Question

A gas is compressed at constant pressure 50 $N/m^2$ from a volume of $10m^3$ to a volume $4m^3$ . Energy 100 J is then added to the gas by heating. Its internal energy is

• A. increased by 100 J
• B. increased by 200 J
• C. decreased by 200 J
• D. increased by 400 J

Answer 1

Answer: (D)

Change in volume $\Delta V_2-V_1=4-10=-6m^3$
A volume decreases, work is done on the gas and so it is negative.
$W=p\Delta V=50\times (-6)$
$=-300J$
Additional heat supplied
$(G=+100J)$ .
So, for first law of thermodynamics change in external energy is given by
$\Delta U=Q-W$
$=100-(-300)=400J$
Internal energy increases by 400 J.