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THermodynamics problem

  1. Jan 25, 2015 #1
    1. The problem statement, all variables and given/known data
    Suppose that a tank contains 680 m3 of neon at an absolute pressure of 1.01x105 Pa. The temperature is changed from 293.2 to 294.3 K. What is the increase in the internal energy of the neon? [3.9x105 J]

    2. Relevant equations
    delta Q = delta U + delta W
    pV = nRT

    3. The attempt at a solution
    there is no potential energy between them. all the kinetic of neon is kinetic since it is ideal gas
    how do i find delta Q and delta W. should they be positve or negative?
    i assume that an increase in temperature increases kinetic energy. so internal energy increase. delta U is positive?
    can you plz tell me how to proceed
     
  2. jcsd
  3. Jan 25, 2015 #2
    If the tank is rigid, what can you say about its change in volume? Given this, how much work is done on the surroundings?

    Chet
     
  4. Jan 25, 2015 #3
    the volume won't change. but the pressure will.
    i used P1 / T1 = P2 / T2 to find new pressure, then work = (delta p) V
    but the answer was not correct
     
  5. Jan 25, 2015 #4
    Actually, if the volume doesn't change, zero work is done on the surroundings.

    Irrespective of whether any work is done, what is the effect of temperature on the internal energy of an ideal gas? How many moles of neon are present in the tank?

    Chet
     
  6. Jan 25, 2015 #5
    internal energy increases since kinetic energy increases
     
  7. Jan 25, 2015 #6
    How many moles of neon are there in the tank? Quantitiatively (in terms of an algebraic equation), if the temperature of an ideal gas changes by ΔT, what is the change in the internal energy ΔU.

    Chet
     
  8. Jan 25, 2015 #7
    is it 1/2 k (delta T)?
    i ca't obtain the answer with this too
     
  9. Jan 25, 2015 #8
    Actually, it's 3/2 kΔT, and that's per molecule. What is the change in internal energy per mole? If you have n moles, what is ΔU? You still didn't tell me how many moles of neon are in the tank. You need this to determine the change in U. Do you know how to use the ideal gas law to calculate the number of moles in the tank?

    Chet
     
  10. Jan 25, 2015 #9
    from pV = nRT
    but i don't know which temperature to use
     
  11. Jan 25, 2015 #10
    The number of moles in the tank doesn't change, and you're using the starting pressure and the starting volume in your calculation of the number of moles. So, does it make sense to you to use the final temperature or the starting temperature?

    Chet
     
  12. Jan 25, 2015 #11
    the starting one
     
  13. Jan 25, 2015 #12
    So how many moles of neon are there in the tank? And you still haven't answered my question about the algebraic relationship between ΔU and ΔT for n moles of an ideal gas.

    Chet
     
  14. Jan 25, 2015 #13
    ok. thanks I got it.
    but I still don't understand why work = (delta p) V cannot be used? if hte pressure changes, should not there be a work?
     
  15. Jan 25, 2015 #14
    Since when is work equal to distance times the change in force?

    Chet
     
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