We have seen that when two parallel conductors carry current, there is a magnetic field produced between them, which cause force between the conductors. It can be regarded that one conductor produced magnetic field and sets up the side thrust on the other conductor. If the flux density is denoted by B, the force can also be expressed in term of B as—
F = B I l newton
If I is Amperes, l is in metes, and F in newton, the flus density B is in Weber per square meter. Thus if I=1A, l=1m, F=B. the unit flux density can therefore be defined as that which gives force of one newton per meter length of conductor when the conductor carries the current of one Ampere.
If a magnetic flux is entering an area sq. meter perpendicular and the flux density in the area of value B the total magnetic flux is given by—
Flux φ = B A webers
The magnetic flux density increases with the use of magnetic materials. Thus more flux is produced in core of magnetic material due to current in the coil above the magnetic material due to current in the coil above magnetic material than when the medium is air only.
The magnetic flux density B = µH
Where µ is the permeability of the material medium. In general the expression can be put as B = µµ0H
Where, µ0 = permeability of free space = 4π x 10-7
Example:- A long straight conductor carries a steady current of 10 A. Find (a) the intensity of magnetic field produced at a point 15 cm from the axis of conductor & (b) flux density of the field at that point.
Solution:-
(a) H = I/2πr
= 10/ 2π x0.15
=10.6 amp/meter
(b) B = µµ0H
= 1 x 4π x 10-7 x 10.6
= 1.34 x 10-5 webers/sq.m.