Heat Internal Energy and Work – Class 11 | Chapter – 12 | Physics Short Notes Series PDF for NEET & JEE

Heat Internal Energy and Work: Heat, energy, and work are the core elements of thermodynamics. They all have innumerable uses in our daily lives and various scientific processes. Therefore, it is essential to understand the underlying concepts. There are mainly two types of Energy – Potential Energy and Kinetic Energy. They are further divided into various forms, one of which is Heat Energy. 

Heat Internal Energy and Work


Heat can be defined as kinetic energy produced by the random motion of matter, i.e., vibratory and rotary motions of molecules. Heat tends to flow from higher to lower temperatures, making it useful in various mechanical and chemical processes. The amount of heat flow is determined by mainly three factors:

  • Mass of the substance (m)
  • The difference in temperature between two objects (ΔT)
  • Nature of the substance

Based on this, we can conclude that Q ∝ mΔT or Q = cmΔT, Where c is proportionality constant, it is known as the specific heat capacity and is determined by the nature of the object. The SI unit of heat is Joule (J).


Heat and temperature are often used interchangeably, but this conception is wrong. Heat is the total kinetic and potential energy of the molecules in an object. On the other side, the temperature is defined as the average kinetic energy of the molecules. Therefore, it is denoted by the letter T. It is vital to understand that, unlike heat, it does not tell about the total energy of a thermodynamic system. A body with a higher temperature doesn’t need to have higher heat energy. For instance, an iceberg has a lower temperature than a burning matchstick, but the total heat energy in the iceberg is higher than that in a burning matchstick. Temperature is an intensive property and is not affected by the quantity of the matter in the context.

Thermal Expansion

Thermal expansion refers to the tendency of a material to expand or contract in response to changes in temperature. When a material is heated, its particles absorb energy and begin to move faster, causing the material to expand. Conversely, when a material is cooled, its particles slow down and move closer together, causing the material to contract.

The amount of expansion or contraction that a material undergoes depends on its coefficient of thermal expansion (CTE), which is a measure of how much its length, area, or volume changes per unit of temperature change. The CTE varies from material to material and can be affected by factors such as the material’s composition, structure, and crystal orientation.

Thermal expansion can have practical implications in various fields. For example, in construction, it is important to account for the expansion and contraction of building materials due to temperature changes to prevent damage to the structure. In engineering, the effects of thermal expansion must be considered when designing machines and devices to ensure that they operate properly over a range of temperatures.

Thermal expansion is broadly divided into three types:

  • Linear Expansion

When there is a change in length caused by heat, it is known as linear expansion. The formula for linear expansion is, L/Lo=LT where Lₒ = Original length, L = Expanded length, L = Coefficient of length expansion, ΔT = Difference in temperature and ΔL = Change in length.

  • Area Expansion

When there is a change in the area caused by the change in temperature, it is known as area expansion. The formula for area expansion is, A/Ao=AT where Aₒ = Original area, A = Expanded area, A = Coefficient of area expansion, ΔT = Difference in temperature and ΔA = Change in area.

  • Volume Expansion

When there is a change in volume due to temperature change, it is known as volume expansion. The formula for volume expansion is, V/Vo=VT, where Vₒ = Original volume, V = Expanded volume, V = Coefficient of volume expansion, ΔT = Difference in temperature and ΔV = Change in volume.

Internal Energy

Internal energy refers to the total energy of a system due to the kinetic and potential energies of its particles (atoms, molecules, or ions). It is a state function, meaning that it depends only on the current state of the system and not on how the system got there.

The internal energy of a system can be changed by adding or removing heat or work from the system. When heat is added to a system, the internal energy of the system increases because the particles move faster and their potential energy increases. Similarly, when work is done on a system, its internal energy can increase because the particles move against a force and their potential energy increases.

The internal energy of a system can also change due to changes in its volume, pressure, and temperature. For example, when a gas is compressed, its particles move closer together, increasing their potential energy and the internal energy of the system. Conversely, when a gas expands, its particles move farther apart, decreasing their potential energy and the internal energy of the system.

It is denoted by the letter U, measured in joules (J). It depends on the state of the molecules and their random movements. We can increase the internal energy by increasing the supply of heat. The formula for internal energy is given as, ΔU = Q + W, where ΔU = Internal Energy of the system, Q = Heat supplied to the system, and W = Work done by the system.


In thermodynamics, everything is divided into two parts – system and surroundings. While the system refers to the object of interest, surroundings refer to everything else. The total energy exchanged between the system and the surroundings is considered the work done. The amount of work done is affected by various factors such as pressure, volume, temperature, etc. The work can be done by the system and on the system, depending upon the path and the system’s initial and final state. The work done on the system is negative, while the work done by the system is positive. The amount of work done is calculated as, W = – ∫ P.dV

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