Physics · Electric Charges And Fields · NEET
On the axis, the field from the near charge and the far charge point the same way and add up, giving E_axial = 2kp/r³. On the equator, the point is the same distance from both charges, so their fields are equal in size but point in different directions. When you add them as vectors, only the horizontal parts survive and the vertical parts cancel. This leaves E_equatorial = kp/r³, which is exactly half of the axial value. Same distance, half the field.
It points anti-parallel to the dipole moment p. The dipole moment p points from the negative charge to the positive charge. On the equatorial line, the net field points the opposite way, from the positive side back toward the negative side. On the axis the field is parallel to p. So axial and equatorial fields point in opposite directions relative to p.
For a short dipole, both axial and equatorial fields fall as 1/r³ (r is distance from the centre). A single point charge falls as 1/r². The dipole falls faster because the two opposite charges nearly cancel far away. If a NEET option says the dipole field varies as 1/R³ for R >> L, that is correct.
Exact equatorial field: E = (1/4πε₀) · p / (r² + a²)^(3/2), where 2a is the separation and r is the distance from the centre on the perpendicular bisector. For a short dipole r >> a, so (r² + a²)^(3/2) ≈ r³, giving E = kp/r³. NEET numerical questions almost always use the short-dipole form.
Yes. The equatorial line (or equatorial plane) is the set of points on the perpendicular bisector of the line joining the two charges. Any point there is equidistant from +q and −q. The axial line is the straight line passing through both charges. These two directions give the two standard dipole field results you must know for NEET.
Two point charges −q and +q are placed at a distance L apart. The magnitude of the electric field intensity at a distance R (R >> L) varies as
Try the real previous-year questions from this chapter — each with the answer and a full solution.
E = (1/4πε₀) · (p / r³), where p is the dipole moment and r is the distance from the centre of the dipole. The field points anti-parallel to p.
At the same distance r, E_equatorial = E_axial / 2. Axial is 2kp/r³, equatorial is kp/r³.
It is anti-parallel to the dipole moment p, i.e. it points from the positive-charge side toward the negative-charge side.
Yes. In a medium of permittivity ε, replace ε₀ by ε (or divide by dielectric constant K): E = kp/(K r³). A larger K reduces the field.
The point is equidistant from +q and −q, so both fields have equal magnitude. Their components perpendicular to the dipole axis are equal and opposite, so they cancel; only the components parallel to the axis survive and add.
