Isothermal process

用戶2457

2011-04-12 10:20 am

By 1st law of thermodynamics ,U= Q + W
In isothermal process ,u=0 as change in temp.=0,if work is done by the gas then heat is gained by the gas ,
As heat can only be converted to internal energy (1.KE or 2.intermolecular PE) of gas
Q: if the state (phase) of gas unchange, what form will the heat gained by gas from isothermal process convert to be since there are no gain in PE (state unchange) or KE (constant temp.) by gas

OR
my deduction is wrong

THX for answering!!

回答 (1)

天同

2011-04-12 10:01 pm

✔ 最佳答案

You have amde a mistake,

The first law of thermodynamics shoul dbest be written as,

dQ = dU + dW
where dQ is the heat flow in to/out from the gas
dU is the change of internal energy of the gas
dW is the work done by the gas

In an isothermal expansion of the gas, the gas does work dW. Because the process is isothermal, dU = 0. Thus we have
dQ = dW

Heat flows into the gas as a result of its expansion.

When the gas expands, it tends to cool down. But in so doing, there is a temperature diference between the gas and the surroundings. This difference in temperature thus casues heat to flow into the gas so that the internal energy of the gas remains unchanged.

Be aware that an isothermal process must be quasi-static, i.e. it must be proceeded at a slow pace.

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