Physics · Electric Charges And Fields · NEET
It is a vector. Magnitude is p = q x 2a, and its direction is fixed in space (from -q to +q). Because it is a vector, in torque and energy problems you must use the angle it makes with the field: torque = pE sinθ and energy U = -pE cosθ. Treating it as a scalar loses the angle and gives wrong answers.
By NEET/NCERT convention the dipole moment points from the negative charge to the positive charge (from -q toward +q). Note this is opposite to the direction of the electric field between the two charges, which runs from + to -. Do not mix the two.
2a is the full separation between the two charges, i.e. the distance from -q to +q. Here a is the half-length (distance from the centre to each charge). So p = charge x total separation. If a question gives 'dipole length L', then 2a = L and p = qL.
Total charge is +q + (-q) = 0, so far away it is neutral overall. But the + and - charges sit at slightly different places, so their fields do not fully cancel nearby. The leftover field is the dipole field, and it falls off as 1/r^3 (faster than a single charge's 1/r^2).
Charge q (unit coulomb) tells you how much electricity a body carries. Dipole moment p (unit C m) tells you how strongly a neutral pair is separated - it needs both the charge and the gap. A big charge with almost no separation can have a small p, and vice versa.
An electric dipole is placed at an angle of 30° with an electric field of intensity 2 x 10^5 N/C. It experiences a torque of 4 N m. If the dipole length is 2 cm, the charge on the dipole is:
An electric dipole of moment 5 x 10^-6 C m is aligned with a uniform field of 4 x 10^5 N/C. It is rotated by 60° from the field. The change in its potential energy is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
p = q x 2a, where q is the magnitude of either charge and 2a is the distance between the two charges. Its unit is coulomb-metre (C m) and it is a vector pointing from -q to +q.
The SI unit is coulomb-metre (C m). For example, charges of 10 μC separated by 5 mm give p = 10^-5 x 5 x 10^-3 = 5 x 10^-8 C m.
From the negative charge to the positive charge (from -q to +q). This is the standard NEET/NCERT convention and is opposite to the field line direction between the charges.
The +q and -q fields nearly cancel far away because the net charge is zero. What is left depends only on the product q x 2a (the dipole moment), and this leftover field decreases as 1/r^3, faster than a single point charge's 1/r^2.
An ideal dipole is the limit where the separation 2a shrinks to zero and the charge q grows so that the product p = q x 2a stays finite. It is a useful idealisation used in most dipole formulas.
