Magnetism

Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms particularly its electrons. All magnetic phenomena result from forces between electric charges in motion

Magnetism makes a magnet:

• Attract and repel other magnets

• Attract iron, nickel, cobalt and some other substances

Magnets are in most electronic devices, in fact, anything that has a motor uses a magnet.

Magnetic Poles

The magnetic effect is greatest at the ends or poles of the magnet. Each magnet has at least two poles a north and a south pole. The north pole is usually red while the south pole is usually blue.

When two magnets are close, they create a pushing or pulling force on each other.

When two like poles interact, they repel each other.

When two unlike poles interact, they attract each other.

Magnetic Field

A magnetic field is the region or space around a magnet where the effects of the magnet can be experienced. The magnetic field lines describe a curved loop from the north pole to the south pole of the magnet. The concentration of field lines is greatest near the poles.

The density of field lines indicates the strength of the field.

Magnetic Flux

Field lines represent the magnetic flux , which measures the flow of magnetism around a magnet. Unit : Weber (Wb)

A strong magnet has a greater magnetic flux than a weaker magnet.

Magnetic flux density is the magnetic flux per unit area. It is the best way to measure the strength of a magnet. Unit : Tesla (T or Wb/m2)

Magnetic Forces

A force exists between magnets. This force acts at a distance (it is a non- contact force).

The force of attraction between magnets depend on

• The strength of the magnets

• On their distance apart → the force is inversely proportional to the square of the distance

The magnetic properties of materials

Permeability is a measure of the ease with which magnetic flux passes through a material.

Example: Iron is more permeable than steel. We say iron is magnetically soft. Steel is described as magnetically hard.

All this means is that it is easier to magnetize iron than it is to magnetize steel.

Steel retains magnetism better than iron. Steel is said to have higher magnetic retentivity. Iron is used where temporary magnetism is necessary. Steel is used in permanent magnets.

Electromagnetism

The magnetic effect of an electric current

When an electric current passes through a wire , a magnetic field is produced around the wire.

Magnetic field of current carrying wire

• The field lines make a pattern of concentric circles symmetrical around the conductor(wire) at the center.

• The pattern is densest nearest the wire.

• The magnetic field is strongest near the conductor but gets weaker with distance.

• The magnetic field is in a plane perpendicular to the direction of current through the conductor.

The direction of a field around a current carrying conductor can be found using the Right Hand Grip Rule. The rule is as follows:

If a conductor carrying current is gripped with the right hand , with the thumb pointing along the conductor in the direction of the conventional current , the curl of the fingers around the conductor indicates the direction of magnetic lines of force.

The magnetic effect of an electric current

Solenoids

A solenoid is an arrangement in which a wire is looped into a helix or a spiral. The solenoid concentrates magnetic field lines inside the coil. Opposite magnetic poles exist at the ends of the solenoid. The field inside the solenoid is uniform, the field outside the solenoid is like that of a bar magnet.

The force on a current carrying conductor in a magnetic field.

Remember that current has an associated magnetic field so when a current carrying conductor is placed in a magnetic field interaction between the two fields occur. This interaction results in a force on the current carrying conductor.

The force experienced by a current carrying conductor in a uniform magnetic field.

The force, F, on the current carrying conductor placed at right angles to the magnetic field is given by → F = BIl

In general, if the angle between the conductor and the magnetic field is θ, then the Force, F, is given by → F = BIl sinθ

The direction of the force is at right angles to both the conductor and the magnetic field (Fleming’s LH Rule).

When the field is parallel to the conductor, θ = 0^o, F = 0.

When the field is perpendicular to the conductor, θ = 90^o, F = BIl.

The direction of the force which results from the interaction can be predicted by Flemings left–hand rule. It is as follows: If the forefinger , second finger and thumb of the left hand are held at right angles to one another , and if the forefinger points in the direction of the field and the second finger in the direction of the current, then the thumb points in the direction of the force which is acting on the current carrying conductor.

