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Question 2: Distinguish between the three forms of energy; work, heat and internal energy.


Heat, work and internal energy are three different but inter-related concepts. All the three are related to the temperature of the body.


Heat is the energy which flows from one body to another due to temperature difference between the two bodies.

When a water pot is placed on a flame, we observe that it heats up. We can say that something has flown from the flame to the water, which has raised the temperature of water. This particular ‘something’ which has flown from the flame to the water and has warmed it is called heat.
Thus heat is energy which transfers from the hotter body to the colder one.
According to molecular theory, heat is the total kinetic energy of the molecules of a body.


There is another method of raising the temperature of a body. This method is to perform work on somebody.This can be explained by the following examples.

  • When we rub our hands with one another we feel our hands become warm.
  • Pumping up a tire with a hand pump, we observe the pump heats up.

In both these examples, work is done on the bodies that increases the temperature of the body without a direct heat supply.


Both heat and work increase the temperature of a body though they are distinct forms of energies. However, in the case of work to increase the temperature of a body, unlike heat, it is not the temperature difference to direct heat from one body to another but the overall displacement (rubbing) of the bodies.

Internal Energy

The sum of kinetic and potential energies associated with the random motion of molecules of a substance is called the internal energy of that substance.

Matter consists of atoms and molecules which continuously move around. In case of solids, the atoms or molecules execute to-and-fro motion about their mean positions. In case of liquids, however, molecules are free to move. During their motion, they collide with one another and transfer/exchange energies. Whereas in the case of gases, molecules are moving freely and suffer elastic collisions with one another. The kinetic energy of the molecules may be translational, vibrational or rotational.

When a substance is heated, the random motion of atoms or molecules increases and hence the K-E too. When the kinetic energy increases, it increases the internal energy of the substance too; the heat energy is converted to the KE of the body.
In the same way, internal energy may be increased by doing work. The random motion of molecules may also be increased by performing work on the substance.
We should note that when heat or work energy, or both of them, are transferred to a substance, they cannot be distinguished as heat energy or work energy in the substance. Heat and work are not state functions.
Internal energy is a state variable and it has a particular value at a particular state of the substance. It is represented by ‘U’.

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