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Homework Help: Physics: the carnot cycle

  1. May 16, 2005 #1
    hey everyone,
    I need some help solving the following problem. I am kind of confused on where to start. here is the question from the book.

    A carnot refrigerator is operated between two heat reservoirs at temperatures of 320K and 270K.

    a) If in each cycle the refrigerator receives 415J of heat energy from the reservoir at 270k, how many joules of heat energy does it deliver to the reservoir at 320k?

    b)If the fridge goes through 165 cycles each minute, what power input is required to operate the refrigerator?

    Thanks in advance for any help.
  2. jcsd
  3. May 16, 2005 #2
  4. May 16, 2005 #3
    What is the efficiency expression for a carnot refrigerator in terms of source and sink temperatures; heat absorbed from source and heat rejected to sink. Equate them and simplify.

    How can you know work done from the heat abosrbed and heat rejected? After that it is simple algebra.
  5. May 17, 2005 #4


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    You need to show some work. Think about the two quantities that are conserved in the ideal carnot cycle (hint: it's a reversible process).
  6. May 18, 2005 #5

    Andrew Mason

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    Over one cycle: [tex]Q_H/T_H + Q_C/T_C = \Delta S[/tex]

    What is [itex]\Delta S[/tex] over one cycle? What does that tell you about Q_H?

    The co-efficient of performance for a refrigerator is:

    [tex]COP = \frac{Q_C}{W} = \frac{Q_C}{Q_H - Q_C}[/tex]

    Can you work W out in terms of temperatures and COP? Find W from that (one cycle) and then work out work consumption in 165 cycles. That is the energy / minute or power.

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