Electromagnets and their applications

An electromagnet is a temporary magnet which has magnetism only when current is passing through a coil of wire. Usually a soft iron core wrapped with coil of wire.

The strength of the magnetic field associated with an electromagnet can be increased by:

• Increasing the current

• Increasing the number of turns of coil per unit length

• Including a soft iron core

Applications of Electromagnets:

• used to lift iron rods and steel bars and sheets in steel in steelworks, cranes in scrap yards

• Transportation – Maglev trains

• In hospitals , to remove iron and steel splinters from eyes

• They are used in many electrical devices like

  • electrical bells,

  • loudspeakers

  • magnetic locks

Electromagnetic Induction

Magnetic Induction

The process by which a magnet can induce a magnetic field in an unmagnetized iron bar or other material is called magnetization by induction.

The Domain Theory tries to explain why metals get magnetized. The magnetic elements (e.g. Iron) have little molecular magnets inside them. These are randomly orientated in an unmagnetized piece of metal but point in a particular direction in a magnetized piece.

A moving or changing magnetic field which cuts across a conductor will generate a movement of charge and induces an electro magnetic field or voltage between the ends of the conductor.

Relative Motion → The direction of the magnet determines the direction of the induced current.

Current is produced in a conductor when it is moved through a magnetic field because the magnetic field applies a force on the free electrons in the conductor and causes them to move.

Inductive Coupling

Inductive coupling can be defined as:

• The transfer of energy from one circuit component to another through a shared magnetic field

• A change in current flow through one device induces current flow in the other device.

Factors that affect the size of the induced electro magnetic field

• The strength of the magnetic field

• The number(N) of turns of coil per unit length of the solenoid.(greater N greater e.m.f)

• The speed with which the magnetic field lines cut the conductor.( greater speed greater e.m.f)

• Type of core e.g. a soft iron core will increase the e.m.f

Transformers

A transformer is a device for increasing or decreasing an alternate current voltage.

Structure of a Transformer

A transformer is a device composed of two unconnected coils, usually wrapped around a soft iron core, that can increase or decrease the voltage of ac current.

All transformers have three parts:

1. Primary coil – the incoming voltage V_p (voltage across primary coil) is connected across this coil.

2. Secondary coil – this provides the output voltage V_s (voltage across the secondary coil) to the external circuit.

3. Laminated iron core – this links the two coils magnetically.

Notice that there is no electrical connection between the two coils, which are constructed using insulated wire.

Transformers are either step voltage up or to step it down.

Principle of the Transformers

Energy can be efficiently transferred by electromagnetic induction from one winding to another winding.

This is done by a varying magnetic field produced by an alternating current . The magnetic field expands and contracts at each half cycle. An electrical voltage is induced when there is a relative motion between a wire and a magnetic field.

NB: Direct current (DC) is not transformed, as DC does not vary its magnetic fields.

Transformer Operation

• One of the windings is designated as the primary and the other winding as the secondary.

• Since the primary and secondary are wound the on the

  same iron core,

• when the primary winding is energized by an AC source, an alternating magnetic field called flux is established in the transformer core.

• The flux created by the applied voltage on the primary winding induces a voltage on the secondary winding.

Transformer equation → N_p / N_s = V_p / V_s = I_s / I_p. P denotes primary and s denotes secondary.

Transformer Losses

Eddy currents → are circulating electric currents within the iron core.

Copper losses- → heating effect of current in coils.

Hysteresis → is the name given to the reluctance of a material to undergo changes in magnetism. At each cycle the core reverses polarity of its magnetism which requires energy.

Magnetic flux leakage → some of the changing magnetic flux may not link with the secondary coil.

Conditions for Ideal Efficiency

• Coils are made of low resistance material e.g. Copper

• The core is laminated

• The core allows good flux linkage between primary and

  secondary

• The core is made of soft magnetic material.