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What are laws of Thermodynamics?

by Harpreet_Physics


Thermodynamics is the branch of physics which deals with heat and other forms of energy and also gives relationships between them. In other words, it describes how thermal energy is converted to and from other forms of energy and how it affects matter. There are four laws of thermodynamics. These laws give new thermodynamic properties. These properties are helpful in  understanding and predicting the system.

Zeroth law

The Zeroth law of thermodynamics  was first developed by R.H Fowler in 1931 stated as follows.

If two systems are in thermal equilibrium with a third system separately, then those systems are in thermal equilibrium with each other.

 Let us suppose if a system A and B is separately in thermal equilibrium with system C. Then A and B are also in thermal equilibrium with each other.

Zeroth law tells us that all systems have a common property when they are in thermal equilibrium with each other. And This common property is Temperature.

With the help of  temperature, we can determine whether or not a system is in thermal equilibrium with other systems.

Thermal equilibrium means equal temperatures. And  unequal temperatures simply means no Thermal equilibrium. This law gives the important concept of temperature.

First law

The first law of thermodynamics is based on the principle of conservation of energy. 

According to the First law of thermodynamics,  If a quantity of heat dQ is supplied to a body, then in general it is used in three ways. 

(i) Partially, System can use it in raising the temperature of the body i.e., increasing its internal kinetic energy dUK.

(ii) System can use it in  increasing the internal potential energy dUp by doing internal work against molecular attraction.

(iii) And rest is used in expanding the body against the external pressure i.e., in doing external work dW.

So, above statement become

dQ = dUK + dUp + dW                                            ……….. (1)

But we know

dU = dUK + dUp

Thus we have

dQ = dUK + dW                                                           ……….. (2)

This can take the form

  ΔQ = ΔU+ΔW                                                           ……….. (3)

We should note that, Equation (2) represents the differential form whereas equation (3) stands for change in respective quantities.

However, we can measure Q, U and W in the same unit i.e., all the three either in joules or in calories.

If the heat is taken by the body then Q is positive and if it is given by the body Q is negative.

Similarly, if the work is done by the system then W is positive and if the work is done on the system then W is negative.

Limitations of the first law of thermodynamics 

  • It does not tell about the direction of heat flow However, it does not indicate whether heat can flow from a cold end to a hot end or not. 
  • Does not tell if the process is feasible or not. 

Thus the Second law of thermodynamics overcomes these limitations. 

Second law

Different scientists have given statements for the second law of thermodynamics – (i) Kelvin-Planck statement (ii) Claussius statement

Kelvin-Planck’s Statement

It is impossible to construct a device which, operating in a cycle, will take heat from a body and convert it completely into work, without leaving any change anywhere.

Claussius Statement

“It is impossible to construct a device which, operating in a cycle, will take heat from a cold body and reject it to a hot body without expenditure of work by an external energy source”. In other words-heat cannot flow from a colder body to a hotter body on its own.

Third Law

For the study of chemical equilibrium in chemical reactions. It is necessary to determine the change in the entropy of the system when the reaction takes place at 0 K temperature.

Nernst assumed that this change in entropy is zero. His this assumption is termed as Nernst Heat Theorem. Nernst and Simon have given this concept in the form of law, stated as

“The entropy change associated with any isothermal reversible process of condensed system (solid or liquid) approaches zero as the temperature is reduced to absolute zero.”

This is the statement of the Third law of thermodynamics.

Other name for the third law of thermodynamics is the law of unattainability of absolute zero of temperature. 

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