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Magnetization is a vector quantity that measures the density of a permanent or an induced dipole moment in any magnetic field. Magnetization is also known as magnetic polarization.
Growing up we have all played with magnets at some point or another. But have you ever wondered what causes these magnets to be magnetic or why certain materials are drawn towards these metals? And another point to be pondered upon is, why don’t all the materials possess their own magnetic field.
The magnetic behaviour of a magnet is marked by the alignment of the atoms within a substance. When a ferromagnetic substance is brought in contact with a strong external magnetic field, it experiences a torque in which the substance aligns itself in the direction of the magnetic field applied and thus gets strongly magnetized in the direction of the magnetic field. The materials can be classified very easily based on their magnetic properties with the help of magnetization.
Read More: Magnetic Pole
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Key Terms: Magnetization, Magnetic fields, Dipole moment, Magnetic dipole, Electron, Atoms, Torque
What causes magnetization?
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Magnetization is caused as a result of a magnetic moment, which in turn is a result of the movement of electrons in an atom or by the spin of electrons. The net magnetization occurs as the response of that material to its external magnetic field combined with the unbalanced magnetic dipole moment that was innate in that material due to the movement of the electrons inside of it. Magnetization helps us in identifying different materials according to their magnetic properties. It is a vector quantity.
Causes of Magnetization
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What is Magnetization and its Mathematical Formula:
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Magnetization is denoted by M and is the net magnetic moment of any given material per unit volume.
Mathematically, Magnetization can be defined as:
M=mnet/V
Where, mnet is the magnetic moment of the material and V is the volume.
The concept of magnetization can be further explained by taking Solenoid as an example.
Integral Path for Ampere’s Law
Let's consider a solenoid with n number of turns per unit length. The current passing through it can be denoted by I. The magnetic field inside the solenoid can be calculated by:
B0= μ0nI
Let’s fill the interior of the solenoid with a material that has non-zero magnetization. Now the magnetic field inside the solenoid is greater than what it was before. The net magnetic field inside the solenoid can be now calculated by:
B=B0+Bm
Where Bm = magnetic field of the core material
Here, Bm is proportional to the magnetization of the material M and it can be mathematically calculated as:
Bm=μ0M
Here, μ0 denotes the constant of permeability of a vacuum.
By introducing another vector field H, we can further this concept. The H represents the magnetic intensity of a given material. Magnetic intensity can be defined as a vector quantity that gives the strength of a magnetic field at any given point. We can calculate the magnetic intensity of a given material by:
H=B/μ0 –M
The total Magnetic Field of a given material can also be calculated as:
B=μ0(H+M)
Where H= magnetic field due to external factors; (eg. current in the solenoid)
M= magnetic field due to the nature of the core
The nature of the core in the above formula is dependent upon the external forces and can be calculated by:
M=χH
Here, χ is known as the magnetic susceptibility of the material, which gives the response of any magnetic material to an external field. It is a dimensionless quantity.
The magnetic susceptibility is different for different materials. It is small and positive for paramagnetic materials and small and negative for diamagnetic materials. It can be calculated as:
B=μ0(1+χ)H
=μ0μrH
=μH
μr= 1 + χ; it is a dimensionless quantity called the relative magnetic permeability of any material. The Magnetic permeability of a substance (depicted by μ) can be defined as:
μ = μ0μr = μ0(1+χ)
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Sample Questions
Ques. What is magnetic permeability? (1 mark)
Ans. Magnetic permeability is the ratio of magnitude of the total field that is inside of the material to the magnetic intensity of the magnetizing field. The S.I. unit of magnetic permeability is H/m.
Ques. What is magnetic susceptibility? (1 mark)
Ans. Magnetic susceptibility is the ratio of the intensity or the degree of magnetization to the applied magnetic field. It is a dimensionless proportionality constant that has no unit. 400.
Ques. A solenoid has a core of a material with relative permeability 400. The windings of the solenoid are insulated from the core and carry a current of 2A. If the number of turns is 1000 per metre, calculate (a) H, (b) M, (c) B and (d) the magnetising current Im . (3 marks)
Ans.(a) The field H is dependent of the material of the core, and is
H = nI = 1000 × 2.0 = 2 ×103 A/m.
(b) The magnetic field B is given by
B = µr µ0 H
= 400 × 4π ×10–7 (N/A2 ) × 2 × 103 (A/m)
= 1.0 T
(c) Magnetisation is given by
M = (B– µ0 H)/ µ0
= (µr µ0 H–µ0 H)/µ0 = (µr – 1)H = 399 × H
≅ 8 × 105 A/m
(d) The magnetising current IM is the additional current that needs to be passed through the windings of the solenoid in the absence of the core which would give a B value as in the presence of the core. Thus B = µr n (I + IM ). Using I = 2A, B = 1 T, we get IM = 794 A
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