Question

In a thermodynamic process, pressure of a fixed mass of gas is changed in such a manner that the gas releases $20\text{J}$ of heat and $8\text{J}$ of work is done on the gas. If initial internal energy of the gas was $30\text{J}$, then the final internal energy will be

• A. $42\text{J}$
• B. $18\text{J}$
• C. $12\text{J}$
• D. $60\text{J}$

Answer 1

The correct option is B $18\text{J}$

Given,
Heat given by the system $Q=-20\text{J}$
Work done on the system $\left(W\right)=-8\text{J}$
Initial internal energy $\left(U_i\right)=30\text{J}$
From $1$st law of thermodynamics,
$Q=\Delta U+W$
$\Rightarrow \Delta U=Q-W$
$\Rightarrow U_f=U_i+Q-W$
$=30+(-20)-(-8)$
$=18\text{J}$