Define intensity of electric field at a point. Derive an expression for the intensity due to a point charge.

Explanation

Intensity of electric field (E) : Intensity of electric field at any point in an electric field is defined as the force experienced by a unit positive charge placed at that point.
Expression : (1) Intensity of electric field is a vector. Its direction is along the direction of motion of positive charge.
(2) Consider point charge q. Electric field will exist the given charge. A test charge `q_(0)` is placed at P.
(3) Force acting on `q_(0)` due to q is `F=1/(4 pi epsi_(0)). (q q_(0))/r^(2)`
(4) Intensity of electric field at theta point is equal to the force experienced by a test charge `q_(0)`. Intensity of electric field, `E=F/q_(0)`
`E=1/(4 pi epsi_(0)). q/r^(2)` N/C or V/m