Electromagnetism

Magnetic Effect of a current


Th­e basic idea behind an electromagnet is extremely simple:Pass an electrical current through a wire and something amazing happens: a magnetic field is produced! You can show this by passing current through an insulated wire near a compass, although iron filings will also do.

Turning the current on and off will cause the compasses to deflect. Shown below are four compasses near a wire that comes towards the camera.



Magnetic Field around a straight wire


The shape of the magnetic field around a current-carrying conductor is rather curious: it is circular. The strongest deflection of compasses happens nearest to the wire.

We can draw the field as a series of concentric circles around the wire as shown below. Notice that the circles are closer together nearer the wire: this indicates a stronger field.



Rather than awkwardly draw the field in some sort of 3D image, we normally draw them as if seeing the wire from above or below, as shown here:





The animation clearly shows that reversing the direction of the current reverses the direction of the magnetic field.

The magnetic field of a long, straight current-carrying wire is stronger:
(the field becomes stronger so we would draw more lines closer together)

1.increase the size of the current

2.when a larger current flows through the wire


Field Directions

The direction of the field lines shows how a compass would line up if placed at that point. We can use the right hand grip rule to remember the relationship between current and field around a wire:



Grab the wire with your right fist (as shown), thumb pointing up: this is the direction of conventional current ( + to - ).

Your curled up fingers show the direction of the field.

An alternative way of remembering this is called the corkscrew rule: when screwing in a corkscrew to a bottle of wine, the point indicates the direction of the current, the the turning the field direction.

N.B. This does not, of course, work with left-handed corkscrews!


Magnetic field pattern around a flat coil and solenoid

When an electric current flows in a wire it creates a magnetic field around the wire.
By winding the wire into a coil we can strengthen the magnetic field. Electromagnets are made from coils like this, Making an electromagnet stronger.



A strong field can be made by coiling the wire around a piece of soft iron.
This electromagnet is sometimes called a solenoid. The shape of the magnetic field is the same as a bar magnet.



We can make an electromagnet stronger by doing these things:

* wrapping the coil around an iron core

* adding more turns to the coil

* increasing the current flowing through the coil.




Force on Current-carrying conductors


Place a current-carrying conductor in an external magnetic field (perhaps between two bar magnets), and the fields will push against each other. This force is incredibly important: it is the basis for all electric motors!



Normally the external field is fixed in place, so the force is only seen to act on the wire: moving it up or down. However, remember that forces come in pairs - the permanent magnets will experience the force too.



In the image above, the wire is at 90° to the direction of the magnetic field (which is between the poles of a horseshoe magnet).

Current is flowing to the left; the magnetic field diagonally towards us. This results in an upwards force on the wire.


If we reverse the direction of the current, the magnetic field around the wire reverses (surprise!). This reverses the force, so the wire deflects down:



A rather neat result of this effect is that if we pass alternating current (ac) through the wire, the direction of the force will continuously change. In the UK, ac is at a frequency of 50 Hz, so the wire can be seen vibrating up and down. This effect is used in speakers!

Fleming's Left Hand Rule




We have three directions to worry about: current, field and force. At GCSE we make sure the wire is held at 90° to the direction of the field. This makes life easier: the force is either up or down.

Current: dc - this is always from + to -
Field: direction is always from N to S.
Force: depends on how the other two align.

The left hand rule can be used to show how these three are related in a motor.

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.