Question

Define electric dipole moment. Is it a scalar or a vector? Derive the expression for the electric field of a dipole at a point on the equatorial plane of the dipole.

Answer 1

Electric dipole moment: The strength of an electric dipole is measured by the quantity electric dipole moment. Its magnitude is equal to the product of the magnitude of either charge and the distance between the two charges.

Electric dipole moment, p = q × d

It is a vector quantity.

In vector form it is written as`vecp = qxxvecd`, where the direction of `vecd`is from negative charge to positive charge.

Electric Field of dipole at points on the equatorial plane:

The magnitudes of the electric field due to the two charges +q and −q are given by,

`E_(+q) = q/(4piε_0) 1/(r^2+a^2)    ...... (1)`

`E_(-q) = q/(4piε_0) 1/(r^2+a^2)    ...... (2)`

`therefore E_(+q) =E_(-q)`

The directions of E_+q and E_−q are as shown in the figure. The components normal to the dipole axis cancel away. The components along the dipole axis add up.

∴ Total electric field

`E = -(E_(+q) + E_(-q)) cos theta hatp`

[Negative sign shows that field is opposite to `hatp` ]

`E = -(2qa)/(4piε_0 r^3)hatP        ........ (3)`

At large distances (r >> a), this reduces to

`E = -(2qa)/(4piε_0 r^3)hatP     ....... (4)`

`because vecp = q xx 2veca  hatp`

`because E = (-vecp)/(4piε_0 r^3)`   ( r>>a )