Define magnetic flux density.
A stiff copper wire is balanced horizontally on a pivot, as shown in the figure below.
Sections PQ, QR, and RS of the wire are situated in a uniform magnetic field of flux density 
produced between the poles of a permanent magnet. The perpendicular distance of PQRS from the pivot is 
. When a current of 
is passed through the wire, a small mass of 
is placed at a distance 
from the pivot in order to restore the balance of the wire, as shown in the figure below
Explain why, when the current is switched on, the current in the sections PQ and RS of the wire does not affect the balance of the wire.
The length of section QR of the wire is 
. Calculate the magnetic flux density B.
.
