Question

In a thermodynamics process pressure of a fixed mass of a gas is changed in such a manner that the gas molecule give out $30$ joule of heat and $10$ joule of work is done on the gas. If the initial internal energy of the gas was $40$ joule, then the final internal energy will be :

• A. -20 J
• B. 20 J
• C. 80 J
• D. 3 J

Answer 1

Answer: (B)

Here $:dQ=-30J$
(negative sign appears as the heat is going out from the system)
$dW=-10J,U_i=40J$
Using the relation
$dQ=dU+dW$
$-30=\left(U_f-40\right)+(-10)$
$U_f=20J$