The diagram shows a non-uniform electric field near a positively charged and a negatively charged sphere. Four electrons, A, B, C and D,

Q#25 (Past Exam Paper – November 2011 Paper 12 Q31)

The diagram shows a non-uniform electric field near a positively charged and a negatively charged sphere.

Four electrons, A, B, C and D, are shown at different positions in the field.

On which electron is the direction of the force on the electron shown correctly?

Solution:

Answer: A.

The direction of electric field lines is the direction of the force on a positive charge. That is, the direction is from (away) a positive charge towards a negative charge. It represents the force acting on a positive charge (a positive charge would be attracted towards a negative charge).

An electron, which has a negative charge, will be attracted towards the positive sphere [B incorrect], NOT towards the negative charge [D incorrect] since like charges repel and unlike charges attract.

The resultant field between the two charges is non-uniform.

In a non-uniform electric field, the direction of the electric field on a charge at any specific point is given by tangent to the electric field line at that point in the electric field. [C incorrect since it is not a tangent] 

Choice C represents the direction of the force on the charge if only the positive charge was present.