Magnetic Field

A magnetic field is a region in which a magnetic object, placed within the influence of the field, experiences a magnetic force.



A pattern of this directional force can be obtained by performing an experiment with iron filings. A piece of glass is placed over a bar magnet and the iron filings are then sprinkled on the surface of the glass. The magnetizing force of the magnet will be felt through the glass and each iron filing becomes a temporary magnet.

If the glass is now tapped gently, the iron particles will align themselves with the magnetic field surrounding the magnet just as the compass needle did previously. The filings form a definite pattern known as the magnetic field pattern, which is a visible representation of the forces comprising the magnetic field. The magnetic field is very strong at the poles and weakens as the distance from the poles increases. It is also apparent that the magnetic field extends from one pole to the other, constituting a loop about the magnet.


Magnetic field lines between two magnets

Attraction

When two magnets or magnetic objects are close to each other, there is a force that attracts the poles together.



Magnets also strongly attract ferromagnetic materials such as iron, nickel and cobalt.

Repulsion

When two magnetic objects have like poles facing each other, the magnetic force pushes them apart.



Magnets can also weakly repel diamagnetic materials.


Temporary and Permanent magnets

• Permanent magnets — are able to retain their magnetism for long periods. They can be found around us as fridge magnets, bar magnets or button magnets used in games, or lodestones (natural magnets).

• Temporary magnets — are sometimes called induced magnets. They refer to magnetic materials that have been placed within a strong magnetic field and become magnets. These magnets lose their magnetism once they are removed from the magnetic field. Temporary magnets can be found in telephones, electric motors, and cranes at refuse dumps.

• Materials that are more easily magnetised tend to lose their magnetism more quickly. They are referred to as ‘soft’ magnetic materials. Examples include iron and alloys like MumetalTM (a nickel–iron alloy). ‘Hard’ magnetic materials, on the other hand, are much less easily magnetised, but they retain their magnetism for a longer time, e.g. steel.

• The Earth behaves like a giant magnet. Just like any magnet, it has two magnetic poles — North and South. These poles are not the same as the geographic North and South Poles that we see on world maps. The north-pole of a freely suspended bar magnet, such as that in a compass, points to the Earth’s magnetic North, which is near to its geographic North.

Magnetisation and Demagnetisation

Theory of Magnetism and Magnetic Domains

A popular theory of magnetism considers the molecular alignment of the material. This is known as Weber's theory. This theory assumes that all magnetic substances are composed of tiny molecular magnets.



Any unmagnetized material has the magnetic forces of its molecular magnets neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction, and the south pole faces the opposite direction. A material with its molecules thus aligned will then have one effective north pole, and one effective south pole.

An illustration of Weber's Theory is shown in figure 1-11, where a steel bar is magnetized by stroking. When a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.

Ways of Making magnets

1. ‘Stroke’ method

A piece of magnetic material can be turned into a magnet if it is stroked by a magnet. As the magnet moves along the magnetic material, it causes the magnetic dipoles in the magnetic material to become aligned in one direction and give rise to a magnetic field.

2. Electrical method using a direct current

When a large direct current is passed through the solenoid, the unmagnetised steel bar will become magnetized after a while. This is because when an electric current flows through the solenoid, it produces a strong magnetic field which magnetizes the steel bar.

The poles of the magnet can be determind by a simple method known as Right-hand grip rule.


Ways of demagnetizing magnets

1. Heating

Heating a piece of magnetized metal in a flame will cause demagnetization by destroying the long-range order of molecules within the magnet. By heating a magnet, each molecule is infused with energy. This forces it to move, pushing each molecule out of order within the magnet and leaving the piece of metal with very little or no magnetization.

2. Hammering

When a magnet is hammered or dropped, the vibrations caused by the impact on the magnet randomize the magnetic molecules within the magnet, forcing them out of order and destroying the long-range order of the unit magnet.

3. Alternating Current (AC) Field

Using an AC current produces a magnetic field which can be moved and reduced to demagnetize materials. The field created by the AC current drags the magnetic molecules of the magnet in different directions. When the AC current is altered or reduced, the molecules within the magnet do not all return to previous positions, causing randomization of the molecules and reducing the force of the magnet.

Magnetism

Magnets and magnetic

Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Some are attracted to a magnetic field (paramagnetism); others are repulsed by a magnetic field (diamagnetism); others have a much more complex relationship with an applied magnetic field. Substances that are negligibly affected by magnetic fields are known as non-magnetic substances. They include copper, aluminium, gases, and plastic.

Magnets attract (never repel) the , ferromagnets, "magnetic metals".
Ferromagnetic materials have the highest magnetic susceptibilities.Examples of ferromagnetic materials are Iron, Nickel, Cobalt, hematite, magnetite and ionized gases (such as the material stars are made of). Magnets also attract or repel other permanent magnets,depending on which way they are facing each other. Permanent magnets usually have some iron in them.

There is a kind of dark-gray brittle ceramic called "ferrite" (pronounced like
"fair-right"), which has iron, oxygen, and some other metals with oxygen.
Ferrite can be magnetic too, because of the iron in it.


Properties of magnets

1.Magnetic Poles



The magnetic force surrounding a magnet is not uniform. There exists a great concentration of force at each end of the magnet and a very weak force at the center.

Proof of this fact can be obtained by dipping a magnet into iron filings (fig. 1-8). It is found that many filings will cling to the ends of the magnet while very few adhere to the center.

The two ends, which are the regions of concentrated lines of force, are called the POLES of the magnet. Magnets have two magnetic poles and both poles have equal magnetic strength. The poles are where the magnetic effects are the strongest.

2.North and South Poles



If a bar magnet is suspended freely on a string, it will align itself in a north and south direction. When this experiment is repeated, it is found that the same pole of the magnet will always swing toward the north magnetic pole of the earth. Therefore, it is called the north-seeking pole or simply the NORTH POLE. The other pole of the magnet is the south-seeking pole or the SOUTH POLE.

A practical use of the directional characteristic of the magnet is the compass, a device in which a freely rotating magnetized needle indicator points toward the North Pole.

3.Laws of magnetic poles

The realization that the poles of a suspended magnet always move to a definite position gives an indication that the opposite poles of a magnet have opposite magnetic polarity.The law previously stated regarding the attraction and repulsion of charged bodies may also be applied to magnetism if the pole is considered as a charge.

The north pole of a magnet will always be attracted to the south pole of another magnet and will show a repulsion to a north pole. The law of magnetism is:

Like poles repel, unlike poles attract.


How is magnet identified?

If an object attracts another object, it cannot be concluded to be a magnet as it may either be a magnetic material (not a magnet) that is attracted by the suspended magnet or a magnet itself with the opposite pole on the approaching end. To be certain, we would have to test the other end with the N pole of the suspended magnet to see if repulsion occurs.