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Homework Help: Determine the change in internal energy, of the gas during this process

  1. Mar 8, 2009 #1
    1. The problem statement, all variables and given/known data

    A gas sample heated in a cylinder used 650 kJ of heat. A piston that compresses the gas does 800 kJ of work. What is the change in internal energy, ΔE, of the gas during this process


    2. Relevant equations

    ΔE = q + w

    3. The attempt at a solution

    ΔE = q + w
    ΔE = (650kJ) + (800 kJ)
    ΔE = 1450 kJ

    a result of the gas being heated and compressed, the energy of the gas increases by 1450 kJ. Is this at all correct? Or should it be -650 kJ??? Thank you for your help.
     
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  3. Mar 8, 2009 #2

    Mapes

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    Hi new^2^physics, welcome to PF. Your answer looks fine.
     
  4. Mar 8, 2009 #3

    Andrew Mason

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    To avoid problems, you have to use signs correctly. You should state the first law as:

    [tex]\Delta Q = \Delta U + W[/tex]

    where positive [itex]\Delta Q[/itex] is the heat flow INTO the gas, positive [itex]\Delta U[/itex] is the INCREASE in internal energy of the gas, and positive W is the work done BY the gas (so if work is done on the gas, W is negative).

    If you make careful use of the signs (+ or -) you will avoid confusion.

    In this case, you would state the first law as:

    [tex]\Delta U = \Delta Q - W[/tex]

    where [itex]\Delta Q = 800 J [/itex] and W = -650 J. (since work is done ON the gas).

    AM
     
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