The first law of thermodynamics states that energy can neither be created nor destroyed, only transformed from one form to another. In an isothermal process, where the temperature of a system remains constant, the first law dictates that any heat added to the system is entirely used to do work, as the internal energy change is zero.

Isothermal Process:
An isothermal process occurs when the temperature of the system remains constant throughout the process.
To maintain constant temperature, heat is exchanged between the system and its surroundings.

Application in Isothermal Process:
In an isothermal process, the change in internal energy (ΔU) is zero because the temperature remains constant.
Therefore, the first law equation simplifies to: 0 = Q – W.
This means that the heat added to the system (Q) is equal to the work done by the system (W).
In other words, all the heat energy supplied to the system is used to perform work.
In essence, in an isothermal process, the heat absorbed by the system is entirely converted into work, as the temperature remains constant, and thus the internal energy does not change.