 2 x 10^5
    c. 7.5 x 10^4
    d. 18 x 10^5
    e. 420

    The answer is a, but what I calculate
    Q=mcdeltaT
    =5*4186*1
    =20930
    is b, am I doing anything wrong?
     
  2. jcsd
  3. Nov 7, 2007 #2

    CompuChip

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    What is the boiling point of water?
    What is Delta T ?
     
  4. Nov 7, 2007 #3

    malty

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    Em, i suppose you mean [tex] Q=5*4186*10 [/tex] and in that you included your division by four to change the joules into calories.

    I can't see anything wrong anyways, unless it wanted you to include the latent heat of vaporization into the formula, though I doubt that would change your answer by much.

    I may be wrong too though..
     
    Last edited: Nov 8, 2007
  5. Nov 7, 2007 #4
    O...I think the answer in original question is not correct, it should be
    it should be
    Q=5000g * 1 J/(calories*celcius) * 40 celcius
    =20000J

    However the answer is still b though, what is the problem?
     
  6. Nov 8, 2007 #5

    CompuChip

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    You're quite close, but I have the feeling you're mixing up your units.
    Here's the correct calculation (note how I leave in the units and let them cancel at the end).

    Q = m * C * dT
    = 5 kg * 4186 J/(kg C) * 40 C
    = 837,200 J.

    Now taking 1 cal = 4.184 J (which seems to be the accepted value),
    Q = 200,095 cal = 2 x 105
    in the correct number of significant digits.
    So the answer is b) -- if it says it's a) then it is wrong :smile:
     
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