Q98 / 125 · BITSAT 2014 · Physics

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

  • A
    $2 \mathrm{~J}$
  • B
    $42 \mathrm{~J}$
  • Correct answer
    C
    $18 \mathrm{~J}$
  • D
    $58 \mathrm{~J}$

Solution

Given $\Delta \mathrm{Q}=-20 \mathrm{~J}, \mathrm{~W}=-8 \mathrm{~J}$ Using Ist law $\Delta \mathrm{Q}=\Delta \mathrm{U}+\Delta \mathrm{W}$ $$ \begin{array}{l} \Rightarrow \Delta Q=-20+8=-12 \mathrm{~J} \\ \mathrm{Uf}=-12+30=18 \mathrm{~J} \end{array} $$

Hence, the correct answer is option: 'C'.

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