Domain theory of magnetism.

Domain theory of magnetism.

In ferromagnetic substances dipoles (small atomic magnets) form large groups called domains. These dipoles face one direction where the direction varies from one domain to another. In an un-magnetized material, dipoles in different domains face in different directions hence their resultant magnetism is zero.

When a magnetic material is placed in a magnetic field the dipoles get aligned and eventually all domains face the same direction. When this happens then the material becomes magnetized. When a material is fully magnetized we say it is saturated. This means that the magnetism of the material cannot be increased by any other method.

Hard and soft magnetic materials

Hard magnetic materials that that are difficult to magnetize but retains magnetism for long. Such materials are used to make permanent magnets. Steel is an example.

Soft magnetic materials are those that are easily magnetized but do not retain magnetism for long. Such materials are to make temporary magnets. An example is iron.

Characteristics of a PN Junction Diode

A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in one direction only. However, unlike a resistor, a diode is non ohmic; does not behave linearly with respect to the applied voltage as the diode has an exponential I-V relationship and therefore we cannot described its operation by simply using an equation such as Ohm's law.

If a suitable positive voltage (forward bias- positive terminal connected to the p-type side) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased. 

By applying a negative voltage (reverse bias- positive terminal connected to the n-type side) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction.

Junction Diode Symbol and Static I-V Characteristics

Before using a PN junction as a practical device or as a rectifying device we need to firstly bias the junction, i.e. connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases the potential barrier. As the potential barrier is increased, current is then blocked from flowing, only very little current (leakage current), at very large external voltage, passes through.

An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction. As the potential barrier, current can easily flow through the diode (the diode is more conductive). Current increases as voltage is increased however, not linearly (non-ohmic)

In summary;

The PN junction region of a Junction Diode has the following important characteristics:

1. A semiconductor can be doped with donor impurities (Pentavalent atoms) such as Antimony (N-type doping), so that it contains mobile and free electrons- the charge carries. 
2. A semiconductor may be doped with acceptor impurities (trivalent atoms)such as Boron (P-type doping), so that it contains holes- the charge carriers. 
3. Semiconductors contain two types of mobile charge carriers, Holes and Electrons. The holes are positively charged while the electrons negatively charged.
4. The p-n junction region has no charge carriers and is known as the depletion layer.
5. The junction (depletion) layer has a physical thickness that varies with the applied voltage. A reverse bias voltage widens it while a forward bias voltage decreases it.
6. When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer.
7. When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow. 
8. When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).

Magnetization and Dagnetization

Magnetization

Magnetization is the process of making magnets from magnetic materials. The following are methods used to make magnets.

1. Magnetizing by induction

This is a process by which magnets are made by placing ferromagnetic materials in a magnetic field. Materials like iron lose their magnetism easily and are said to be soft while others like steel gain magnetism slowly but retain it longer and are therefore said to be hard and are used to make permanent magnets.

2. Magnetizing by stroking

The object to be magnetized is placed on a bench then a bar magnet is dragged along the length of the bar from one end to the other. This is repeated several times and the object becomes magnetized. There are two types of stroking; single-stroke and double-stroke

Procedure:

1. The steel bar is stroked with the same pole of the permanent magnet from one end to the other end in one direction.
2. The stroking magnet has to be lifted sufficiently high above the steel bar between successive strokes.
3. The steel bar will become a magnet with pole produced at the end where the strokes finish is opposite to the stroking pole used as the atomic magnets in the domain are attracted to the stroking pole.

Note:

• When using two magnets, the stroking pole used in each magnet has to be opposite, and they stroke the steel bar in opposite direction.
• Using two magnets to stoke is faster than that using one magnet.
• Stroking method only produces weak magnets.

3. Magnetizing using an electric current

This is the use of magnetic effect of an electric current through a solenoid (insulated wire of many turns).

When direct current flows through a solenoid, a magnetic field is created. If a core made from a magnetic material is placed inside a solenoid, the core will become magnetised. A relatively small current can create a strong magnet.

A core made from a hard magnetic material will keep its magnetism after the current has been turned off.

The magnetic poles can be determined using the right hand grip rule:

·         Curl the fingers of your right hand in the direction of the conventional current in the solenoid and extend your thumb.

·         The thumb points to the north pole .

The strength of the electromagnet is affected by:

     i.        Amount of current- large current causes a stronger electromagnet

    ii.        Length of the solenoid: a longer solenoid produces a stronger electromagnet

  iii.        Type of the core

   iv.        Shape of the core: a u-shaped core causes a stronger electromagnet than a straight core

 Demagnetizing

Demagnetizing is the process of removing magnetic properties of a magnet. The following methods are which a magnet can lose its magnetism;

                     i.        Hammering them hard with their poles facing E-W direction

                    ii.        Heating them strongly

                  iii.        Placing a magnet inside a solenoid and passing an a.c. current through it for a short time.

Storing magnets

·         Magnets should be stored in pairs with unlike poles adjacent to each other attached to pieces of soft iron called keepers.

The keepers ensures that dipoles are arranged in closed loops hence maintaining their alignment. This therefore retains magnetism in the magnets.

• Magnets should not be hammered especially with their poles facing E-W direction
• Magnets should not be heated strongly or dropped roughly on hard surfaces.
•  Magnets should not be placed near alternating currents.
• Magnets should be kept dry and clean since rust can make them lose their magnetism.