A sample of ideal gas `(gamma = 1.4)` is heated at constant pressure. If an amount `140 J` of heat is supplied to the gas, find (a) the changes in internal energy of the gas, (b) the work done by the gas.

Answer

`100J ; 40J`

Explanation

Here, `gamma= 1.4, dQ= 140J, dU= ?, dW=?`
Let the sample of gas contain `n` moles,
`dQ= nc_(p) (T_(2)-T_(1))= 140J`.
Change in internal energy of gas,
`dU= nC_(v)(T_(2)-T_(1))= n(C_(v))/(C_(p)).C_(p)(T_(2)-T_(1))`
`=(nC_(p)(T_(2)-T_(1)))/(C_(p)//C_(v))=(DeltaQ)/(gamma)= (140)/(1.4)= 100J`
From 1st law of thermodynamics
`dW=dQ-dU=140-100= 40J`