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First Law of Thermodynamics and Melting-Ice Scenario?

  1. Jan 12, 2016 #1
    In order to explain the change in energy of an ice cube removed from a refrigerator and placed on the kitchen counter, I consider it as a closed system (can exchange only energy but not matter).
    For the first law of thermodynamics, ∆U = Q+W. I'm pretty sure that the sign of Q is positive due to the temperature difference that results in the energy transfer. What I'm not sure about is the work. In the process that ice melts, I figure there might be a increase in gravitational potential energy because its center of gravity descends, so should the sign of W be positive, too?
    I'm also not sure about the nature of those three types of energy. I guess the nature of internal energy is the microscopic motion of the particles in the system, but what about the other two?
    Any help would be appreciated!
  2. jcsd
  3. Jan 12, 2016 #2
    As you are writing the question, to me, it seems not totally defined: what is the system?
    For example, you are talking about an ice cube resting on a counter..., and the ice
    melting. So, presumable you are considering the counter top, as part of the system, and likely
    the air around the ice cube, etc... So, the question to me is ill defined..

    Thermo dynamics deals with closed systems... Yours is not closed, so you need to try and understand what
    a closed system is, as far as thermo goes..
  4. Jan 12, 2016 #3
    Thanks for your reply!
    And I'm still confused about the definition of closed system. May I consider the ice cube itself as a closed system while the counter and air as the surroundings?
  5. Jan 12, 2016 #4


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    As the ice cube melts, heat moves from the environment into the system and the internal energy of the water increases as it changes from ice to liquid. Work is done on water by the gravitational field of the Earth, but this is a small effect compared to the heat transfer.
  6. Jan 12, 2016 #5
    Thank you!
    And do you think the ice cube itself can be counted as a closed system?
  7. Jan 12, 2016 #6
    Yes. Yes. Yes!!! The ice cube plus then any water that forms from it is your closed system. This system is very well defined.

    A closed system is one in which no mass enters of leaves.
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