202/3 – Know the principles of electro-magnetism

Outcome 3.1 – Flux patterns of electromagnets

Unit 202 Principles of electrical science

Preliminary understanding

Last session

3.1 Describe the magnetic flux patterns of electromagnets

Current carrying conductor, solenoid, magnetic poles, relays.

This session

When current flows in a conductor, a magnetic field is created around it.

Magnetic field around a current carrying conductor

Magnetic field around a current carrying conductor

Wood screw turning?

Magnetic field around a current carrying conductor

Magnetic field around a solenoid

There is only a magnetic field when current is flowing.

This is called an electro-magnet.

When it is used operate something it is called a solenoid.

SolenoidsA solenoid occurs when a number of strands of wire are formed into a series of turns with nothing in them but air.

If the turns of wire are wound around a core it is termed as a coil.

If each turn of the coil is a conductor, then increasing the number of turns increases strength of the magnetic field.

If an iron-core were added then the strength of the magnetic field would increase dramatically.

Magnetic field around a solenoid

Magnetic field around a solenoid

The Strength Of The Magnetic Field

The strength of the magnetic field is determined by the number of times the conductor is wound around the coil.

If the number of turns is increased then so effectively has the number of conductors. This has increased the magnetic field.

It is also dependent on the size of the current.

If an iron core was added instead of air then the magnetic field would increase dramatically.

Solenoid uses

Door entry systemsNo current when locked, energised to open.

Gas safety valveEnergised while open, closes when emergency stop is pressed or power fails.

Door bellEnergised when pressed, ding. Dong when released.

Coils

If we want the extra magnetism that comes from having the current passing in the same direction we use a coil.

This is an air filled coil with very few lines of magnetic flux

A coil former made out of copper has more lines of magnetic flux than one of air.

Coils

The former here is made out of iron and there are many more lines of magnetic flux.

Coils

Material obviously plays an important part, remember all materials have their own individual magnetic properties.

Increasing The Current

There is a direct link between the current that a conductor carries and the strength of the magnetic field surrounding it.

When we combine the number of turn in the coil and increase the current we are dealing with magnetomotive force (m.m.f)

The magnetomotive force is the force that causes a magnetic field to be built up or

established

m.m.f (F) = current (I) x number of turns (N)

F = IN

Magnetic field used in a relay

Draw this circuit

Inside a relay

Draw a 12v circuit that will turn a

230v light on and another 230v light off

class task

Draw a 24v relay circuit that will turn a 230v light on and operate another

24v relay.

(it will need 2 x 2way switches)

Individual task

consolidation

3.1 Describe the magnetic flux patterns of electromagnets

Current carrying conductorsolenoid, magnetic poles, relays.

3.2 Apply Flemings right hand rule to the operating principles of a simple alternator

3.3 Calculate magnitudes of a generated EMF

Next session