# Second Law of Thermodynamics

Second Law of Thermodynamics is also known as Entropy Law or Law of Entropy. It is a central concept not just for Thermodynamics, but also for Information Theory (where we talk of Information-Entropy) and various other fields of Physics.

## Statements of Second Law of Thermodynamics

### Rudolf-Clausius Statement

It is impossible to make heat flow from body at a lower temperature to another at a higher temperature without doing external work on the system.

### Kelvin-Planck Statement

It is impossible to construct an engine, operating in a cycle, which will produce no effect other than extracting heat from a reservoir and reforming an equivalent amount of work.

Carnot’s TheoremThis in simple words means that no heat engine can be built that can have 100% thermal efficiency.

Carnot’s Theorem is also called Carnot’s Principle or Carnot’s Rule

No heat engines operating between two heat reservoirs can be more efficient than a reversible heat-engine operating between the same two reservoirs.

## Zeroth Law of Thermodynamics

If two bodies
(P) and (Q) are in thermal equilibrium, and also

(P) and (R) are in thermal equilibrium, then
(Q) and (R) are also in thermal equilibrium.

## First Law of Thermodynamics

The heat energy supplied to a system is equal to sum of increase in internal energy (dU) of the system and the work done (dW) by the system.

## The need for Second Law of thermodynamics

The limitations of the First Law of thermodynamics are:

• The first law does not indicate the ‘direction‘ of heat transfer. Heat always flow from hot to cold body, but first law is silent on the direction of heat-transfer
• The first law is also silent about the ‘conditions‘ under which heat can be converted to work
• It also gives no indication about the ‘extent‘ of change that takes place.
• Low Efficiency‘ of a heat engine is not due to bad design. It is the law of nature, well known as Second Law of Thermodynamics