Physics Important Question: Magnetism

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V.S. A. (1 Mark)

Q1. What is the magnetic moment of the electron due to its orbital motion?

Where e = charge on the electron

v = velocity of the electron



r = radius of the orbit of the electron

Q2. Which magnetic materials have i) relative permeability > 1 ii) relative permeability < 1?

  • A paramagnetic material has a constant relative permeability slightly more than 1.
  • A ferromagnetic material has a variable relative permeability larger than 1.  A ferromagnetic material, such as iron does not have a constant relative permeability. As the magnetizing field increases, the relative permeability increases reaches a maximum and then decreases.
  • A diamagnetic material has a constant relative permeability slightly less than 1.

Q3. Why do magnetic lines of force prefer to pass through iron than air?

  • Air is a diamagnetic material which is weakly repelled by an external magnetic field. While iron is a ferromagnetic material and strongly attracted by the external magnetic field
  • Thus iron allows magnetic lines to pass through them while air weakly repels them. Hence magnetic lines of force prefer to pass through iron than air

Q4. Are the i) diamagnetic materials ii) paramagnetic materials iii) ferromagnetic materials attracted or repelled by the magnet?



  • Diamagnetic materials are weakly repelled by an external magnetic field (magnet).
  • Paramagnetic materials are weakly attracted by an external magnetic field (magnet).
  • Ferromagnetic materials are strongly attracted by the external magnetic field (magnet).

Q5. Write the relation between relative permeability and magnetic susceptibility.

  • The quantity (1 + χ  )  is called relative magnetic permeability and is denoted by μr. It is a dimensionless quantity

∴ μ(1 + χ  )

  • μis called relative magnetic permeability and χ is the susceptibility of the substance.

Q6. Can there be a material which is non-magnetic?

 

Q7. Name two materials which have i) positive susceptibility ii) negative susceptibility.



  • For diamagnetic substances magnetic susceptibility is negative. e.g. Copper, gold
  • For paramagnetic substances, magnetic susceptibility is positive and small. e.g. Aluminium, platinum
  • For ferromagnetic substances, magnetic susceptibility is positive and large. e.g. iron, cobalt

Q8. The relative permeability of a material is i) 0.999 ii) 1.001. Identify the material.

  • If relative permeability of medium is 0.999 (slightly less than 1), it is a diamagnetic substance.
  • If relative permeability of medium is 1.001 (slightly greater than 1), it is a paramagnetic substance.

Q9. Is the magnetic susceptibility dimensionless quantity? Why?

 

Q10. What is a Curie temperature?

  • It is the temperature required to destroy the alignment of domains and to make a ferromagnetic substance demagnetised.
  • Above Curie temperature. a ferromagnetic substance behaves as paramagnetic.

Q11. What is a ferromagnetic substance?



  • Those substances which are strongly magnetised in an external magnetic field in the same direction as the external applied field and retain its magnetic moment even after the removal of external field are Called Ferromagnetic substances.

Q12. What is the gyromagnetic ratio?

  • The ratio of magnetic dipole moment to the angular momentum of revolving electron is constant and is called as the gyromagnetic ratio.

Origin of Magnetism 09

  • Gyromagnetic ratio is also called the magnetogyric ratio.

Q. What is the effect of temperature on the magnetization of paramagnetic material?

  • By Curie’s law of magnetization, The magnetization of a paramagnetic material is directly proportional to the external magnetic field and inversely proportional to the absolute temperature.

S.A.I (2 Marks)

Q1. Explain an analogy between magnetic dipole moment M of a circular current loop and electric dipole moment P of an electric dipole.

Origin of Magnetism



  • The magnetic induction at a point on the axis at a distance of ‘x’ from the centre of a current carrying circular coil of radius ‘a’ is given by

Origin of Magnetism 02

It is directed along the axis of the coil and away from it and perpendicular to the plane of the coil.

For x >>a, we can neglect a² from denominator in the expression.

Origin of Magnetism 03



This is an expression for magnetic induction due to current carrying loop

Now, the electric intensity due to an electric dipole on its axis is given by

Origin of Magnetism 04

  • From the two equations (3) and (4) we can say that the magnetic dipole moment is analogous to electrostatic dipole moment P and the magnetic field is analogous to the electric field. Thus the planar current loop is analogous to a magnetic dipole. i.e. current carrying loop produces a magnetic field and behaves like a magnetic dipole.

Q2. Define magnetization. Obtain relation between magnetization, external magnetic field and absolute temperature for paramagnetic material.

Magnetization:

  • The net magnetic dipole moment per unit volume is called the magnetization of a sample.
  • By definition, magnetization.

Origin of Magnetism 12



Curie’s Law of Magnetization:

  • The magnetization of a paramagnetic sample is directly proportional to the external magnetic field and inversely proportional to the absolute temperature.

