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Water equivalent of a kettle

  1. Mar 29, 2015 #1
    1. Please see the attachments for my attempts at answering the question.

    forum question.PNG

    Please note that this is not a homework question as the answers are provided. I just want to know how to get there. Thanks in advance.
     

    Attached Files:

    Last edited by a moderator: Mar 29, 2015
  2. jcsd
  3. Mar 29, 2015 #2

    haruspex

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    Not sure how you're interpreting the second question. My interpretation is:
    • The 20% of power that was not useful in the first part includes the heat required to raise the kettle to 90C.
    • In the second part, the hot 1 litre of water is poured out and a second litre of water at 15C is poured in.
    • The wasted heat is less the second time because the kettle itself starts hot.
    Your attachments are not easy to read. They come out side-on and the writing is a bit small. The ability to add images is really intended for diagrams and extracts from printed matter. Please take the trouble to type in working.
     
  4. May 9, 2015 #3
    I have the same question in my study guide and I get 10 min and 7 seconds for part b
     
  5. May 9, 2015 #4

    haruspex

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    I get 11, 58 for part a, but maybe 4.2 J per Calorie is not accurate enough. For part b I get 10, 24.
    Please post your working.
     
  6. May 10, 2015 #5
    1000g-150g = 850g

    Eout = mct
    = (.85)(4190)(75)
    = 267112.5J

    Ein = Eout/efficiency
    = 267112.5/0.8
    = 333890.625J


    time = Ein/Pin
    = 333890.625/550
    = 607s = 10 min and 7 sec
     
  7. May 10, 2015 #6

    haruspex

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    Why do you subtract 150 from 1000?
     
  8. May 10, 2015 #7
    The question states that the water equivalent of the kettle is 150g. 1 litre = 1000g. Therefore the kettle only needs to heat 1000g-150g=850g for the second litre of water. I'm not sure if I'm correct though.
     
  9. May 10, 2015 #8

    haruspex

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    There's a litre of water in both cases. The kettle is additional to that.
    You need to think about where the other 20% of the power goes. In each case, the equation takes the form:
    energy in = energy to heat water + energy to heat kettle + energy lost to room/air
    Assign symbols to those (distinguishing a and b) and write the equation for each using the given data.
     
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