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Homework Help: Pistons, Pressure, Work, and Height

  1. Dec 7, 2008 #1

    1. The problem statement, all variables and given/known data

    A circular disc of lead has a mass 50 kg, it acts as the piston of a circular container shown in Figure CP16.69. The disc floats on 0.140 mol of compressed air. (Image Attached)

    Figure CP16.69

    (a) What is the piston height h if the temperature is 30°C?

    (b) How far does the piston move if the temperature is increased by 105°C?

    The following questions involve work, recall conditions for (-) work and (+) work ...
    (c) How much work did the atmosphere do on the piston?

    (d) How much work did the gravity field do on the lead piston

    (e) How much work did the gas do on the lead piston?

    2. Relevant equations

    P= F/A

    3. The attempt at a solution

    I haven't tried anything but A- I'm stuck on that one.

    I first found the pressure using P=F/a. I found the weight by doing 50*9.8m/s2 and then divided by (pi)*r^2. That answer, which is already in Pa, I then added to 101,300 pa, the atmospheric constant for pressure. That pressure I used in PV= nRT to find volume. Then I used the volume of a cylinder formula (V= (pi)*r^2*h) to find height. This answer did not come out right (and I made sure to be in centimeters). Does anyone know where I'm going wrong?

    Thanks in advance!


    Attached Files:

    Last edited: Dec 7, 2008
  2. jcsd
  3. Dec 7, 2008 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    Your approach is correct. If you give us all the information we will be able to check it. We need the radius of the cylinder. Then show us your work. I am not sure why you are working in centimeters. You should be working in MKS.

  4. Dec 7, 2008 #3
    Thanks! I figured out what was going wrong. I had kept my answer in meters instead of centimeters. It's so frustrating when that happens.

    Thanks again!
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