Page 478 - Basic Electrical Engineering

Relation between flux density, B and field intensity, H

The ratio of B and H is called the permeability of free space and is
represented by µ 0

This value is for free space or air or for any non-magnetic material like

paper, wood, oil, etc.
The relation, B = µ H is true when the medium is free space or air. When
0
the medium through which flux or flux density is established is changed, the
value of flux, ϕ or flux density, B increases. It has been observed that when

the core of a current-carrying coil is made of iron instead of being an air-core
one, the value of the flux density produced increases many times. The ratio of

the flux density produced with iron core to the flux density produced with air
core by the same magnetic field strength is called the relative permeability,
μ .
r
For air, μ = 1 and for iron or for some alloys of iron, μ can be very high.
r
r
Relative permeability, as mentioned earlier, indicates how many times the
material is more permeable than air (permeability is similar to conductivity in

an electrical circuit. Permeability is the ability of the material to allow the
establishment of flux through it)

For a non-magnetic material or air or vacuum as the medium, we write

B = μ H (5.6)
0

While for a material of the medium having a relative permeability of µ we
r
write,
B = μ μ H = μH
0 r
where

−7
μ = μ μ = 4π×10 ×μ (5.7)
0 r
r