This question attracted lots of candidates. Performance was fairly satisfactory. Part (a)(i); many candidates succeeded in giving the correct statement of coulomb’s law of electrostatics.
In question (a) (ii), a small fraction the candidates gave the correct definition of electric field intensity. Some candidates however missed the answer by mentioning ‘force’ without qualifying it with the word “electric” since gravitational and or magnetic force(s) could also be present at the point of interest. In (a) (iii) only few candidates got the correct field pattern and indicated the neutral point and correct direction of the field with arrows.
In (b), this question electrostatic force was attempted by many candidates. A good number of the candidates performed fairly well. Symbols in the given equation were identified correctly by many candidates, and the S. I. units given. However, some could not correctly identify k.
Questions (c) (i) and (ii) are numerical problems on electric potential. Here, candidates’ performance was quite fair. Most candidates failed to put the unit of their answers.
The expected answers include:
(i) Coulombs law states that the (electrostatic) force between two (point) charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them
(ii) Electric field intensity at a point is the electric force per unit (positive) charge at that point
(iii)
- correct shape
- correct arrows direction
- neutral point correctly indicated
(b)(i) k = constant of proportionality
1
(accept 4πεo. Provided εo is defined as permittivity of free space).
q1 and q2 = point charges
r = distance between the charges
(ii) unit of k – N m2 C-2 OR m F-1
unit of q1 and q2 - coulomb (C)
unit of r - metre(m)
(c)(i) V = 1 x q
4πεo. r
= 9.0 x 109 x 1.6 x 10-12
5.0 x 10-2
= 0.288 V
(ii) w = QV
= 1. 6 x 10-12 x 0.288
= 4.6 x 10-13 J
