Question

A perfect gas goes from state $A$ to 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 on gas is $10^{5}J$
• B. work done on gas is $0.5\times 10^{5}J$
• C. work done by gas is $10^{5}J$
• D. work done by gas is $0.5\times 10^{5}J$

Answer 1

Answer: (B)

From first law of thermodynamics
$dU=dQ-dW$
$dU=8\times 10^5-6.5\times 10^5$
$dU=1.5\times 10^{5}J$
In the second process, $dU$ remains the same.
$\therefore dW=dQ-dU=1\times 10^5-1.5\times 10^5$
$dW=-0.5\times 10^{5}J$
Negative sign shows that work is done on the gas.