CBSE Class 12-science Answered

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.

OR

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.