First slide

Heat ;work and internal energy

Question

P-V diagram of a cyclic process ABCA is as shown in the given figure. Choose the correct alternative.

Moderate

A

${ΔQ}_{A \to B} is negative$
B

${ΔU}_{B \to C} is positive$
C

${ΔU}_{C \to A} is negative$
D

${ΔW}_{CAB} is negative$

Solution

During process AB pressure and volume both are decreasing. Therefore, temperature and hence internal energy of the gas will decrease $(T \propto PV) or {ΔU}_{A \to B} =$ negative. Further, ${ΔW}_{A \to B}$ is negative.
In process B to C, pressure of the gas is constant while volume is increasing. Hence, temperature should increase or ${ΔU}_{B \to C}$ = positive.
During C to A volume is constant while pressure is increasing. Therefore, temperature and hence internal energy of the gas should increase or ${ΔU}_{C \to A} =$ positive.
During process CAB, volume of the gas is decreasing. Hence, work done by the gas is negative.

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