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Physics > Thermodynamics > Second Law of Thermodynamics

Second Law of Thermodynamics

We keep growing old, never younger. Although how we wish we could. We clean our rooms once and within no time they get dirty again. These are some things that work in one direction, not the other way round. These examples entail the second law of thermodynamics. But what does it state? Let’s find out more.

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What is the Second Law of Thermodynamics?

If you ever drop a glass and watch it shatter, you know there is no way of going back in time and getting back the unbroken glass. This is irreversibility. The second law of thermodynamics states that the heat energy cannot transfer from a body at a lower temperature to a body at a higher temperature without the addition of energy. This is why running an air conditioner for a long period of time, costs you money.

Browse more Topics under Thermodynamics

Kelvin-Planck statement

It is impossible to convert all the heat extracted from a hot body into work. In the heat engine, the working substance takes heat from the hot body, converts a part of it into work and gives the rest to the cold body. There is no engine that can convert all the heat taken from the source into work, without giving any heat into the sink. This means that for obtaining continuous work, a sink is necessary.

Clausius statement

It is not at all possible to transfer heat from a cold body to a hot body without the expenditure of work by an external energy source.

Equivalence of Two Statements

For example, there is a refrigerator that transfers an amount of heat from a cold body to a hot body without having any supply of external energy. So this is the violation of Clausius statement. Now suppose an engine working between the same hot and cold bodies takes in heat from hot body converts a part W into work and gives the remaining heat to the cold body.

The engine alone does not violate the second law of thermodynamics. But if the engine and refrigerator combine together, they form a device that takes up all the heat from the hot body and converts all into work without giving up any amount to the cold body. It violets the Kelvin-Planck statement. So we say that the two statements of the second law of thermodynamics are equal in all respects.

Laws of Thermodynamics


Second Law of Thermodynamics

Increase in Entropy

Let us see what is entropy, and its relation to the second law of thermodynamics. The entropy of the system is measured in terms of the changes the system has undergone from the previous state to the final state. Thus the entropy is always measured as the change in entropy of the system denoted by ∆S. If at all it is necessary to measure the value of the entropy at a particular state of the system, then zero value of entropy is assigned to the previously chosen state of the system.

The process during which the entropy of the system remains constant is an isentropic process. During the isentropic process, the value of entropy of the system at initial and final state remains constant. Thus during the isentropic process the value of ∆S=0. The reversible isentropic process never really occurs, it is only an ideal process. In actual practice whenever there is a change in the state of the system the entropy of the system increases.

Causes of increase in entropy of the closed system are

  • In a closed system, the mass of the system remains constant but it can exchange the heat with surroundings. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system.
  • Due to internal changes in the movements of the molecules of the system, there is disturbance inside the system. This causes irreversibilities inside the system and an increase in its entropy.

Let us study thermodynamics in detail. 

Question For You

Q. The second law of thermodynamics implies :

a. The whole of heat can be converted into mechanical energy

b. No heat engine can be 100% efficient

c. Every heat engine has an efficiency of 100%

d. A refrigerator can reduce the temperature to absolute zero

Sol: b. No heat engine can be 100% efficient

Total conversion of heat into work is not possible i.e 100% efficiency is not possible as it will lead to a negative change in entropy of universe which is not valid according to the Second Law of Thermodynamics.

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