A molecule behaves as an electric dipole consisting of two equal point charges, of opposite sign, separated by a fixed distance.

Question 7

A molecule behaves as an electric dipole consisting of two equal point charges, of opposite sign, separated by a fixed distance. The molecule moves with constant horizontal velocity as it enters a vertical uniform electric field, as shown.

The positive and negative charges of the molecule enter the field at the same time.

Which row describes the velocity of the molecule in the field?

Reference: Past Exam Paper – June 201 Paper 13 Q30

Solution:

Answer: B.

Initially, before entering the electric field, the velocity of the molecule is horizontal. That is, it has no vertical component of velocity (= 0).

The electric field is vertically downwards. This would produce a vertical force on a charge.

Electric field lines are drawn from positive to negative. So, the positive charge of the molecule is attracted downwards while the negative charge is attracted upwards in the field. 

But since the charges are separated by a fixed distance, the forces on the charges of the molecule cancel out.

Thus, the vertical velocity of the molecule (initially zero) does not experience any resultant force. The vertical velocity remains the same (it remains zero).

 

The electric field is vertical. So, it does not have any effect on the horizontal component of velocity. The horizontal component of velocity remains constant.