The Second Law of Thermodynamics

The “engine” statement of the second law of thermodynamics states that

“It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began.”

This is also referred to as the Kelvin-Planck statement.

The concept of the second law of thermodynamics is based on the difference between the nature of the internal energy and the macroscopic mechanical energy. When a body is in motion, the molecules in that body is also in random motion in which the direction of motion is the same as the direction of the body’s velocity. The kinetic energy possessed by the moving molecules is referred to as the kinetic energy of the moving body. The internal energy is composed of this kinetic energy and the potential energy along with the random motion of the molecules.

When a body is moving in a surface, it has organized motion. As this body stops due to friction, its organized motion is converted into the random motion of its molecules. Since the motion of every molecules cannot be controlled, the conversion of random motion back to organized motion cannot be completely done, and so, there will only be partial conversion of that will happen. The partial conversion of random motion to organized motion can be done by the heat engine.


Another statement of the second law of thermodynamics is known as the “refrigerator” statement. It states that

“It is impossible for any process to have as its sole result the transfer of heat from a cooler to a hotter body.”

It is also called as the Clausius statement.

The flow of heat is from hotter to colder bodies, and which is cannot be reversed, but some devices such as refrigerators take heat from a colder to hotter body. The “refrigerator” statement of the second law implies that, although such device takes in energy from a cold reservoir and expels energy to a hot reservoir, but in order for it to operate, it requires an input of mechanical energy or work.