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 This direction may be
obtained by the directional property of cross product of vectors