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Old May 25th 2019, 01:29 PM   #1
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How can a truly isolated system change its volume?

A question for thermo experts to think about.

How can a truly isolated system change its volume?
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Old May 25th 2019, 01:47 PM   #2
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How about this: imagine a large elastic balloon floating in deep space. The balloon is initially uninflated, occupying a small volume. Inside the balloon is a tank of compressed air. The valve on the tank has a timer, and at a predetermined time the timer opens the valve, releasing the compressed air, and the balloon expands.
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Old May 25th 2019, 01:51 PM   #3
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So how is the system isolated since work must be done expanding the balloon?
Oh and thank you for your reply.
This was a serious muse.
I was wondering if a truly isolated system implied a fixed volume.
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Old May 25th 2019, 02:28 PM   #4
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The isolated system consists of the balloon and the compressed air. Work is done by the air solely on the balloon material to stretch it. There are no other outside agencies (i.e. the balloon is not doing work on an outside atmosphere, because there is no outside atmosphere).
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Old May 25th 2019, 02:42 PM   #5
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Originally Posted by ChipB View Post
The isolated system consists of the balloon and the compressed air. Work is done by the air solely on the balloon material to stretch it. There are no other outside agencies (i.e. the balloon is not doing work on an outside atmosphere, because there is no outside atmosphere).
Yes that is what I meant, I suppose it boils down to what you include as part of the system.
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Old May 26th 2019, 02:58 AM   #6
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Is it possible to define a truly isolated system?
I would contend that all you can do is (as ChipB did) is define a system which interacts minimally with the rest of the universe.
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