Electric and magnetic elds are produced by electric charges and their motion. Electric charge may be positive or negative. e force Fe between two charged spherical bodies whose radii are

small compared to the distance between them, r, and which are remote from other dielectric media is

Fe = 1

4piε rq q

where q1 and q2 are the charges and ε is the permittivity of the medium in which they are located. In free space ε = ε0 = 8.854 × 10−12 F m−1. Bold typeface indicates a vector quantity. When the charge q2 is distributed over a region of space the force exerted on q1 is the vector sum of the forces due to all the elemental charges dqn that make up q2 and so

F rq dq r

1 4e

where rn is the distance between q1 and dqn and rn is the unit vector along the line joining them. From Equation 4.2, the force per unit charge at the location of q1 is

q dq r r

1 4 .