Question

If heat is supplied to the system it
A. does work only
B. increase the internal energy
C. either does work or increase the internal energy or both
D. does nothing

Answer 1

From the concept of thermodynamics' first law, we know that heat supplied to a system in a thermodynamic cycle is equal to the work done by that system in a complete cycle. It is also known as the law of energy conservation.

Complete step by step answer:
In other words, we can say that when the heat is supplied to a system, it is converted into work and internal energy is that system. We can express it mathematically as below:
$Q = \Delta U + W$
Here $\Delta U$ is the change in internal energy, and W is the work done.

Based on the above expression, we can say that heat is converted into the system's work and internal energy.

Let us consider an isolated system that means the system will not exchange mass and energy with its surroundings that are heat supplied to the system, and the system's work is zero.
$0 = \ Q$
$0 = \ W$

Substitute 0 for Q and W in equation (1).
$0 = \Delta U + 0$
$0 = \Delta U$

From the above expression, we can say that the internal energy of an isolated system is constant. It means that there is no change in the isolated system's internal energy if heat supplied and work done is zero.

As the temperature of the isothermal process is constant and internal energy is a function of temperature. Therefore, the change in the internal energy of the isothermal system is zero.

Therefore, from the first law concept, we can conclude that if heat is supplied to a system, it either does work or increases the internal energy or both.

So, the correct answer is “Option C”.

Note:
We can observe that the heat supplied to an isothermal system is converted into work completely, which is a perpetual machine of the second kind and not possible according to the second law