Second Law of Thermodynamics

The second law of thermodynamics states that any process that occurs spontaneously will always increase the entropy (S) of the universe. In other words, it says that the entropy of an isolated system will never decrease with time. The second law of thermodynamics is also called the Law of Increased Entropy.

Although the first law of thermodynamics tells about the quantity of energy transfer and its conservation, it fails to provide any insight about the direction of flow of energy or transfer of energy. The second law of thermodynamics also puts a question mark on the direction of heat transfer and efficiency of heat engines that can be achieved.

However, when a system is in thermodynamic equilibrium or a reversible process is going on, then the total entropy of the system and its surroundings remains constant.

According to the concept of heat and work done, the second law of thermodynamics can have the following definition;

Heat energy cannot be converted into mechanical energy with 100% efficiency or without wasting a certain amount of energy. For example, in an engine, the gas is heated which increases its pressure and the increased pressure drives the piston. However, there is always some heat of the hot gas that is not used for driving the piston or some heat is wasted or it has to be wasted. In an engine, this wasted heat is transferred to a heat sink. Similarly, in a car engine, the unused heat is discarded in the form of fuel and air mixture into the air. The heat generated due to friction also remain unused and removed from the system.

So, the second law of thermodynamics says what is not possible, it does not tell what is possible. For example, according to this law, the efficiency of a machine can never be 1 or 100%. We can say that it is not possible that all the heat taken from the source is converted into work done without any wastage of heat. The unused heat is sent to the heat sink. It also says that without using a pump we cannot send the head back from the sink to the source. The following scientists' statements also support this law.

Clausius Statement

The heat flows from a hot body to a cold body independently but it cannot flow back or from a cold body to a hot body without any external work.

Kelvin Plank Statement

It is impossible to construct an engine or machine that can convert all the heat energy into work without wasting or releasing some heat into the surroundings.

Second law of thermodynamics in relation to entropy

According to this law, the entropy of the universe is continuously increasing.

What is entropy?

Entropy of a system refers to the randomness or disorder of a system. The concept of entropy was introduced by German physicist Rudolf Clausius in the year 1850.

Properties of Entropy

It is a thermodynamic state function as it depends on the state of a system, not on the path followed to achieve the final state. It is generally represented by S. The SI unit of entropy is J/Kmol and CGS unit is cal/Kmol.

According to Thermodynamics second law, any process that occurs spontaneously always leads to an increase in the entropy (S) of the universe. For example, the transfer of heat from the hot cup of tea to the cold surrounding is a spontaneous process and it increases the entropy of surroundings. The heat lost by the hot cup increases the temperature of surroundings so the particles in the surroundings will move faster and will lead to a more disordered arrangement or entropy.

All natural processes that occur spontaneously without using energy are irreversible. For example, the flow of heat does not occur from a cold body to a hot body; a hot cup of coffee at room temperature will lose heat to the surrounding, the transfer of heat from surrounding to cup will not occur.

Applications of the second law of thermodynamics

This law says that heat flows from high temperature to low temperature. The working of all heat engines is based on this principle. Various advanced modern times vehicles are also developed following this law.

The working of air conditioners and refrigerators and freezers is also based on this law. As per this law heat cannot be transferred from a cold body to a hot body, without using energy. So, refrigeration involves the transfer or removal of heat from a substance at a low temperature than surroundings with the help of external work

So, in such appliances, electricity is used to facilitate the transfer of heat from a cold reservoir to a hot reservoir so we are able to reduce the temperature of food items in a freeze and to reduce the room temperature on a hot day by an air conditioner.