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Two uniformly large parallel thin plates having charge densities +σ and -σ are kept in the X-Z plane at a distance 'd' apart. Sketch an equipotential surface due to electric field between the plates. If a particle of mass m and charge '-q' remains stationary between the plates, what is the magnitude and direction of this field?


Two small identical electrical dipoles AB and CD, each of dipole moment 'p' are kept at an angle of 120º as shown in the figure. What is the resultant dipole moment of this combination? If this system is subjected to electric field (begin mathsize 11px style straight E with rightwards arrow on top end style) directed along +X direction,

what will be the magnitude and direction of the torque acting on this?


Asked by Topperlearning User 24th April 2015, 11:00 AM
Answered by Expert


Here the red dotted lines represent the parallel equipotential surfaces along X-Z plane.

If a charge q has to be held stationary between the two plates, it will have to be balanced by two forces.

Gravitational force: mg, downwards

Electrostatic force = 2qE, acting upwards

This implies that in XZ plane, the upper plate is + charged plate and lower plate is -charged plate.

So, E field lines have to be directed along -y axis.


Resultant dipole moment,

Direction of resultant dipole moment :


That is, 30 degrees with +x axis.

Given applied E is along +x axis,

So torque on resultant dipole will be

τ = pEsin30 = pE /2

Direction will be along -z axis. 

Answered by Expert 24th April 2015, 1:00 PM
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