Class 11 Physics: Thermodynamics

Class 11 Physics: Thermodynamics

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Last updated on September 10th, 2019 at 05:30 am

What is Thermodynamics?

Thermodynamics is a branch of physics which is concerned with heat and its relationship with another form of energy. William Thomson first coined the term thermodynamics in the year 1749 in the context of heat flow. This branch of physics does not discuss the microscopic properties and is concerned only with the macroscopic behavior. These macroscopic properties are governed by four laws, namely Zeroth, First, Second, and Third Law of thermodynamics. We can apply these laws of thermodynamics to a wide variety of topics, including several branches of engineering and chemistry.

Laws of Thermodynamics:

Zeroth Law:

If two bodies A and B are independently in thermal equilibrium with another body C, then A and B are also in thermal equilibrium with each other.

Zeroth law helps us to define the concept of temperature to measure the hotness or coldness of a body. According to the zeroth law of thermodynamics, at thermal equilibrium, we can assign all bodies with equal temperature. Therefore, a hotter body has a higher temperature, and a colder body has a lower temperature.

First Law:


The internal energy of an isolated system is constant. This definition can be formulated by saying that the change in internal energy of a closed system is equal to the difference between the heat supplied to the system and the amount of work done by the system.

The first law of thermodynamics is a restatement of the principle of conservation of energy. Suppose, an amount ΔQ of heat is given to the system, and during the process, an amount ΔW of work is done by the system. If the change in internal energy is ΔU, then according to First law,


Second Law:

It states that for an isolated system, the total entropy can never decrease over time. Alternatively, there are two other definitions associated with the principle of heat engine and refrigerator. They are known as Kelvin–Planck statement and Clausius Statement of the second law of thermodynamics, respectively.

Kelvin–Planck statement: It is not possible to design a heat engine which operates on a cycle and whose sole result is to take heat from a body and convert it entirely into mechanical work.

Clausius Statement: It is not possible to design a device which operates on a cycle and whose sole result is to transfer heat from a colder body to a hotter body.

Third Law:

It states that the entropy of a system approaches a constant value as the temperature approaches absolute zero. The entropy of a system at absolute zero is typically zero and in all cases is determined only by the number of different ground states it has.

Nernst–Simon statement: The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as the temperature at which it is performed approaches 0 K.

What is Entropy?

Entropy is a thermodynamic variable which defines the randomness or disorder of a system. In a given equilibrium state, any system has a definite value of entropy. For example, the entropy of a solid where the particles are constrained to move is less than the entropy of a gas where the disorder is much pronounced.

If the temperature of a system remains fixed at T when a small amount of heat ΔQ is given to it, we define the change in entropy as

    \[ \boxed{\Delta S=\frac{\Delta Q}{T}} \]

Usually temperature changes during a process. Thus for a reversible process, the change in entropy is

    \[ \boxed{S_{f}-S_{i}=\int_{a}^{b}{\frac{\Delta Q}{T}}} \]

Different Measures of Energy:

There are four different measure of energy defined for the various thermodynamical environment. They are known as internal energy, enthalpy, Helmholtz free energy, and Gibbs free energy.

  • Internal energy (U) of a system is the total energy contained within the system excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. We can not measure the total amount of internal energy associated with a system. However, the difference in internal energy during a process can be measured.

    \[ \boxed{U=\int{TdS}-\int {PdV}} \]

  • Enthalpy (H) is a measure of energy associated with a thermodynamic system. It is the sum of the internal energy and the product of the pressure and volume of a thermodynamic system. Enthalpy is energy whose value is determined entirely by the temperature, pressure, and composition of the system.


  • Helmholtz free energy (F) is a thermodynamic potential that measures the useful work obtainable from a closed thermodynamic system at a constant temperature and volume.


  • Gibbs free energy (G) is a thermodynamic potential that can be used to calculate the maximum amount of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure.