Question

(Figure) shows that two `P-V` curves AB and BC for a gas: one is for isothermal process and other is for adiabatic process.
(i) Which curve denotes which change and why?
(ii) How much work will be done by the gas in change AB? Will the internal energy of gas increase or decrease? How much heat will the gas give or take?
(iii) Discuss all the above things in change BC.

Answer 1

(i) As is clear fron (figure), slope of curve AB is less than the slope of curve BC. Therefore, AB represents an adiabatic curve, BC involves change in temperature from `T_(1) "to" (T_(2)`.
(ii) Work done by the gas in change `AB= area A B G E A`. It is positive work
As temperature remains constant in an isothermal change, internal energy `U= "constant" dU=0`.

From first law of thermodynamics,
`dQ=dU+dW=0+dW= dW`
`=area ABGEA`
The system will take this much of heat
(iii) Work done by the gas in change `BC= area BC HGB`. It is positive work.
As temp. falls from `T_(1) "to" T_(2)` internal energy decreases.
As the change is adiabatic, `dQ=0`. Heat is neither given not taken.