Internal energy

  • #1
So the internal energy of a system changes if work or heat is transferred between itself and its environment. Say you compress a gas and therefore do work on it, increasing its internal energy. According to Newton's 3rd law, shouldn't the gas also do work on its surroundings. How come this isn't accounted for?
 

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  • #2
phinds
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So the internal energy of a system changes if work or heat is transferred between itself and its environment. Say you compress a gas and therefore do work on it, increasing its internal energy. According to Newton's 3rd law, shouldn't the gas also do work on its surroundings. How come this isn't accounted for?
Who says it's not accounted for? Given a specific setup, you could for example, have one piston compressing gas and another piston using that compression but with a different ratio. If on the other hand, you have a fixed container, nothing is moving so no work is being done on the container.
 
  • #3
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So the internal energy of a system changes if work or heat is transferred between itself and its environment. Say you compress a gas and therefore do work on it, increasing its internal energy. According to Newton's 3rd law, shouldn't the gas also do work on its surroundings. How come this isn't accounted for?
It is accounted for. The gas is doing negative work on the surroundings.

Chet
 
  • #4
Andrew Mason
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The forces (of the surroundings on the system and of the system on the surroundings) are equal and opposite. But the displacement over which those forces operates is the same for both forces. The force exerted by the surroundings on the system is in the direction of the displacement, so the work done by the surroundings on the system is positive. But the force exerted by the system on the surroundings is in the opposite direction of displacement so the work done by the system on the surroundings is negative.

AM
 
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