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.

Also read:

First law of thermodynamics

Concept of energy

What is Gibbs phase rule?

What is a thermodynamic property?

What is thermodynamics?

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