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Homework Help: Heat and light

  1. Sep 27, 2010 #1
    50kg of water at 0ºC have to be frozen into ice in a refrigerator. The room temp. is 20ºC. What is the minimum work input to the refrigerator to achieve this?
    (latent heat of fusion of water=3.33x10^5J/kg)
    ans:1.22x10^6J

    It is proposed to heat a house using a heat pump operating between the house and the outside. The hose is to be kept at 22ºC, the outside is at -10ºC and the heat loss from the house is 15kW. What is the minimum power required to operate the pump?
    ans:1.63kW

    Please help me to show the methods. I don't understand!!
     
  2. jcsd
  3. Sep 27, 2010 #2

    rock.freak667

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    The energy needed goes into decreasing the temperature of the water to 0°C (sensible heat) AND into changing the phase of the water (latent heating)

    Given the inside and outside temperature, what is the Carnot efficiency of the pump?

    Carnot efficiency = 1 - (Tc/Th) (h=hot temp., c=cold temp.)
     
  4. Sep 27, 2010 #3
    about question1, I do not know the method. I have tried many times but the ans is not matched.
     
  5. Sep 27, 2010 #4

    rock.freak667

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    I think my method should work, are you sure that answer you posted is correct?
     
  6. Sep 27, 2010 #5
    yes. This is my assignment needed to submit tomorrow. Please thanks
     
  7. Sep 27, 2010 #6
    no.2 i still cannot prove the answer correct. I have used your calculation which is also my content of recent learning. But, I cannot do anything....
     
  8. Sep 27, 2010 #7
    no ideas....oh my god
     
  9. Sep 27, 2010 #8

    rock.freak667

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    For question 2, when you get the efficiency; you also know that efficiency = Heat/Work

    so you can find the work like that.
     
  10. Sep 27, 2010 #9
    May u write the procedure, thank you. If u are busy, it is not matter
     
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