Absolute Permeability (μ) and Relative Permeability (μr)
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
= µ0 µr H Wb/m2 ...(i)
When H is established in air (or vacuum), then corresponding flux density developed in air is
B0 = µ0 H
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 m Wb 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
Hence, total flux density in the iron rod consists of two parts
(i) B0 –flux density in air even when rod is not present
(ii) Bi–induction flux density in the rod
B = B0 + Bi
= µ0 H + m/A
Eq. (i) above may be written as
B = µr . µ0 H
= µr B0
µr =B/B0
Flux Density (B)
It is given by the flux passing per unit area through a plane at right angles to the flux. It is usually designated by the capital letter B and is measured in weber/meter2 . It is a Vector Quantity. It ΦWb is the total magnetic flux passing normally through an area of A m^ 2
, then
B = Φ/A Wb/m^2 or tesla (T)
Intensity of Magnetisation (I)
It may be defined as the induced pole strength developed per unit area of the bar. Also, it is the magnetic moment developed per unit volume of the bar.
Let
m = pole strength induced in the bar in Wb
A = face or pole area of the bar in m^2
Then
I = m/A Wb/m^2
Hence, it is seen that intensity of magnetisation of a substance may be defined as the flux density
produced in it due to its own induced magnetism.
If l is the magnetic length of the bar, then the product (m × l) is known as its magnetic moment M.
I= m/A
= m×l / A×l
= m/V
= Magnetic Moment / Volume
 |
| Magnetic lines |
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
= µ0 µr H Wb/m2 ...(i)
When H is established in air (or vacuum), then corresponding flux density developed in air is
B0 = µ0 H
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 m Wb 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
Hence, total flux density in the iron rod consists of two parts
(i) B0 –flux density in air even when rod is not present
(ii) Bi–induction flux density in the rod
B = B0 + Bi
= µ0 H + m/A
Eq. (i) above may be written as
B = µr . µ0 H
= µr B0
µr =B/B0
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 magnetising force.
Flux Density (B)
It is given by the flux passing per unit area through a plane at right angles to the flux. It is usually designated by the capital letter B and is measured in weber/meter2 . It is a Vector Quantity. It ΦWb is the total magnetic flux passing normally through an area of A m^ 2
, then
B = Φ/A Wb/m^2 or tesla (T)
Intensity of Magnetisation (I)
It may be defined as the induced pole strength developed per unit area of the bar. Also, it is the magnetic moment developed per unit volume of the bar.
Let
m = pole strength induced in the bar in Wb
A = face or pole area of the bar in m^2
Then
I = m/A Wb/m^2
Hence, it is seen that intensity of magnetisation of a substance may be defined as the flux density
produced in it due to its own induced magnetism.
If l is the magnetic length of the bar, then the product (m × l) is known as its magnetic moment M.
I= m/A
= m×l / A×l
= m/V
= Magnetic Moment / Volume
0 comments: