Question

A perfect gas goes from state A to another state B by absorbing $8\times {10}^{5}J$ of heat and doing $6.5\times {10}^{5}J$ of external work. It is now transferred between the same two states in another process in which it absorbs ${10}^{5}J$ of heat. In the second process

• A. work done by the gas is ${10}^{5}J$
• B. work done on gas is ${10}^{5}J$
• C. work done by gas is $0.5\times {10}^{5}J$
• D. work done on the gas is $0.5\times {10}^{5}J$

Answer 1

The correct option is D work done on the gas is $0.5\times {10}^{5}J$

$dU=dQ-dW=(8\times {10}^5-6.5\times {10}^5)=1.5\times {10}^{5}J$
$dW=dQ-dU={10}^5-1.5\times {10}^5=-0.5\times {10}^{5}J$
- ve sign indicates that work done on the gas is $0.5\times {10}^{5}J$