Dipole in a uniform external field

1. Consider a dipole of charge +q and –q, separated by a distance
2. Consider an external electric field Ē . This field is external which means it is not induced by the dipole. The dipole is placed in the external field Ē
3. This dipole experiences a rotating force, which is torque
4. If the points q and –q of the dipole lie very close to each other, then the force +q Ē and - q Ē on +q and –q respectively will cancel out as the force acts on the same point. There will also not be any rotating or torque in such a case
5. However, as the charges are separated by some distance, the forces acts on different points and hence torque comes into picture here
6. The pencil diagram illustrates the points 4 and 5 respectively

 Let us take the questions asked:

• They are forces acting on both ends of the dipole. So should the dipole move along the midpoint?

In fact when the forces act at a single point may be the midpoint, the forces of equal magnitude and opposite direction cancel each other. So there will be no rotation or torque at the midpoint.

• Explain why the distance is taken as AC?

The magnitude of the torque is each force multiplied by the perpendicular distance between the two forces

• Explain – The one on top should come down and one on bottom should go up to form a straight line. But why does this not happen?

This is due to the direction of the forces, as they are opposite to each other but magnitude wise both are same. Hence, the ends are pulled – one in upward direction and one in downward direction

Infact the direction of external field is important. If external field is parallel to dipole and the angle between them is zero, then no torque is experienced by the system. The up down movement  and the torque is due to the fact that external field  is not in the same direction as the dipole.