Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Internal Energy Problem

  1. Jan 7, 2008 #1
    [SOLVED] Internal Energy Problem

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

    Given .15meters cubed of Helium gas at 300K and 3Pa in a chamber with a frictionless piston. How much heat is required to double the internal energy of the gas at constant pressure?
    2. Relevant equations

    I really dont know what equations to use, this problem is giving me a lot of trouble any help is greatly appreciated
  2. jcsd
  3. Jan 7, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    You need to show some work to get help here. Those are the rules. You do not need to have calculations, but you must show that you have put some thought into the problem. I will give you some questions to consider to get you started:

    1) The internal energy of a gas means the cummulative total of the kinetic energy of the gas molecules. Do you know a formula for the average kinetic energy of a molecule of a gas?

    2) Once you know the kinetic energy of one of the Helium molecules, how can you use this to find the total average internal energy of the gas? HINT: The volume given will come into play here.
  4. Jan 7, 2008 #3
    Sorry, im new to the forum but this take home test has driven me to look for help.
    So far I found the moles of Helium originally in the Chamber using PV=nRT. I know i can use the Urms equation to find the average kinetic energy of a molecule and therfore the original amount of internal energy. My problem comes when Finding out how much of the heat added is turned into work compared to a temperature (internal energy) increase so that i can eventually use (delta)U= Q - W
  5. Jan 7, 2008 #4
    Istead of the Urms equation im using Ke=3/2Rt to find average Ke per mole
  6. Jan 7, 2008 #5
    I guess all that Im asking is if there is an equation that shows how when Heat is added to an isobaric system how much of the heat goes into work and how much goes into temperature change. If anyone knows if there is such an equation or other ones i could use to figure it out that would be great.
  7. Jan 7, 2008 #6
    solved it on my own thanks for the start GO1
  8. Jan 7, 2008 #7


    User Avatar
    Homework Helper
    Gold Member

    This is what I would use as well.

    Try starting off like this. Double the internal energy and find what the temperature of the gas would be. Then, knowing that the pressure must remain the same, you should be able to find the new volume of the gas.

    Now, knowing the volume of the gas, can you find the work done by the gas?

    If you know change in internal energy and the work done, can you find the heat that would need to be added.
  9. Jan 7, 2008 #8
    thats how i ended up solving it, thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook