Skip to main content

Magnetism and electromagnetism

In this chapter you will learn:

  1. Magnetism

  2. Types of magnet

  3. Electromagnetism and Solenoid

  4. The motor effect

  5. Fleming’s left hand rule

  6. Electric motors and Loud speakers

  7. The generator effect

  8. Transformers

Magnetism

All magnets create a magnetic field.

All magnets have two poles - north pole and a south pole.

Magnetic materials are iron, steel, nickel and cobalt.

There are two types of magnet- permanent and induced magnets.

Permanent magnet

Permanent magnet produces its own magnetic field.

Permanent magnet produce their magnetic fields all the time. It cannot be turned on and off.

Bar magnets and horseshoe magnets are examples of permanent magnets.

Permanent magnet is used in loudspeakers.

Induced magnet

An induced magnet is a material that turns in to a magnet when it is placed in a magnetic field.

Induced magnetism always causes a force of attraction.

An induced magnet loses all of its magnetism quickly when removed from the magnetic field

Magnetic forces are non-contact forces this means The force between a magnet and a magnetic material is always one of attraction.

The strength of the magnetic field depends on the distance from the magnet. The magnetic field is strongest at the poles of the magnet.

The closer the lines of force are together, the stronger the magnetic field. The further away from a magnet, you get the weaker magnetic field.

Electromagnetism

When a current flows through a conducting wire a magnetic field is produced around the wire.

The strength of the magnetic field depends on the current and the distance from the wire.

The strength of the magnetic field is increased by increasing the current.

Solenoid

A solenoid is a coil of wire. Wrapping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.

The magnetic field inside a solenoid is very strong and uniform.

Outside the coil, the magnetic field is weak just like a bar magnet.

Electromagnet

An electromagnet is a solenoid with an iron core. An electromagnet is used in devices such as door locks and electric bells that can be controlled remotely.

An electromagnet is used in some cranes for picking up scrap iron or steel in a recycling yard. The 'magnetic' crane can pick up these items and dump them down.

The motor effect

When a current-carrying wire in a magnetic field experiences a force, this is called motor effect.

The following equation can be used to calculate the force on a wire carrying a current at right angles to a magnetic field.

force = magnetic flux density × current × length

The units used in the equation above are as follows:

  • force is measured in newtons, N

  • magnetic flux density is measured in tesla, T

  • current is measured in ampers, A

  • length is measured in metres, m

Question

A wire has a current of 2.5A flowing through it as it passes through a magnetic field of 0.6T. What force will the 0.35m wire experience?

Answer:

force = magnetic flux density × current × length

F = B I L

force = 0.6 x 2.5 x 0.35

force = 0.525 N

The force always acts at right angles to the magnetic field of the magnet and the direction of the current in the wire.

The wire should be at 90 degrees to the direction of the magnetic field for getting the maximum force.

There is no motor effect force if the wire runs parallel to the magnetic field.

The magnitude of the force increases with the increase in current flow and the strength of the magnetic field.

The size of the force acting on the conductor depends on three things: the magnetic flux density, the current in the conductor and the length of the conductor in the magnetic field.

Fleming’s left hand rule

The direction of a motor effect force can be found using Fleming’s left hand rule.

  • if you reverse the direction of the current, you reverse the direction of the force.

  • if you reverse the direction of the magnetic field, you also reverse the direction of the force.

Electric motors and Loud speakers

A coil of wire carrying a current in a magnetic field tends to rotate.This is the basis of an electric motor.

You can reverse the direction of the motor rotation either by swapping the polarity of the d.c. supply to change the direction of current flow or swapping the magnetic poles of the permanent magnet to change the direction of the magnetic field.

Loud speaker works using the motor effect.

A loudspeaker convert electrical energy into sound energy by way of a vibrating cone.

An a.c. current is sent through a coil of insulated copper wire attached to the base of a paper-cardboard cone.

Therefore the variation in the current signal makes the cone vibrate, which creates pressure variations in the air and forms sound waves.

Microphones produce current from sound waves.

The generator effect

If there is a change in the magnetic field relative to the position of a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.

Electricity is generated using the generator effect.

If you want to change the size of the induced potential difference you must change the rate at which the magnetic field changes.

An induced potential difference or induced current can increased by increasing the speed of movement or increasing the strength of the magnetic field.

A generator consists of a coil of wire rotating in a magnetic field.

A.C. Generators use slip rings and brushes so that the contacts don't swap every half turn. Due to this method of connection they are able to produce alternating current.

Alternators produce alternating current.

Dynamos produce direct current.

Dynamos have a spilt-ring commutator. In this method of connection the contacts swap every half turn, producing direct current.

Transformers

Transformers change the potential difference of an alternating current.

Transformer is made from two coils of wire, a primary coil and a secondary coil.

The wire is usually made of copper with a thin coating of insulation material.

A Step-Down Transformer changes a high-voltage supply into a low-voltage.

The ratio of the potential differences across the input primary coil and the output secondary coil of a of a transformer, Vp and Vs, depends on the ratio of the number of turns on each coil, np and ns.

This equation can be used to calculate the output potential difference from a transformer.

The units used in the equation above are as follows:

Vp is the potential difference in the primary (input) coil in volts (V)

Vs is the potential difference in the secondary (output) coil in volts (V)

np is the number of turns on the primary coil

ns is the number of turn on the secondary coil

Question

A transformer has 100 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 420 V

Answer:

Vp / Vs = Np / Ns

420 ÷ Vs = 100 ÷ 50

Vs = 420 ÷ 2

Vs = 210 V

The National Grid system uses transformers to efficiently deliver electricity to all parts of a country.

Comments