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Various Forms of Energy: The Law of Conservation of Energy

Energy can be classified into two distinct categories: one based on motion, namely kinetic energy; the another one is based on position namely potential energy. Energy comes in many a forms which transforms into one another in ways which may not often be clear to us.

Internal Energy

The mechanical energy (kinetic and potential) of an object discussed above refers to the object as a whole. Every substance (solid, liquid or gas) possesses a certain amount of energy due to the motion and configuration of the molecules that constitute it. This energy is called the internal energy. The internal energy of a solid, liquid or gas consists of two parts
  1. kinetic energy due to the motion of the molecules; this motion can be translational, rotational or vibrational, and
  2. potential energy due to mutual attraction and repulsion between molecules; this depends upon inter-molecular separation. The sum total of the kinetic and potential energies of the molecules of a substance is the internal energy of the substance.
If the internal energy of a body is increased, its temperature increases. For example, if we rub our hands together, we do work against the friction between the hands. The kinetic energy of the motion of our hands is converted into heat energy and we feel warm. The heat energy increases the kinetic energy of the molecules of our hands thus increasing the internal energy. A decrease in internal energy implies a fall in temperature.

Heat or Thermal Energy

Heat is a form of energy called thermal energy. A body has more thermal energy when it is hot than when it is cold. As a hot body cools, it can be made to perform mechanical work at the expense of its thermal energy.

For example, in a steam engine steam expands and cools. The steam loses thermal energy which is converted into mechanical energy of the piston which moves the wheels of the engine. Thus energy is associated with heat. The thermal energy of a body is related to the disorderly motion of its molecules as well as to its internal energy. The relationship between thermal energy, internal energy and temperature is the foundation of the subject of thermodynamics.

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