Simulation · Physics · Class 12

Torque on a dipole in a uniform field

From the lesson Dipole in a uniform external field in Electric Charges and Fields. Change the values and watch what happens.

The idea behind it

NCERT §1.11

In a uniform field the forces qE and −qE on the two charges are equal and opposite, so the net force on a dipole is zero.
The two forces act at different points and form a couple: torque τ = p × E, of magnitude τ = pE sin θ, where θ is the angle between p and E.
The torque turns the dipole towards alignment with E. It is maximum (pE) at θ = 90° and zero when p is parallel or antiparallel to E.
In a non-uniform field there is also a net force. With p parallel to E the dipole is pulled towards the stronger field; with p antiparallel it is pushed towards the weaker field.
A comb run through dry hair attracts uncharged bits of paper: the comb's field polarises the paper, inducing a dipole along the field, and because the comb's field is non-uniform the paper is pulled towards the comb.

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