Explain the different forms of energy.

$(a)$ Heat Energy

$(b)$ Chemical Energy

$(c)$ Electrical Energy

$(1)$ Heat Energy :

The frictional force is not a conservational force and work is associated with the force of friction. A block of mass $m$ sliding on a rough horizontal surface with speed $v_{0}$ comes to a halt over a distance $x_{0}$. The work done by the force of kinetic friction $f$ over $x_{0}$ is - $f x_{0}$.

By the work energy theorem $\frac{m v_{0}^{2}}{2}=f x_{0}$.

If one think in mechanics, the kinetic energy of the block is lost due to the frictional force. When the block is sliding on a table, the slight temperature of block increases and hence internal energy of block and table increases.

In winter, in order to feel warm, we generate heat by vigorously rubbing our palms together. A quantitative idea of the transfer of heat energy is obtained by noting that $1 \mathrm{~kg}$ of water releases about $42000 \mathrm{~J}$ of energy when it cools by $10^{\circ} \mathrm{C}$.

 $(2)$ Chemical Energy:

Chemical energy arises from the molecules participating in the chemical reaction have different binding energies.

A stable chemical compound has less energy than the separated parts. A chemical reaction is basically a rearrangement of atoms.

Hence, in fact chemical energy is a electromagnetic energy.

If the total energy of the reactants is more than the products of the reaction, heat is released and the reaction is said to be an exothermic reaction.

If the total energy of the reactants is less than the products of the reactions, heat is absorbed and the reaction is endothermic.

Coal consists of carbon and a kilogram of it when burnt releases about $3 \times 10^{7} \mathrm{~J}$ of energy. Chemical energy is associated with the forces that give rise to the stability of substances. These forces bind atoms into moleclues into polymeric chains.

The chemical energy arising from the combustion of coal, cooking gas, wood and petroleum is indispensable to our daily existence.

$(3)$ Electrical Energy :

Electric energy is consumed due to the flow of electric current in electric circuit. Electric energy = Electric potential $\times$ Electric charge.

Energy is associated with an electric current.

An urban Indian household consumes about $200 \mathrm{~J}$ of energy per second on an average.

$(4)$ The Equivalence of Mass and Energy :

Till the end of the nineteenth century, physicists believed that in every physical and chemical process, the mass of an isolated system is conserved matter is neither created nor destroyed.