First Law of Thermodynamics and Melting-Ice Scenario?

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  • #1
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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!
 

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  • #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..
 
  • #3
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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..
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?
 
  • #4
Khashishi
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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.
 
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  • #5
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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.
Thank you!
And do you think the ice cube itself can be counted as a closed system?
 
  • #6
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Thank you!
And do you think the ice cube itself can be counted as a closed system?
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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