An introduction to Thermodynamics is essential before discussing the second law of thermodynamics. Thermodynamics is an integral part of physics. The study of thermodynamics deals with heat transformation, the relationship of heat with other factors like temperature, work and energy.

More precisely, thermodynamics is a field of study that explores the topic of heat transformation as well as heat transformation, work, temperature and energy. And the first and second law of thermodynamics helps get the deeper essence of heat transformation and how it works. 

What is the Second Law of Thermodynamics?

There are two major laws explaining the process of energy transformation and why this happens. These laws are called the first law of thermodynamics and the second law of thermodynamics. Both of these thermodynamic laws deal with energy transformation and heat flow.

The second law of thermodynamics says that heat transformation occurs spontaneously, but the heat only flows from high to low temperatures (technically the word used is entropy)

Naturally, the flow of heat only transfers from the high temperature to the low temperature.

For example, if you put two glasses of water together so that both of their surfaces are in touch with each other. And one of the glasses is filled with hot/boiling water, and the other is filled with normal temperature. After a few minutes, you will see that both glasses have an equal temperature. 

The high temperature of the glass of hot water has been transferred to the glass of normal temperature water. This happens according to the second law of thermodynamics. 

Different Directions of thermodynamics

Different thermodynamic processes happen naturally in one direction and generally do not proceed from the opposite direction. It means that heat always naturally flows from the warmer object to the colder object among the different temperatures. But the fact is that even if the direction of the thermodynamic process changes/reverses ( from a cold object to a warm object), this would not violate the first law of thermodynamics.

 Therefore, this reversal will cause energy conversion that does not occur naturally. For instance, energy conversion happens when gasoline or fuel is burned to start a car. In this case, the chemical energy present in gasoline is converted to thermal energy, and then it changes into the mechanical energy that helps the car start. And then, the mechanical energy changed into Kinetic energy. 

So, when we hit the brakes, the kinetic energy reverses to the friction heat or thermal energy. But there cannot be a total conversion of heat into mechanical energy as this process is characterised by significant energy loss.

The second law of thermodynamics or, more specifically, the Clausius statement of the second law, discusses the direction of thermodynamics processes.

Statements of the second law of thermodynamics

Various statements by scientists and physicists express the second law of thermodynamics. Here we will tell you some of the major statements of the second law:

• Carnot’s Principle-  Carnot’s principle is the first promulgation using the second law of thermodynamics to give a theory of efficiency. It was presented by French physicist and engineer Nicolas Leonard Sadi Carnot in 1824. According to this principle, the maximum efficiency of a thermodynamic cycle of reversible engines relies completely on the temperature difference between the high and low-temperature reservoirs.
• Clausius’s Statement- In 1850, R. Clausius made one of the most notable early comments on the second law of thermodynamics. Clausius states that heat cannot naturally flow from a cooler to a warmer system without any external force. Examples of this rule can be refrigerators and coolers that need external work to extract heat from a hot object to cool it down.
• Kelvin and Planck’s statements- Kelvin’s and Planck’s statements about the second law of thermodynamics are read together because of their shared major concepts of the law. Both of them propose the idea of a single energy reservoir. Heat flow derived from a thermal reservoir cannot produce a positive amount of work if the system goes through a conversion cycle.

Entropy and the second law of thermodynamics

Change in entropy (S) determines the direction of thermodynamics. It refers to the physical property of a matter; an idea developed from the second law of thermodynamics. According to the second law, there cannot be a total conversion of heat energy into work as there is always some energy loss. This lost energy that cannot perform work is called entropy.

Clausius states that if Q or an amount of heat is added to a system by a reversible process against a stable temperature, the entropy of that system changes: dS= dQ/T.

Equations of thermodynamics

There are many equations of thermodynamics related to heat, work, inner energy, entropy etc. Here, we will use equations of the three laws:

• Equation of the first law- ∆U= q+w
• Equation of the second law- ∆Suniv= ∆Ssys+ ∆Ssurr

Conclusion- 

The study of thermodynamics is an extended branch of physics. Thermodynamics deals with the concept of heat transformation and the relationship between heat, work and energy. And three thermodynamic laws help to explain the process of heat transformation and its connection with other aspects like work, pressure, inner energy of a system etc. 

The second law of thermodynamics states that heat flow is always transferred from the warmer object to the colder object. And the reversal of this process does not occur naturally. This law is used as the base for building many electrical and mechanical systems.