Absolute Permeability and Relative Permeability
In Figure given below it is shown a bar of a magnetic material, say, iron placed in a uniform field of strength H N/Wb. Suppose, a flux density of B Wb/m2 is developed in the rod.
Then, the absolute permeability of the material of the rod is defined as
μ = B/H henry/metre or B = μH = μ0μr H Wb/m2 …………………..(i)
When H is established in air (or vacuum), then corresponding flux density developed in air is
B0 = μ0H
Now, when iron rod is placed in the field, it gets magnetised by induction. If induced pole strength in the rod is m Wb, then a flux of mWb emanates from its N-pole, re-enters its S-pole and continues from S to N-pole within the magnet. If A is the face or pole area of the magentised iron bar, the induction flux density in the rod is
Bi = m/A Wb/m2
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Hence, total flux density in the iron rod consists of two parts [Figure given below].
(i) B0 –flux density in air even when rod is not present
(ii) Bi –induction flux density in the rod
B = B0 + Bi = μ0H + m/A
Eq. (i) above may be written as B = μr . μ0 H = μr B0
………………….. for same H
Hence, relative permeability of a material is equal to the ratio of the flux density produced in that
Material to the flux density produced in vacuum by the same magnetizing force.
Read article – flux density
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