## Law of Equipartition of Energy – Class 11 | Chapter – 13 | Physics Short Notes Series PDF for NEET & JEE

Law of Equipartition of Energy: The law of equipartition of energy is a fundamental principle in thermodynamics and statistical mechanics that states that, at thermal equilibrium, each degree of freedom of a molecule in a gas or a solid has an average energy of (1/2)kT, where k is the Boltzmann constant and T is the absolute temperature of the system.

## Law of Equipartition of Energy

The law applies to any system in which energy is uniformly distributed among its degrees of freedom. For example, in a gas of diatomic molecules, each molecule has five degrees of freedom: three translational degrees of freedom (movement in three dimensions), and two rotational degrees of freedom (rotation about two axes). At thermal equilibrium, the average energy associated with each degree of freedom is (1/2)kT, meaning that each molecule has a total energy of (5/2)kT.

Similarly, in a solid, each atom or molecule has three translational degrees of freedom (movement in three dimensions) and three vibrational degrees of freedom (motion of the atoms or molecules relative to each other). At thermal equilibrium, each degree of freedom has an average energy of (1/2)kT, meaning that each atom or molecule has a total energy of (3/2)kT.

The law of equipartition of energy is a consequence of the equipartition theorem, which states that at thermal equilibrium, the total energy of a system is uniformly distributed among all its degrees of freedom. The theorem assumes that the system is in thermal equilibrium, meaning that it is at a constant temperature and has reached a steady state in which its energy is conserved.

## Applications of Law of Equipartition of Energy

The law of equipartition of energy is a fundamental principle that has many practical applications in various fields of science and engineering. Some of the most notable applications include:

• Thermal insulation: The law of equipartition of energy is used in the design of thermal insulation materials, such as blankets, foam, and fiberglass. These materials are designed to trap air molecules between the fibers, which reduces heat transfer by conduction and convection. The law of equipartition of energy helps to explain how these materials work by describing the transfer of energy between the air molecules and the fibers.
• Specific heat capacity: The law of equipartition of energy is used to calculate the specific heat capacity of materials, which is the amount of heat required to raise the temperature of a unit mass of the material by one degree Celsius. The law states that each degree of freedom of a molecule in a material contributes (1/2)kT of energy, and therefore, the specific heat capacity is directly proportional to the number of degrees of freedom.
• Chemical reactions: The law of equipartition of energy is used to study chemical reactions by calculating the activation energy required for a reaction to occur. The activation energy is the minimum amount of energy required for the reactant molecules to overcome the energy barrier and form products. The law of equipartition of energy helps to explain how the energy is distributed among the molecules, and how this affects the rate of the reaction.
• Spectroscopy: The law of equipartition of energy is used in spectroscopy to interpret the vibrational and rotational spectra of molecules. The spectra provide information about the energy levels of the molecules, and the law helps to explain how the energy is distributed among the different modes of motion.

## Limitations of Law of Equipartition of Energy

The law of equipartition of energy is a fundamental principle in thermodynamics and statistical mechanics that assumes that each degree of freedom of a molecule in a gas or a solid has an average energy of (1/2)kT at thermal equilibrium. While this law is generally useful for understanding the behavior of many systems, there are some important limitations to its applicability.

• Low temperatures: The law of equipartition of energy assumes that the temperature of the system is high enough such that all degrees of freedom are excited and contribute to the total energy of the system. However, at low temperatures, some degrees of freedom may not be excited and may not contribute to the total energy of the system. In this case, the law of equipartition of energy may overestimate the energy of the system.
• Quantum effects: The law of equipartition of energy assumes that energy is continuously distributed among the different degrees of freedom. However, in quantum mechanics, energy is quantized, meaning that energy is only transferred in discrete units called quanta. Therefore, the law of equipartition of energy is not applicable to systems where quantum effects dominate, such as atoms, molecules, and particles at very low temperatures.
• Molecules with large moments of inertia: The law of equipartition of energy assumes that each degree of freedom of a molecule has the same energy, regardless of its mass or moment of inertia. However, for molecules with large moments of inertia, such as linear molecules, rotational degrees of freedom may not be equally excited. In this case, the law of equipartition of energy may not accurately predict the total energy of the system.
• Non-harmonic potentials: The law of equipartition of energy assumes that the potential energy of the system is a quadratic function of the displacement from equilibrium. However, for systems with non-harmonic potentials, such as molecules with strong intermolecular interactions, the energy distribution among the degrees of freedom may deviate from the law of equipartition of energy.

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