Origin of Magnetism Magnetization

  • This is the relation between magnetization, external magnetic field and absolute temperature for paramagnetic material.

Q3. Distinguisn between diamagnetic and paramagnetic substances.

Diamagnetic Substance Paramagnetic Substance
The magnetic moment of every atom of diamagnetic substance is zero. Every atom is a magnetic dipole having a resultant magnetic moment.
They are weakly repelled by an external magnetic field. They are weakly attracted by an external magnetic field.
When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field. When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field In an external magnetic field, they get weakly magnetized in the same direction to that of the field
When a rod of diamagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length perpendicular to the directions Of the field. When a rod of a paramagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.
For diamagnetic substances magnetic susceptibility is negative. For paramagnetic substances, magnetic susceptibility is positive and small.
In absence of an external magnetic field, the net magnetic moment of diamagnetic substance is zero. In absence of an external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero.
If a watch glass containing a small quantity of diamagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows a depression in the middle. If a watch glass containing a small quantity of paramagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows an elevation in the middle.
If a magnetic field is applied to a diamagnetic liquid in one arm of U-tube, the liquid level in that arm is lowered. If a magnetic field is applied to the paramagnetic liquid in one arm of U-tube, the liquid level in that arm rises.
If diamagnetic gas is introduced between pole pieces of magnet, it spreads right angle to the magnetic field. If paramagnetic gas is introduced between pole pieces of magnet, it spreads in the direction of the magnetic field.

Q4. Write any four properties of a paramagnetic substance.



  • Every atom is a magnetic dipole having a resultant magnetic moment.
  • They are weakly attracted by an external magnetic field.
  • When placed in a non-uniform magnetic field, they tend to move from the weaker to the stronger part of the field.
  • In an external magnetic field, they get weakly magnetized in the same direction to that of the field
  • When a rod of a paramagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length parallel to the directions of the field.
  • In absence of external magnetic field, the magnetic moments of atomic magnets are randomly arranged, hence the net magnetic moment of the paramagnetic substance is zero.
  • On removal of the external magnetic field, paramagnetic substances lose their magnetism.
  • If a watch glass containing a small quantity of paramagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows an elevation in the middle.
  • If a magnetic field is applied to the paramagnetic liquid in one arm of U-tube, the liquid level in that arm rises.
  • If paramagnetic gas is introduced between pole pieces of magnet, it spreads in the direction of the magnetic field.
  • For paramagnetic substances, magnetic susceptibility is positive and small.
  • The susceptibility decreases with increase in the temperature.

Q5. What are domains in a ferromagnetic material? What is the effect of temperature on the domain structure of ferromagnetic material?

  • According to the domain theory, a ferromagnetic substance consists of a large number of small units (regions) known as Domains.  A domain ‘is an extremely small region containing a large number of atomic magnets having magnetic axes aligned in the same direction due to a strong exchange coupling.
  • In one domain the magnetic dipole moments of all the atoms are aligned in the same direction. Hence each domain has a resultant magnetic dipole moment.

Effect of Temperature:

  • Curie temperature is the temperature required to destroy the alignment of domains and to make a ferromagnetic substance demagnetized.
  • When a ferromagnetic substance is heated, the exchange coupling between neighbouring atoms becomes loose and ultimately the domain structure gets vanished.
  • If the heating is continued then at Curie temperature, the exchange coupling disappears and the domain structure is destroyed and hence the substance becomes paramagnetic.

Q6. Write any four properties of a diamagnetic substance.

  • The magnetic moment of every atom is zero.
  • They are weakly repelled by the external magnetic field.
  • When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field.
  • In an external magnetic field, they get weakly magnetized in the direction opposite to that of the field
  • When a rod of diamagnetic substance is suspended in a uniform magnetic field, it comes to rest with its length perpendicular to the directions Of the field.
  • For diamagnetic substances magnetic susceptibility is negative.
  • In absence of an external magnetic field, the net magnetic moment of diamagnetic substance is zero.
  • On removal of the external magnetic field, diamagnetic substances lose their magnetism.
  • If a watch glass containing a small quantity of diamagnetic liquid is placed on two dissimilar magnetic poles, the liquid shows a depression in the middle.
  • If a magnetic field is applied to a diamagnetic liquid in one arm of U-tube, the liquid level in that arm is lowered.
  • If diamagnetic gas is introduced between pole pieces of magnet, it spreads right angle to the magnetic field.

S.A. II (3 Marks)

Q1. Explain magnetization of ferromagnetic material with the help of toroid. Hence obtain the relation between relative permeability and magnetic susceptibility.

  • The magnetization of a ferromagnetic material such as iron can be studied with an arrangement called Toroid with an iron core.

