Question

9. A gas in a closed cylinder contracts isothermically as a piston moves inward. A total of 850 J of heat are released by the gas as it contracts. (a) What is the work done by the gas? (b) What is the work done on the gas?

Answer 1

Work is done on the gas as the piston moves inwards in the cylinder and the gas contracts isothermally.

Only when work is done on the gas isothermally is when heat is released by the gas.

Keeping that in mind, we will look at (b) first.

(b) Work done on the gas during this contraction can be determined by the first law of thermodynamics.

U= Q-W

As temperature is constant, U= 0

Therefore, Q=W

I.e. work done on the gas W_b= 850 J

(a) Work done by the gas is equal and opposite to the work done on the gas.

Therefore, W_a= -W_b= -850 J