Two long straight vertical wires X and Y are separated by a distance of 
, as illustrated in the figure below.
The wires pass through a horizontal card PQRS. The current in wire X is 
in the upward direction. Initially, there is no current in wire Y.
The magnetic flux density 
at a distance 
from a long straight current-carrying wire is given by the expression
where 
is the current in the wire and 
is the permeability of free space. Calculate the magnetic flux density at wire Y due to the current in wire X.
A current of 
is now switched on in wire Y. Use your answer in (i) to calculate the force per unit length on wire Y.
The currents in the two wires in (a)(ii) are not equal. Explain whether the force per unit length on the two wires will be the same, or different.
.
.
and 
in the wires X and Y respectively. Therefore, the force per unit length on the two wires will be the same.
