Question

(a) Which physical quantity has its SI unit (1) Cm (2) N/C.
(b) Two point charges q and -q is placed at a distance $2a$ apart. Calculate the electric field at a point P is situated at a distance r along the perpendicular bisector of the line joining the charges. What is the electric field when r >> a? Also, give the direction of electric field w.r.t. electric dipole moment?

Answer 1

a) 1) Electric dipole moment
2) Electric field
b) Field at P due to charge q:
along BP.
Field at P due to charge -q:
along PA.
Clearly, the components of these 2 forces with equal magnitudes cancel along the direction OP but add up along the direction parallel to BA.
Therefore, the resultant electric field at P due to the 2 charges at A and B is:
E = - *
When
E = -
the direction of electric field in this case is opposite to the direction of the dipole moment vector.