Some gas in a cylinder is supplied with thermal energy q.

Question 34

Some gas in a cylinder is supplied with thermal energy q.

The gas does useful work in expanding at constant pressure p from volume V0 to volume VF, as shown.

Which expression gives the efficiency of this change?

Reference: Past Exam Paper – June 2016 Paper 11 Q16

Solution:

Answer: C.

The gas is supplied with thermal energy q.

(Input) thermal energy = q

Work done by expanding gas = pΔV

Work done by expanding gas = p × (V_final – V_initial) = p (V_F – V_O)

This is the output energy.

Output energy = p (V_F – V_O)

Efficiency = output energy / input energy

Efficiency = p (V_F – V_O) / q