# Second Law of Thermodynamics [Kelvin-Plank & Clausius Statement]

By | February 10, 2019

## Second law of thermodynamics

Second law of thermodynamics is perhaps one of the most important law of science (more specifically physics) because it tells us about the directional constraint of a process.

Here one question may arise.

What is a directional constraint?

We all know that all-natural (and artificial) processes always proceed spontaneously towards equilibrium and take place in a particular direction.

For example:

• Liquids always flow from a high point to a lower point
• Heat always flow from high temperature to low temperature
• Gases always diffuse from high concentration to low concentration
• A moving wheel can be stopped by applying the brakes but a stopped wheel cannot be moved by releasing the brakes
• Electrical heater converts electricity into heat but cooling the heater cannot generate electricity

Reverse of all the above-mentioned processes does not happens naturally. However, some of the above-mentioned processes can be reversed but only with the help of some external aid.

For example:

• Liquids can flow from a low point to a high point with the help of a pump
• Heat can flow from low temperature point to high temperature point with the help of a heat pump
• Gases can flow from low concentration to high concentration with the help of a fan

There are two statements which describe second law of thermodynamics.

## Kelvin-Plank statement of second law

It is impossible for a heat engine to produce net work in a cycle if it exchanges heat only with bodies at a single fixed temperature.

## Clausius statement of second law

It is impossible to construct a device which, operating in a cycle, will produce no effect other than the transfer of heat from a cooler to a hotter body.