Basic Principles of Electromagnetism

C h a p t e r 4 | 974.6.4 PermeabilityPermeability is the measure of the ability of the material to allow the magneticfield to exist in it. Absolute permeability, µ of a material is the ratio of the fluxdensity to magnetic field strength.µ = µ 0µr[4.6]If the magnetic fields exist in the vacuum, the ratio of the flux density to themagnetic field strength is a constant called the permeability of free space. Forair or any other non-magnetic medium, the ratio of magnetic flux density toBmagnetic field strength is constant , = a constant. The equation forHpermeability of free space in non-magnetic medium is as shown in equation4.7 below.BH= µ 0[4.7]The permeability of free space, µ 0 is equal to 4π x 10-7 H/m. In the air or anynon-magnetic material µr = 1, this is the same magnetic properties as avacuum as shown in equation 4.7. µ r is relative permeability and it isconsidered when the different type of material is used. µr is defined as theratio of the flux density produced in the material that produced in the air or asdefine in equation 4.8.flux density in materialµr=[4.8]flux density in vacuumµr varies with the type of magnetic material. The approximate ranges ofrelative permeability for some common magnetic materials are as follows:Cast iron: µ r = 100 – 250Mild steel: µ r = 200 – 800Cast steel: µ r = 300 – 900Therefore the permeability for all media other than free space in a magneticmedium or material is as shown in equation in 4.9 below.BH= µ 0µ[4.9]rBasic Principles of Electromagnetism