4 Putting this value of k in Eq (1 1), we
see that for q1 = q2 = 1 C, r = 1 m
F = 9 × 109 N
That is, 1 C is the charge that when placed at a distance
of 1 m from another charge of the same magnitude in vacuum
experiences an electrical force of repulsion of magnitude
*
A torsion balance is a sensitive device to measure force