# 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.

nasu
Gold Member
At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.

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/JouleExperimentOnFreeExpansion/

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.