# Internal energy

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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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.

Chestermiller
Mentor
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

Andrew Mason