What is Energy?

Energy is defined as the ability to do work. Work is defined as the distance covered by an object in response to an applied force. For example, when a book is lifted from the ground and put  on a table, work is done against the force of gravity. 

Energy is categorized into two main types － potential and kinetic.

Potential energy is the energy associated with the position, composition, or condition of an object. It is also known as the energy at rest or the stored energy. For example — the book held above the ground on the table possesses potential energy due to its height. This potential energy remains stored in the book as long as it is held in that position.

Different forms of potential energy include gravitational potential energy, electrical potential energy, chemical potential energy, nuclear energy, and elastic energy.

Gravitational potential energy is the most apparent form of potential energy, and it depends on the relative position of the object under the influence of the gravitational field. For instance, the book held above the table has gravitational potential energy stored in it. 

Electrical potential energy comes from the attraction or repulsion of electric charges, such as, the energy stored inside a capacitor in an electrical circuit is in the form of electric potential energy.

Chemical potential energy is the energy stored in molecules. For example, dry wood stores chemical potential energy which is released and converted into other forms of energy when the wood burns. 

Nuclear energy comes from the atomic nucleus...and elastic energy is the energy stored in an object due to its configuration, such as a stretched slingshot.

Kinetic energy is that which is associated with the motion of an object. For instance, when the book falls from the table, its potential energy gets converted into kinetic energy due to its motion.

Kinetic energy itself also exists in different forms — namely mechanical, electrical, thermal, and radiant energies.

While mechanical energy is due to the mechanical movement of an object like a moving car, electrical energy is constituted by the flow of electric charges, as seen in lightning strikes. When kinetic energy is in the form of electromagnetic waves, such as the energy from sunlight, it becomes radiant energy. 

Thermal energy is the kinetic energy associated with the motion of atoms and molecules in an object.  As the atoms and molecules within an object move or vibrate quickly and collide more frequently with each other, the temperature of the object increases. The heat from a burning flame is an example of thermal energy.

The opposite happens when the vibrations or motions of these atoms and molecules slow down. The object is then said to be cold and have low thermal energy - like when water freezes.

All these different forms facilitate the flow of energy from one type to another, or from one object to another, to get the work done.

The law of conservation of energy states that energy is neither created nor destroyed — it only transforms from one type to another. The total energy of the system, therefore, remains conserved. 

During archery, for example, an arrow held in a stretched bow contains elastic potential energy. When the bow is released, this potential energy gets converted into kinetic energy of the arrow, and the arrow is propelled to hit the target. Again, the total energy remains conserved.