Q137 / 160 · AP EAMCET 2015 · Physics
A gas does $4.5 \mathrm{~J}$ of external work during adiabatic expansion. If its temperature falls by $2 \mathrm{~K}$, then its internal energy will be
- Aincreased by $4.5 \mathrm{~J}$
- Correct answerBdecreased by $4.5 \mathrm{~J}$
- Cdecreased by $2.25 \mathrm{~J}$
- Dincreased by $9.0 \mathrm{~J}$
Solution
From first law of thermodynamics,
$\begin{aligned}
& d Q=d U+d W \\
& \Rightarrow \quad d U=-d W \quad \text { [In adiabatic process } d Q=0 \text { ] } \\
& d U=-4.5 \mathrm{~J}
\end{aligned}$
As, the temperature falls so internal energy will decrease by $4.5 \mathrm{~J}$
Hence, the correct answer is option: 'B'.