Question

The internal energy of a gas is 20 J. It undergoes
a thermodynamic process such that 35 J of heat
is added to the system and it does 5 J of work on
atmosphere. The final value of the internal energy
of the gas is
(2) 25 J
(3) 50 J
(4) 40J
(1) – 10J​

Answer 1

The final value of the internal energy of the gas is 50 J.

Explanation:

1. From first law of thermodynamics equation

  $U_{f}-U_{i}=Q_{1-2}-W_{1-2}$      ...1)

  Where

  $U_{f}$ = final internal energy of system

  $U_{i}$ = initial internal energy of system = 20 J

  $Q_{1-2}$ = Heat interaction to system = +35 J

  Here positive sign is taken because heat is given to system.

  $W_{1-2}$ = Work interaction to system = +5 J

  Here positive sign is taken because work is done by system.

2. Now again from equation 1)

  $U_{f}-U_{i}=Q_{1-2}-W_{1-2}$

    $U_{f}-20=35-5$

   $U_{f}$ =50 J

3. So Final internal energy of system = 50 J, Option Third is correct.