Magnetism
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MAGNETISM Magnetism: The branch of physics which deals with the properties of magnetic field produced by the permanent magnetic (bar magnet) is called magnetism.
Atomic Origin of the magnetism: Why some materials are magnetic and others are not? Similarly, what is the origin of magnetic field? These are very complex questions. However, a qualitative answer can be given in a fairly simple terms. A bar magnet as well as current cause magnetic fields and current are due to moving charges. We know that a current loop acts like a short magnet. A spinning electron considered as a charge sphere also behaves as a tiny bar magnet. The atoms that make up all matter contain moving electrons. Every electron in an atom can produce magnetic field in two ways.
Orbital motion of electron: Each electron orbiting the nucleus behaves like an atomic sized loop of current that produces a small magnetic field.
Spinning motion of electron: Each electron possess a spinning (rotating about an axis though it) motion which gives rise to magnetic field. As a result, the resultant (net) magnetic field produced by the electrons with an atom is due to the combined effect created by their orbital and spin motions. Since there are many electrons in as atom, their currents or spins may be so placed as to cancel the magnetic effects mutually or strength the effects of each other. An atom, in which there is resultant magnetic field, behaves like a tiny magnet and is called magnetic dipole. Hence magnetism is due to the spin and orbital motion of the electrons surrounding the nucleus and is thus a property of all the substances.
Magnetism by nucleus: The charged nucleus (proton) also spins about its axis and so produces a magnetic field. But this magnetic field is so weak that it can be neglected.
Classification of Magnetic Materials On the basis of their magnetic properties different materials are classified as:
Diamagnetic substance
Paramagnetic substance
Ferromagnetic substance
Diamagnetic Substance Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet.
Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. [Their orbital and spin magnetic moment add vector ally to become zero]. The atoms of such material however acquire induced dipole moments when they are placed in an external magnetic field.
Origin on the basis of electron spin orbital motion:
The resultant magnetic produced by orbital and spin motions of the electrons add up to zero. Substances with such atoms are called diamagnetic substances. For vast majority of materials, diamagnetic effects are exceedingly small. Copper is found to be diamagnetic.
Some important properties are: 1)When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown
2) In a non-uniform magnetic material, these substances move from stronger parts of the field to the weaker parts. For e.g. When diamagnetic liquid is put in a watch glass placed on the two pole pieces of an electromagnet and current is switched on the liquid accumulates on the sides. [Note on increasing the distance between the pole, the effect is reversed]
3) A diamagnetic liquid in a U shaped tube is depressed, when subjected to a magnetic field.
4) The lines of force do not prefer to pass through the specimen, since the ability of a material to permit the passage of magnetic lines of force through it is less.
5) The permeability of the substance, that is,
r
< 1.
6) The substance loses its magnetization as soon as the magnetizing field is removed.
7) Such specimen cannot be easily magnetized and so their susceptibility is negative.
Example: Bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen etc.
Paramagnetic Substance Paramagnetic substances are attracted by a magnet very feebly. In a sample of a paramagnetic material, the atomic dipole moments initially are randomly oriented in space. When an external field is applied, the dipoles rotate into alignment with field.The vector sum of the individual dipole moments are no longer zero.
Origin on the basis of electron spin orbital motion: The orbital and the spin axes of the electrons in an atom ate so oriented that their fields support each other and the atom behaves like a tiny magnet. Substances with such atoms are called paramagnetic substances. For vast majority of materials, paramagnetic effects are exceedingly small. Aluminum is found to be paramagnetic.
Some important properties are: The paramagnetic substance develops a weak magnetization in the direction of the field.
When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.
The lines of force prefer to pass through the material rather than air that is
r
> 1 that is their
permeability is greater than one.
As soon as the magnetizing field is removed the paramagnetic lose their magnetization.
In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
The magnetization of paramagnetic decreases with increase in temperature. This is because the thermal motion of the atoms tends to disturb the alignment of the dipoles.
Example:
Aluminum, platinum, chromium, manganese, copper sulphate, oxygen etc.,
Ferromagnetic Substance
Ferromagnetism, like paramagnetic, occurs in materials in which atoms have permanent magnetic dipole moments. The strong interaction between neighboring atomic dipole moments keeps them aligned even when the external magnetic field is removed.
Origin on the basis of electron spin orbital motion: It is a very special class of magnetic materials in which the atoms co-operate with one another in such a way as to show very strong magnetic effects. Iron, nickel, cobalt and their alloys are examples of ferromagnetic materials.
Some important properties are: These substances get strongly magnetized in the direction of field. The lines of force prefer to pass through the material rather than air that is
r
>1 that is their
permeability is greater than one.
In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
For ferromagnetic materials
r
is very large and so its susceptibility i.e., Xm is positive.
Ferromagnetic substances retain their magnetism even after the magnetizing field is removed.
The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418oF, which means above this temperature, iron is paramagnetic.
Example: Iron, cobalt, nickel and number of alloys.
Origin of magnetic domains:
A magnetic domain describes a region within a material which has uniform magnetization. This means that the individual moments of the atoms are aligned with one another. Magnetic domain structure is responsible for the magnetic behavior of ferromagnetic materials like iron. In most other materials, domains do not naturally exist. The materials have to be exposed to a magnetic field, which will cause the individual moments to try and align with the field, which will eventually nucleate domains. The regions separating magnetic domains are called domain walls where the magnetization rotates coherently from the direction in one domain to that in the next domain. Their volume is of 1015
m3
order.
Magnetization curve: Electromagnetism curve traced on a graph of magnetic induction versus magnetic field strength in an originally unmagnified specimen, as the magnetic field strength is increased from zero. Also known as magnetization curve.
