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Thermodynamic Questions

  1. Jan 9, 2005 #1
    Sorry for some many questions here. I'm stuck on all of these problems and it's starting to really frustrate me. I'm not sure what more to do.

    1. A sample of Helium behaves as an ideal gas as energy is added by heat at constant pressure from 273 K to 403 K. If the gas does 20.0 J of work, what is the mass of helium present?

    >> At first I was going to use Q=mct, but I do not know c of helium and it's not in my physics book. Then I tried to use Q=mL which I think should work, but I don't know how to convert W to Q.

    2. A gas is compressed from 10.00 L to 3.00 L at a constant pressure of 0.800 atm. In the process, 330 J of energy leaves the gas by heat. What is the work done by the gas? What is the change in its internal energy?

    >> I know trial and error isn't the way to approach physics, but I'm lost. I tried using W=Pln(Vf/Vi) and got an answer of .963 J. I don't understand why this is wrong...

    3. A thermal window with an area of 6.00m^2 is constructed of two layers of glass, each 4.00 mm thick and separated from each other by an air space of 4.00 mm. If the inside is at 20.0 C and the outside is at -34.0 C, what is the rate of energy transfer by conduction through the window?

    >> I know I need to use the formula H=kA(change in T/L). But... do I have to do this using only the air in the middle of the two panes of glass? Or do I have to do the formula three times: one for the air, and two each piece of glass? And if so, is the change in temperature the same?

    4. Gas in a container is at a pressure of 1.40 atm and a volume of 6.00 m^3. What is the work done by the gas if it expands at a constant pressure to twice its initial volume.

    >> I think I need to use W=Pln(Vf/Vi). When I did this, though, I got .970... which isn't the answer according to webassign. I honestly don't know why this doesn't work. Does P have to be in a different unit?
     
  2. jcsd
  3. Jan 9, 2005 #2
    For 1.... you know that P(deltaV) = work with constant pressure...

    you also know that PV = nRT....

    so PV1 - PV2 = nRT1 - nRT2 = P(deltaV) = nR(delta T) = work

    you can figure out the number of moles... which you can use to determine the mass..
     
  4. Jan 9, 2005 #3

    cronxeh

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    Gold Member

    1. The number c you looking for is 5.193 J/(grams*K)
    Q=mc(delta T)
    where delta T = 403 - 273

    2. PV = nRT
    If you change Volume, and Pressure stays the same - something's gotta give. So T must rise.

    3. http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html
    This should help you understand difference between convection and conduction
     
  5. Jan 9, 2005 #4

    For number 1... even if you use that equation... how are you going to solve it?
    that gives you the energy change .... what good does that do? If a problem doesn't give you the value... i don't think it is needed to solve the problem, since enough information is available...
     
  6. Jan 9, 2005 #5
    For 2... P(deltaV) = work done on the system which is equal to the total energy...

    q(heat) + Work = total energy... <--- solve for Work...
     
  7. Jan 9, 2005 #6
    I understand why the temperature must rise... but how does that help me find the work done by the gas? :confused:
     
  8. Jan 9, 2005 #7
    If anyone helping me on this has AIM, can you IM me at ecksCavator? I'm so lost...
     
  9. Jan 9, 2005 #8
    P should be in Pascals... for P(deltaV) to work... However though i'm not sure about W = Pln(Vf/Vi), but you can try it out....

    I haven't done thermodynamics for energy transfer... so i can't help you on 3...
     
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