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Homework Help: Total heat capacity of an monatomic gas?

  1. Jul 29, 2010 #1
    when working this out, is it just Cv + Cp? giving 4nR???

    thanks
     
  2. jcsd
  3. Jul 29, 2010 #2

    Gokul43201

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    Please use the provided template. We can not help unless we know the exact question that was given to you - sometimes, even a minor rewording of the question can confuse matters.
     
  4. Jul 29, 2010 #3

    Sorry - I wasnt really trying to find the answer to the question. It was more of a: if it says the total heat capacity, does it mean Cv + Cp or just Cp (because this is bigger).

    However the question was:

    a light bulb at 20 degrees is filled with a monatomic gas, as a pressure of 76e3 Pa. When the bulb is switched on the temp changed to 200 degrees.

    a) what is the pressure of the gas at this temp.

    I realised that i had to use pv = nrt, since the volume is fixed the pressure must also go up by a s.f. of 10.

    b) if the volume of the bulb is 125cm^3, estimate the total heat capacity.

    So I rearanged the ideal gas law and found there to be 2.4 moles. All I was unsure about was at the point, do I work out Cv = 3/2nR + Cp = 5/2nR and sum them???
     
  5. Jul 29, 2010 #4

    vela

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    This isn't correct unless the temperatures were given in units of K or R, which is unlikely given the numbers involved.
    No, the question is asking for how much heat the bulb must absorb to raise its temperature by 1 degree.

    By the way, don't write "Cv = 3/2nR + Cp = 5/2nR" unless you really mean that Cv=5/2nR and Cp=R because that's what it means. I know a lot of students tend to do this, but it's really sloppy notation and often leads to mistakes.
     
  6. Jul 29, 2010 #5
    Your corrct it was in degrees celcius.

    Umm... how would I go about working that out then? thanks
     
  7. Jul 29, 2010 #6

    vela

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    Try taking a stab at it and posting your attempt here. You have the right idea for part a; you just have to use the absolute temperatures, i.e. convert them to K.
     
  8. Jul 30, 2010 #7
    OK.

    So I have converte the temps to K:

    T1 = 293K
    T2 = 473K

    So T2/T1 = 1.61

    So by my reasoning the pressure will now be: 12.3ee4Pa

    However how do I go about doing the second part? thanks
     
  9. Jul 30, 2010 #8
    Ok doing this more properly now. I get the pressure to be: 12.3ee4Pa

    Which gives me 0.39moles.

    So working out: Cv = 3/2nR = 4.86???? is this correct?
     
  10. Jul 30, 2010 #9

    vela

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    No, because you didn't specify any units.
     
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