Question

In a thermodynamic process pressure of a fixed mass of a gas is changed in such a manner that the gas releases $30J$ of heat and $10J$ of work was done on the gas. If the initial internal energy of the gas was $30J$, then the final internal energy will be

• A. 2 J
• B. -18 J
• C. 10 J
• D. 58 J

Answer 1

Answer: (C)

Given, heat, $Q=-30J$
Work done on the gas, $W=-10J$
Initial internal energy, $U_i=-10J$
Final internal energy, $U_f=?$
According to first law of thermodynamics,
$Q=W+\Delta U$
$\Rightarrow -30=-10+U_i-U_f$
$\Rightarrow -30=-10-10-U_f$
$\Rightarrow 30=10+10+U_f$
$\Rightarrow 30-20=U_f$
$\Rightarrow U_f=10J$