Physics · Electric Charges And Fields · NEET
In a uniform field E is the same everywhere. The +q end feels +qE and the -q end feels -qE. These are equal in size and opposite in direction, so they cancel: net force = 0 (only a turning torque is left). In a non-uniform field E is bigger at one end than the other. Now +qE and -qE have different sizes, so they do NOT cancel, and a net translational force is left over.
Once the dipole aligns with the field (torque does this first), it moves toward the region where the field is STRONGER. Reason: the potential energy is U = -p·E = -pE (when aligned). The field E is larger in the strong region, so U becomes more negative there. Nature always moves to lower energy, so the dipole is pulled into the stronger-field region.
In general it feels BOTH. The torque tries to align p with E, and the net force tries to drag it toward the strong-field side. NCERT simplifies to two easy cases: when p is exactly parallel or antiparallel to E, the torque is zero, so ONLY a net force remains. This is the case NEET usually pictures.
This is the classic NCERT example. The comb's field is non-uniform (stronger near the comb). It induces a dipole in each paper bit (polarisation), with p pointing along the field. Because the field is non-uniform, this induced dipole feels a net force toward the comb (the stronger-field side), so the paper jumps to the comb.
No. If p points opposite to E, then U = -pE cos180 = +pE, which is HIGHER where the field is stronger. So an antiparallel dipole is pushed toward the WEAKER field. But a free dipole will first rotate (torque) to align p with E, and then it gets pulled to the stronger side. Watch the alignment before deciding the direction.
A dipole is placed in a non-uniform electric field as shown, with the field stronger toward the right. In which direction will it move?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It can rotate because of torque, but its centre does not get a net push (net force = 0). Only a non-uniform field gives a net translational force.
For a dipole aligned along x, the net force magnitude is F = p (dE/dx), the dipole moment times the rate at which the field changes with position. A larger gradient means a larger force. In a uniform field dE/dx = 0, so F = 0.
Because its energy U = -pE is lowest where E is largest (once aligned). Objects settle into lower energy, so it is pulled to the strong-field region.
Yes, it is the exact analogy. A paramagnetic (aligned) dipole moves from weak to strong field, just like this electric dipole. A diamagnetic (anti-aligned) one moves the opposite way.
Yes. NEET has directly asked which way a dipole moves in a non-uniform field (2021) and often combines it with U = -pE reasoning, so knowing the direction rule saves time.
