Magnetism

Properties of Magnets.

A magnet has the property of attracting iron and steel (ferromagnetic materials).The attraction is greatest at the ends of the magnet. When the magnet is suspended so that it can rotate freely it always settles pointing approximately geometrically north and south.

The Earth's Magnetic Poles

The fact that a compass needle always aligns itself in a particular direction, regardless of its location on earth, indicates that the earth is a huge natural magnet. The distribution of the magnetic force about the earth is the same as that which might be produced by a giant bar magnet running through the center of the earth.The magnetic axis of the earth is located about 15° from its geographical axis thereby locating the magnetic poles some distance from the geographical poles.

The ability of the north pole of the compass needle to point toward the north geographical pole is due to the presence of the magnetic pole nearby. This magnetic pole is named the magnetic North Pole. However, in actuality, it must have the polarity of a south magnetic pole since it attracts the north pole of a compass needle. The reason for this conflict in terminology can be traced to the early users of the compass.

Knowing little about magnetic effects, they called the end of the compass needle that pointed towards the north geographical pole, the north pole of a compass.

With our present knowledge of magnetism, we know the north pole of a compass needle (a small bar magnet) can be attracted only by an unlike magnetic pole, that is, a pole of south magnetic polarity.

Experiments show that unlike poles attract and like poles repel (similar to electric charges)

Poles cannot exist independently (unlike electric charges).Every magnet has a north and south pole. If you break a magnet in half each half will still behave like a complete magnet with both north and south poles, no matter how many times you break it in half there will never be a single pole, even when your piece is one atom thick there are two poles.

Magnetic Fields.The space around a magnet in which it exerts a force is known as its magnetic field. The field reduces with distance from the magnet. A magnetic field may be mapped out by lines of force which show the direction of the magnetic force or the direction in which a single north pole would travel if it could exist independently.

Magnetic Flux Density.Defined as the total number of lines of force per unit area perpendicular to the area. The magnetic flux density is an indication of the strength of the magnetic field.Symbol : B S.I. Units : Tesla ( T )

Magnetic Field Lines Map of the magnetic fields.

Bar magnet

North Pole Facing South Pole

North pole facing North Pole

Theory of magnetism.

Magnetic materials have some magnetism before they are magnetised. It is thought that there are regions within the ferromagnetic materials which contain dipoles lined up in one direction. These domains can be made visible using high powered microscopes.

Unmagnetised piece of iron the direction of the magnetism of the domains is random.On magnetisation the domains become aligned

This gives free poles at the ends of the bar which will give rise to the poles of the magnet.

Making Magnets.1. A piece of steel may be magnetised by drawing a

magnet pole along it in one direction only.

2. Using an electric current.The bar to be magnetised is placed in a cylindrical coil of wire. A current is switched on and off when the bar is removed it is found to be magnetised.

3. Induced magnetism.When a piece of unmagnetised steel is placed near a permanent magnet it becomes magnetised. We say that magnetism is induced in the material.

Demagnetisation.1. Electrical Method. Using a solenoid with a.c. current. If

the material is slowly withdrawn it will become demagnetised.

Magnetic Properties.

1. Susceptibility. Ease of magnetisation of a material.2. Retentivity. Measure of the ability of a material to retain

its magnetism once it is magnetised.

Soft iron is easily magnetised but has no retentivity.Steel Not easily magnetised but high retentivity used for making permanent magnets.

Electromagnetism.

An electric current gives rise to a magnetic field.1. Current in a long straight wire. Using the Right Hand

Rule if the thumb is pointing along the direction of the current the direction of the fingers give the direction of the magnetic field.

2. Solenoid. When an electric current is passed through a solenoid the resultant magnetic flux is similar to that of a bar magnet.

Electromagnet. A solenoid is wrapped around a bar of soft iron bent into a U shape. Using soft iron it becomes magnetised when in a magnetic field and looses almost all of its magnetism when the filed is removed. Therefore switching on and off the

electric current effectively switches the magnet on and off. This is the principle of the electromagnet.Application : The electric Bell.

Faradays Law of Electromagnetic InductionWhen ever there is a change in the magnetic flux linked with a circuit an e.m.f. is induced, the strength of which is proportional to the rate of change of the flux linked in the circuit. The flux is a measure of the strength of the field.Application the Transformer. Uses two coils Primary coil and secondary coil wrapped on a single magnetic core.

Principle of Operation. If an a.c. current is passed through the primary coil an alternating magnetic flux will be set up through the iron which will induce and e.m.f. in the secondary coil.

The magnitude of the induced e.m.f. (output voltage) depends on 1. e.m.f. applied at the primary i.e. a.c. input voltage2. the number of turns on the primary coil3. the number of turns in the secondary coil.

It can be shown that

There are two types of transformers 1. Where the number of turns in the secondary is greater

than the number of turns in the primary and the voltage is increased these are called step-up transformers.

2. Where the number of turns in the secondary is less the number of turns in the primary and the voltage is decreased these are called step-down transformers.

Features of a transformer.

The power taken from a transformer can never be greater than the power supplied to it due to the conservation of energy. In practice the output power is always less than the input due to the fact that the transformer is not 100% efficient.

However assuming 100% efficiency

So therefore if the voltage is increased then the current is decreased.

If the transformer is not 100% efficient then some of its input energy is lost.

The energy (or power) may be lost in a number of ways1. Heat being produced in the windings or coils 2. The core is continually being magnetised and

demagnetised which requires some energy3. All the magnetic field is not linked

Questions.1. A transformer steps down the voltage from 220 V to 6.3

V in a radio. If there are 1320 turns in the primary coil how many turns are there in the secondary coil.

2. A step-up transformer is designed to operate from a 20V supply and deliver energy at a voltage of 250V. Determine the current flowing in the primary coil when the output terminals are connected to a 100W lamp, assuming that the transformer is 100% efficient.

3. If the transformer in the above question is only 90% efficient calculate the current flowing in the primary coils.

Let us now consider case where there is both an Electric and Magnetic Field present.

It is found the a current carrying conductor in a magnetic field experiences a Force. Recall that a force has both a magnitude and a direction. Consider the case where a charge q (coulombs) is moving with a velocity v (m.s-1) through a magnetic field of flux density B (Tesla).It can be shown that the magnitude of the Force acting is given by

Where the angle between the magnetic field lines and the direction of the motion of the charge.

The Force is a maximum value when the angle is 90o.

Direction : As the force is a vector quantity we must also give its direction. For the direction of the field we use Fleming’s Left Hand Rule. Place the forefinger, second finger and thumb of the left hand mutually at right angles to each other. If the forefinger points in the direction of the field, the second finger along the direction of the current then the thumb will point in the direction of the motion caused by the force therefore the direction of the force itself.

Also recall that

Questions.1. An electron moving at a speed of 5x107m.s-1 enters a magnetic field of 0.5T. Calculate the magnetic force on the electron if (i) the electron is moving at right angles of the field and (ii) if the angle between the field and the motion of the electron is 60o.2. A wire of length 2m has a current of 25mA flowing through it and is placed in a magnetic field of flux density 0.5T. Calculate the magnetic force on the wire if (i) the current is moving at right angles of the field and (ii) if the angle between the field and the current is 30o.