MCQ Detailed View

An isothermal process is a type of thermodynamic process in which the temperature of the system remains constant throughout the change. The word “isothermal” is made of two parts: iso meaning “same” and thermal meaning “temperature.” So, isothermal means “same temperature.”

In such a process, the internal energy of an ideal gas does not change, because internal energy depends only on temperature. Since the temperature is constant, the internal energy remains constant as well.

When the system expands or contracts isothermally, heat energy is exchanged with the surroundings. If the system expands, it does work on the surroundings, and the required energy comes from heat absorbed. If the system is compressed, work is done on the system, and heat is released to the surroundings. This ensures the temperature stays the same.

It is important to understand what does not remain constant in an isothermal process. Pressure and volume do not stay fixed. In fact, if volume increases, pressure decreases, and vice versa, according to Boyle’s law:

$$P\times V=\text{constant (at constant T)}$$

This is why isothermal processes are often represented as hyperbolic curves on a P–V diagram.

For example, when a gas in a cylinder with a movable piston is slowly expanded while keeping it in contact with a large heat reservoir, the gas absorbs heat to maintain constant temperature.

To summarize in simple words: in an isothermal process, the temperature remains constant, but heat exchange and work take place to maintain that constant temperature.