Atomic Origin of the magnetism: Why some materials are magnetic and others are not? Similarly, what is the origin of magnetic field? These are very complex questions. However, a qualitative answer can be given in a fairly simple terms. A bar magnet as well as current cause magnetic fields and current are due to moving charges. We know that a current loop acts like a short magnet. A spinning electron considered as a charge sphere also behaves as a tiny bar magnet. The atoms that make up all matter contain moving electrons. Every electron in an atom can produce magnetic field in two ways.
Orbital motion of electron: Each electron orbiting the nucleus behaves like an atomic sized loop of current that produces a small magnetic field.
Spinning motion of electron: Each electron possess a spinning (rotating about an axis though it) motion which gives rise to magnetic field. As a result, the resultant (net) magnetic field produced by the electrons with an atom is due to the combined effect created by their orbital and spin motions. Since there are many electrons in as atom, their currents or spins may be so placed as to cancel the magnetic effects mutually or strength the effects of each other. An atom, in which there is resultant magnetic field, behaves like a tiny magnet and is called magnetic dipole. Hence magnetism is due to the spin and orbital motion of the electrons surrounding the nucleus and is thus a property of all the substances.
Magnetism by nucleus: The charged nucleus (proton) also spins about its axis and so produces a magnetic field. But this magnetic field is so weak that it can be neglected.
Classification of Magnetic Materials On the basis of their magnetic properties different materials are classified as:
Diamagnetic substance
Paramagnetic substance
Ferromagnetic substance
Diamagnetic Substance Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet.
Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. [Their orbital and spin magnetic moment add vector ally to become zero]. The atoms of such material however acquire induced dipole moments when they are placed in an external magnetic field.
Origin on the basis of electron spin orbital motion:
The resultant magnetic produced by orbital and spin motions of the electrons add up to zero. Substances with such atoms are called diamagnetic substances. For vast majority of materials, diamagnetic effects are exceedingly small. Copper is found to be diamagnetic.
Some important properties are: 1)When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown
2) In a non-uniform magnetic material, these substances move from stronger parts of the field to the weaker parts. For e.g. When diamagnetic liquid is put in a watch glass placed on the two pole pieces of an electromagnet and current is switched on the liquid accumulates on the sides. [Note on increasing the distance between the pole, the effect is reversed]
3) A diamagnetic liquid in a U shaped tube is depressed, when subjected to a magnetic field.
4) The lines of force do not prefer to pass through the specimen, since the ability of a material to permit the passage of magnetic lines of force through it is less.
5) The permeability of the substance, that is,
r
< 1.
6) The substance loses its magnetization as soon as the magnetizing field is removed.
7) Such specimen cannot be easily magnetized and so their susceptibility is negative.
Example: Bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen etc.
Paramagnetic Substance Paramagnetic substances are attracted by a magnet very feebly. In a sample of a paramagnetic material, the atomic dipole moments initially are randomly oriented in space. When an external field is applied, the dipoles rotate into alignment with field.The vector sum of the individual dipole moments are no longer zero.
Origin on the basis of electron spin orbital motion: The orbital and the spin axes of the electrons in an atom ate so oriented that their fields support each other and the atom behaves like a tiny magnet. Substances with such atoms are called paramagnetic substances. For vast majority of materials, paramagnetic effects are exceedingly small. Aluminum is found to be paramagnetic.
Some important properties are: The paramagnetic substance develops a weak magnetization in the direction of the field.
When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.
The lines of force prefer to pass through the material rather than air that is
r
> 1 that is their
permeability is greater than one.
As soon as the magnetizing field is removed the paramagnetic lose their magnetization.
In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
The magnetization of paramagnetic decreases with increase in temperature. This is because the thermal motion of the atoms tends to disturb the alignment of the dipoles.
Example:
Aluminum, platinum, chromium, manganese, copper sulphate, oxygen etc.,
Ferromagnetic Substance
Ferromagnetism, like paramagnetic, occurs in materials in which atoms have permanent magnetic dipole moments. The strong interaction between neighboring atomic dipole moments keeps them aligned even when the external magnetic field is removed.
Origin on the basis of electron spin orbital motion: It is a very special class of magnetic materials in which the atoms co-operate with one another in such a way as to show very strong magnetic effects. Iron, nickel, cobalt and their alloys are examples of ferromagnetic materials.
Some important properties are: These substances get strongly magnetized in the direction of field. The lines of force prefer to pass through the material rather than air that is
r
>1 that is their
permeability is greater than one.
In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
For ferromagnetic materials
r
is very large and so its susceptibility i.e., Xm is positive.
Ferromagnetic substances retain their magnetism even after the magnetizing field is removed.
The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418oF, which means above this temperature, iron is paramagnetic.
Example: Iron, cobalt, nickel and number of alloys.
Origin of magnetic domains:
A magnetic domain describes a region within a material which has uniform magnetization. This means that the individual moments of the atoms are aligned with one another. Magnetic domain structure is responsible for the magnetic behavior of ferromagnetic materials like iron. In most other materials, domains do not naturally exist. The materials have to be exposed to a magnetic field, which will cause the individual moments to try and align with the field, which will eventually nucleate domains. The regions separating magnetic domains are called domain walls where the magnetization rotates coherently from the direction in one domain to that in the next domain. Their volume is of 1015
m3
order.
Magnetization curve: Electromagnetism curve traced on a graph of magnetic induction versus magnetic field strength in an originally unmagnified specimen, as the magnetic field strength is increased from zero. Also known as magnetization curve.