Origin of Magnetism 11



  • Let the toroid have ‘n’ number of turns per unit length and ‘I’ be the current through it. The magnitude of magnetic field inside the coil when iron core is not present is given by

B0 = μn I

When an iron core is present in the toroid, the magnetic field increases, which is given by

B = B0  +  BM    ……… (1)

Where BM is magnetic field contributed by the iron core.

  • It is found that BM is directly proportional to the magnetization of iron and is given by

BM = μMz        ……. (2)

  • The strength of the magnetic field at a point can be given in terms of vector quantity called magnetic intensity (H).

Thus  B0 = μH  ……… (3)

Where, H = nI. Unit of magnetic intensity is A/m and its dimensions are [L-1M0T0I1].

Substituting values of equations (2) and (3) in (1)

B = μH + μM

B = μ(H + M)  ………….. (4)

Magnetization can be expressed in terms of magnetic intensity as

Mz   = χ H

Where χ (chi) is called the magnetic susceptibility.

Substituting in equation (4)

B = μ(H + χ H )

∴   B = μ(1 + χ  ) H

The quantity (1 + χ  )  is called relative magnetic permeability and is denoted by μr. It is a dimensionless quantity

∴   B = μμr H  = μ H

  • μis called relative magnetic permeability of the substance. μ is called absolute magnetic permeability of the substance. μis called the magnetic permeability of free space.

Q2. Explain ferromagnetism on the basis of domain theory.

  • Ferromagnetism is a special case of Paramagnetism.  In ferromagnetic substances, to the magnetic dipole moment of atoms, the contribution of the spin magnetic moment is very large.
  • According to the domain theory, a ferromagnetic substance consists of a large number of small units (regions) known as Domains.  A domain ‘is an extremely small region containing a large number of atomic magnets having magnetic axes aligned in the same direction due to a strong exchange coupling. When a ferromagnetic substance is kept in the magnetic field, the permanent alignment of the domain due to a strong interaction (force) takes place this force is known as exchange coupling. In one domain the magnetic dipole moments of all the atoms are aligned in the same direction. Hence each domain has a resultant magnetic dipole moment.  This permanent alignment is due to a strong interaction (force) known as exchange coupling.

Ferromagnetic Substances

  • However, in the absence of an external magnetic field, various domains have random orientations and hence their resultant magnetic moment is zero.
  • When a ferromagnetic substance is subjected to an external magnetic field, each domain experience a torque.  As a result of this, some domains rapidly rotates and remains aligned parallel to the direction of the field. This is called as domain rotation or flipping.
  • At the same time, those domains whose magnetic axes are nearly in line with the external magnetic field grow in size at the cost of the neighbouring domains.  This is called domain growth.
  • As the strength of the external magnetic field is increased, more and more domains flip and align in the direction of the external magnetic field.  Finally, at a certain stage, practically all domains get aligned in the direction of the field.  This is known as magnetic saturation.  At this stage, a ferromagnetic substance behaves like a permanent magnet and retains its magnetic property (residual magnetism) even if the external magnetic field is removed.

Q3. What is diamagnetism? Why the diamagnetic substance remains perpendicular when it is suspended freely in a uniform magnetic field? What is the Meissner effect?

  • Those substances which are weekly magnetized when placed in an external magnetic field, in a direction opposite to the applied field are called diamagnetic substances. The magnetism exhibited by these substances is called diamagnetism.
  • They are weakly repelled by the external magnetic field. When placed in a non-uniform magnetic field, they tend to move from the stronger to the weaker part of the field. This repulsion is most experienced nearer to the poles of the substance. So it aligns itself perpendicular to the field, where it faces minimum repulsion.

Diamagnetism 01

  • Meissner effect: The most exotic diamagnetic materials are superconductors. These are metals cooled to a very low temperature which exhibits both perfect conductivity and perfect diamagnetism. The phenomenon of perfect diamagnetism in a superconductor is called the Meissner effect.

Q4. What is magnetization? State Curie’s law of magnetization? Under what condition you can apply Curie’s law for ferromagnetic materials?

Magnetization:

  • The net magnetic dipole moment per unit volume is called the magnetization of a sample.
  • It is denoted by Mz. It is a vector quantity. S.I. unit of magnetization is A/m and its dimensions are [AL-1].
  • By definition, magnetization.

Origin of Magnetism 12

Curie’s Law of Magnetization:

  • The magnetization of a paramagnetic sample is directly proportional to the external magnetic field and inversely proportional to the absolute temperature.

Permeability

Conditions For Application of Curie’s Law:

  • the law holds good for high temperatures and weak magnetic fields.
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One Comment

  1. Sarth pokale

    It is really usefull to everyone who is in 12th std so you are doing good work hod bless you

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