electrostatics – Worksheet 1 problem solution (Q4)

4) A positively charged object Q is placed as shown in the sketch. The potential difference between two points A and B is 4 x 10^-4 V.

(a) Calculate the change in electrical potential energy of a +2nC charge when it moves from A to B.
(b) Which point, A or B, is at the higher electrical potential? Explain.
(c) If this charge were replaced with another object of charge -2nC, in what way would its change in energy be affected?

[ here is the complete worksheet: electrostatics worksheet 1 ]

Solution:

(a) The electrical potential energy of the positive charge decreases as it moves from A to B since it is moving in the direction of the electric field produced by the object Q.

This loss in potential energy is equal to the work done by the field,

Loss in Electrical Potential Energy = W
=VQ = (4 x 10^-4)(2 x 10^-9) = 8 x 10^-13 J

(b) Point A is at the higher electrical potential since work is required by us to move a positive test charge from B to A.

(c) If the charge is replaced by one with -2nC negative charge, the electrical potential energy of the charge will increase in moving from A to B (in this case we would have to do work on the charge).

[ here is the complete worksheet: electrostatics worksheet 1 ]