## First law of thermodynamics applied to Compression and Expansion

How does the first law of thermodynamics apply to the Compression And expansion of a gas at constant temperature ?

If the gas is at a constant temperature and is being compressed, then
ΔQ = 0
ΔU = +
W(on the system) = +

Are those(^^) correct ?

The equation being :
ΔU = ΔQ + W(on the system)
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 Compression or expansion at constant temperature means heat is extracted or added.
 At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.

## First law of thermodynamics applied to Compression and Expansion

 Quote by nasu At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.
would u explain "why" ?

 would u explain "why" ?
Originally this was an experimental fact about gasses. Nowadays there is maths to back it up, and we know that it only applies to ideal or perfect gasses.

http://demonstrations.wolfram.com/Jo...FreeExpansion/

If you can cope with the maths google has lots on the Joule experiment and the Joule-Thompson experiment.

If not you can take it as a definition of a perfect gas.

The internal energy of a perfect gas depends only on its temperature.

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