1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Thermodynamics: Internal Energy

  1. Jun 5, 2009 #1
    1. The problem statement, all variables and given/known data

    I need some help on these problems, i got most of the other problems right, but these ones im not sure what to do with. I dont even know if those are the right equations for it. Someone please help.

    1. A cylinder-and-piston containing an ideal gas is placed in contact a thermal reservoir. The volume of the gas is very slowly changed from 50 liters to 12 liters as 50 J of work is done on it by an external agency. Determine the change in the internal energy of the gas.
    The answer is in Joules

    1.2. Determine the change in the amount of heat flowing into or out of the system. (For reference, a negative value of heat means it is flowing out of the system.)
    the answer is in Joules

    2. A gas in a thin plastic bag at atmospheric presure receives 10480 J of heat and, in the process, puffs up the bag, increasing its volume by 0.90 m3. By how much is the internal energy of the gas altered?
    the answer is in kJ

    3. Heat is added to 4.00 m3 of helium gas in an expandable chamber that thereupon increases its volume by 2.00 m3. If in the process 2.22 kJ of work are done by the gas, what was its original pressure?
    the answer is in kPa

    2. Relevant equations

    change in energy=W
    change in energy=W+Q
    change in energy=W+Q+Em

    3. The attempt at a solution

    1. (50-12)+50
    1.2. 88-50
    Last edited: Jun 5, 2009
  2. jcsd
  3. Jun 5, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hi naspri4, welcome to PF. One serious problem I see is that you're not making sure your units match. For example, your first answer combines Joules with liters, which can never be right.

    I agree with using the equation [itex]\Delta U = W + Q[/itex] for change in energy. The problem is that both [itex]\Delta U[/itex] and [itex]Q[/itex] or [itex]W[/itex] are sometimes unknown. There's also another equation you'll need that connects internal energy to temperature for an ideal gas, and that should be enough to solve these problems.
  4. Jun 5, 2009 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    If the gas is in contact with a thermal reservoir and it is compressed slowly, what can you say about the temperature of the gas during the process?

    Use the answer from the first to determine the heat flow (is it out of or into the gas?).

    I think the question must be wrong because the work done exceeds the heat input by about 9 times.

    I think you have to assume that the pressure is constant here.

  5. Jun 6, 2009 #4
    Ok, so for the first question, the temperature would be constant meaning there wouldnt be any change. So how would that help me solve the problem? Did I have my equation set up right at the bottom of the first thread. I tried emailing my teacher via webassign but he has not responded for some unknown reason.

    The second one was correct being zero, i dont know how but it was.

    What about the third question? Any tips on how to solve it?
  6. Jun 6, 2009 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Apply the first law. If there is no change in U what is the relationship between Q and W?

    The work done is 101KPa x .9 m^3 =91KJ. If there is a heat flow of only 10.480 KJ this can't happen.

    If pressure is constant, what is the relationship between Work and total change in volume?

    Last edited: Jun 7, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook