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Homework Help: Multiple Choice problem (internal energy)

  1. Mar 19, 2010 #1
    What must be true for two objects of the same material to have the same internal energies?
    (Choose all that apply)

    a. The masses of both objects must be equal

    b. Both objects must have the same density

    c. The temperatures of both objects must be equal

    d. Separate objects cannot have identical internal energies

    I guessed a,b,c and was wrong. I am thinking only a &b. Please tell me what you think.
     
  2. jcsd
  3. Mar 19, 2010 #2

    Dick

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    You aren't saying why you think any of those are true. Guessing may be the wrong strategy. How would you calculate internal energy of a system given temperature and masses of the component objects, assuming it's an ideal gas for example?
     
  4. Mar 19, 2010 #3
    Dick,
    I have no clue what "strategy" to go with, that's the whole reason I'm here.
     
  5. Mar 19, 2010 #4

    Dick

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    Right, but can you think of two objects whose masses are equal but have different internal energies? Two objects whose densities are equal but have different internal energies etc. etc. That's the sort of strategy I'm talking about. Are ANY of those true? All I know is guessing won't work. They wouldn't give you this question if you didn't know SOMETHING.
     
    Last edited: Mar 19, 2010
  6. Mar 19, 2010 #5

    Dick

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    You've got the right general idea about what internal energy is, that's a start. But try and make that general idea more concrete. Don't you have ANY specific examples to reason from? Like I said ideal gases would be a good starting point. But why do you think temperature doesn't matter? Doesn't that determine internal kinetic energies and vibrational energies? Are you really supposed to answer this starting from ground zero?
     
  7. Mar 19, 2010 #6
    Yes, ground zero. I am leaning towards mass and temperature as the answer to the question.
     
  8. Mar 19, 2010 #7
    I will have two lectures on thermodynamics in the coming week. I'm trying to complete online homework and can't find a good source of information for this question.
     
  9. Mar 19, 2010 #8
    I think I've got it.

    The objects have to have the same mass in order to have equal heat capacity.

    The objects have to have the same temperature to be in thermal equilibrium.

    If there heat capacity and thermal equilibrium are identical, their internal energy will be the same.
     
  10. Mar 19, 2010 #9

    Dick

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    No, that's not it. If sample A has internal energy E, and sample B has internal energy 2E, then two copies of A have the same internal energy as B, right? Internal energy is an extensive property, yes? Do A and B have to have any properties in common?
     
  11. Mar 19, 2010 #10
    Here it is...
     

    Attached Files:

  12. Mar 19, 2010 #11

    Dick

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    If it's 'separate objects cannot have equal internal energies', that's a bunch of baloney.
     
  13. Mar 19, 2010 #12
    the image shows my response just minutes ago. you can see it gave me credit for my response. (my response in green) the check mark is just in a weird spot, thats not the answer.

    thanks for your efforts.
     
  14. Mar 20, 2010 #13

    Dick

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    Well, congratulations. I really can't agree with their answers. But there you go. You're welcome.
     
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