An essential consequence of the Newton’s third law in mechanics
is conservation of momentum of an isolated system This, however, holds even
for the case of time-dependent situations with electromagnetic fields, provided
the momentum carried by fields is also taken into account Rationalised 2023-24
Physics
124
 EXAMPLE 4 10
The ampere is the value of that steady current which, when maintained
in each of the two very long, straight, parallel conductors of negligible
cross-section, and placed one metre apart in vacuum, would produce
on  each of these conductors a force equal to  2 × 10–7 newtons per metre
of length