1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Homework Helper

    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

    User Avatar
    Homework Helper

    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

    User Avatar
    Homework Helper

    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
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook