Isothermal Processes

A thermodynamic process that occurs at a constant temperature is called an isothermal process.

Conversion of boiling water into steam at a constant temperature of 100 degrees Celsius is an example of an isothermal process. Similarly, the conversion of water into ice at 0 degrees Celsius is also an isothermal process.

Now, consider an ideal gas system enclosed in a sealed gas syringe at a constant temperature.

When the plunger is slowly pushed in a controlled manner, the gas inside the syringe is compressed. The reduced volume of the gas causes an increase in pressure; however, the temperature remains constant.

Releasing the plunger in a controlled manner leads to an expansion of the gas at constant temperature.

The change in internal energy is zero for these processes as the temperature does not change. Thus, using the first law of thermodynamics, the work done equals heat.

The pV curve for any isothermal process is hyperbolic, since pressure is inversely proportional to volume for an ideal gas at a constant temperature.