Question

explain why electric wires are of copper and
coated with plastic

Answer 1

Metal wire is used for conducting electricity because the electrons in metal are able to move well without heat or current losses. Typical wires used in residential construction are made of copper or aluminum. Most metals conduct electricity well, but copper and aluminum are the most economical and easy to work with. 

Metal is used in construction because of strength and durability. 

When electric conductors are free to contact construction metal, water, and other materials, electric current can flow where it isn't wanted. Often this is called a "short circuit" or a "short". The electricity flows where it isn't supposed to flow. 

Plastic and rubber doesn't conduct electricity well. (Something that doesn't conduct electricity well is called an "insulator" or "insulation"). The plastic or rubber coating on wires prevents electricity flow where it isn't wanted.

Answer 2

because copper is good conduter of electricity
and it is much better than the other metal or it is coated with plastic for safety