A system is taken from state `a` to state `c` along the path `adc` (figure). The amount of heat absorbed is `10 cals` and work done on the gas is `138 J`. Calculate the work done on//by the gas when `20 cals` of heat are absorbed in taking the system from state `a` to state `c` along the path `abc`

Explanation

(i) Along the path `adc`,
`dQ= 10 cals= 10xx4.2J=42J`
`dW= - 138J`
`:. dU=dQ-dW= 42- (-138)= 180J`
This is increase in internal energy of gas.
(ii) Along the path `abc`
`dU' = dU= 180J`
`dQ'= 20 cal.= 20xx4.2J= 84J`
`dW=?`
`:. dW=dQ-dU= 84-180`
`= -96J`
This is the work done on the gas.