Class 11 Physics Thermodynamics Internal Energy

Internal Energy

  • It is defined as the sum of kinetic energies and potential energies of the molecules constituting the system as a whole and not of individual molecule.
  • It is macroscopic variable of the system.
  • It is denoted by U.
  • It is thermodynamic state variable.
  • It is an extensive variable as it depends on the size of the system.
  • It can be specified by values of pressure, volume and temperature at that      particular time.
  • It only depends on the state of the system at that particular time.
  • It does not depend on how the system has reached that state.


In figure(a)  shows a box at rest then the Internal energy of ta gas inside the box is sum of the kinetic and potential energies of its molecules.

In figure (b) if the same box is pushed and it starts moving as a whole with some velocity, the kinetic energy of the box is not to be included in U.

Two different modes to change Internal energy are:-

  1. Heat
  2. Work


Heat: - Consider a bottle and a balloon are tied at the neck of the bottle. When we heat the bottle, due to the difference in temperature heat flow takes place, as a result balloon blows up. That is due to transfer of heat kinetic energy and potential energy changes from bottle to the balloon. This results in the change of internal energy.


Work: - We can do some work on the system (in this case system is bottle with a balloon tied) as a result the internal energy of the system changes.

Internal energy increases when the system absorbs heat and some work is done on the system similarly internal energy decreases if we change the conditions.

How Internal energy is different from Work and Heat

  • Heat and work are not state variables unlike internal energy.
  • They are modes of energy transfer to system resulting in change in internal energy. 

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