Physics · Electric Charges And Fields · NEET
A dipole is +q and -q separated by a small distance. In a UNIFORM field the force on +q is qE (along E) and on -q is qE (opposite to E). These two forces are equal in size but opposite in direction, so they add to zero. Net force = 0, so the centre of the dipole does not move. But because the two forces act at DIFFERENT points, they still form a couple that produces a turning effect (torque).
Torque tau = pE sin theta depends on the angle theta between the dipole moment p and the field E. It is MAXIMUM (tau = pE) when theta = 90 degrees, i.e. the dipole is perpendicular to the field. It is ZERO when theta = 0 degrees (dipole aligned with E — this is stable equilibrium) or theta = 180 degrees (anti-aligned — unstable equilibrium). Remember: sin 0 = 0, sin 90 = 1.
Force decides whether the dipole MOVES from place to place (translation). Torque decides whether the dipole ROTATES. In a UNIFORM field: net force = 0 (no translation) but torque is non-zero (it rotates to align). In a NON-uniform field: there is BOTH a net force (so it also drifts toward the stronger field) AND a torque. For NEET, uniform field = only twist.
theta is the angle between the dipole moment vector p (which points from -q to +q) and the electric field vector E. It is NOT the angle with the plates or with any wire. A common mistake is to use the angle the dipole makes with the perpendicular; always measure from the direction of E to the direction of p.
tau = pE sin theta gives only the MAGNITUDE. The full vector form is tau = p x E (cross product). The direction of the torque is perpendicular to both p and E, given by the right-hand rule, and it always acts to rotate the dipole toward alignment with E (to reduce the angle theta). For NEET numericals the magnitude formula is usually enough.
An electric dipole is placed at an angle of 30 degrees with an electric field of intensity 2 x 10^5 N/C. It experiences a torque equal to 4 N.m. The charge on the dipole, if the dipole length is 2 cm, is
An electric dipole with dipole moment 5 x 10^-6 C.m is aligned with a uniform electric field of magnitude 4 x 10^5 N/C. The dipole is then rotated through 60 degrees with respect to the field. The change in potential energy of the dipole is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
tau = pE sin theta, where p is the dipole moment (p = q x 2a), E is the field strength, and theta is the angle between p and E. In vector form tau = p x E.
No. In a uniform field the forces on +q and -q are equal and opposite, so the net force is zero. Only a torque acts, which rotates the dipole to align with the field.
At theta = 90 degrees (dipole perpendicular to the field). Then sin 90 = 1 and tau = pE, the maximum value.
U = -pE cos theta. It is minimum (-pE, stable) when theta = 0, zero when theta = 90 degrees, and maximum (+pE, unstable) when theta = 180 degrees.
The two equal and opposite forces act at different points (the two charges), so they do not lie on the same line. Two equal, opposite, non-collinear forces form a couple, whose effect is pure rotation with no translation.
