Question

A fixed mass of gas undergoes a change represented by $ABCDA$ as shown in figure. In some of the changes, work is done on the gas and in others, work is done by the gas. In which pair of the changes, work is done on the gas?

• A. $AB$ and $CD$
• B. $AB$ and $BC$
• C. $BC$ and $CD$
• D. $CD$ and $DA$

Answer 1

The correct option is D $CD$ and $DA$

In the given figure,
$AB\to$ isothermal expansion
$BC\to$ adiabatic expansion
$CD\to$ isothermal compression
$DA\to$ adiabatic compression


Work is done on the gas during isothermal compression and adiabatic compression (area under graph is negative) i.e for processes $CD$ and $DA$.
Hence, option (d) is correct.