What is the force per
unit length on it when it is placed on a horizontal table and the
direction of the current is (a) east to west; (b) south to north Solution  F = Il × B
F = IlB sinq
The force per unit length is
f = F/l = I B sinq
(a) When the current is flowing from east to west,
q = 90°
Hence,
f = I B
  = 1 × 3 × 10–5 = 3 × 10–5 N m–1
This is larger than the value 2×10–7 Nm–1 quoted in the definition
of the ampere Hence it is important to eliminate the effect of the
earth’s magnetic field and other stray fields while standardising
the ampere The direction of the force is downwards