Science: Thermal Energy: Heat, Temperature and Thermal Expansion (For CBSE, ICSE, IAS, NET, NRA 2022)

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Thermal energy is also called heat. We receive heat directly from the sun along with light. The heat from the sun dries our clothes, ripens our crops and evaporates water from water bodies to cause rain. We need heat to cook our food, to light the fire, to run a thermal power station.

Thermal Energy


  • It is a form of energy; Heat can do work. The equivalence of work and heat was noticed and experimentally established by J. P. Joule. One Calorie (Unit of heat prevalent at that time) of heat is equivalent to 4.2 Joule of work.
  • e. g. When water is boiled in a kettle the steam built up in the kettle raises its lid up and when the steam escapes out the lid falls down. Heat thus can-do work.


Temperature is the degree of hotness of a body which determines the direction of flow of heat. Heat always flows from a body at higher temperature to a body at lower temperature.

  • The thermometer that a doctor uses to measure the temperature of human body is called Clinical thermometer.
  • The thermometer that we use for measuring temperature in science experiments is called laboratory thermometer.
  • The thermometer that the meteorologists use for determining the maximum and minimum temperature during a day is called as maximum – minimum thermometer.

Effects of Temperature

When a body is heated changes may occur in some of its properties . These changes are the effects of heat. Some of the effects of heat, as you might have observed are:-

  • Rise in temperature: When a body is heated its temperature increases.
  • Change of state: When heat is supplied to a substance in solid state its temperature rises till at a particular temperature it may change into its liquid state without any further change in its temperature. This characteristic constant temperature at which a solid changes into its liquid state is called melting point of the solid.
  • Conversion of a solid into its liquid state at its melting point is called change of state from solid to liquid (fusion) and the heat that is transferred to the substance during melting is called Latent Heat of Fusion.
  • when heat is supplied to a substance in liquid state its temperature rises but there is a possibility that it changes into its vapour state at a constant temperature. The heat supplied in this case is called Latent Heat of Vaporization.

Thermal Expansion

The increase in the size of a body on heating is called thermal expansion. It can be seen that increase in length of a metallic bar will be more for a longer bar and also for a greater rise in temperature of the same bar. Let us consider a metallic bar of length L0 at temperature 0° C. Increase in its length L at a temperature t is given by:

  • Here α is a constant for the material of the bar and is called as the Linear expansivity of the bar.
  • The Linear expansivity (or Coefficient of Linear expansion) of a material is defined as the change in length per unit original length per degree Celsius rise in temperature.
  • The SI Unit of coefficient of expansion is per kelvin (which is same as per degree Celsius in magnitude) .
  • The Volume expansivity of a material may be defined as change in volume per unit original volume per degree Celsius rise in temperature.


Thermal Equilibrium

  • When two bodies at different temperatures are kept in contact, heat is transferred from the hot body to the cold body till both of them acquire the same temperature. The two bodies then are called in thermal equilibrium.
  • It can be seen that if the temperature of hot body is more, the rise in the temperature of cold body will also be more i.e.. heat transferred from a hot body to a cold body is directly proportional to their temperature difference,

Similarly, it can be shown that if the mass of cold body is more it will absorb more heat from the cold body




  • Where s is a constant of proportionality which depends on the nature of the material of the body. This is also called as the specific heat capacity of the material
  • The SI Unit of specific heat capacity (or simply specific heat) is J kg – 1 K – 1

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