Electricity and magnetism 9: Magnetism - magnetic materials - temporary (induced) magnets and permanent magnets - uses

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 This page will help you answer questions such as ...

  What are the rules concerning magnetic poles?

  How can you make a magnet without electricity?

 What is magnetic induction?

 What is the difference between permanent and temporary magnets?

 What do we use magnets for? (excluding electromagnets)


Introduction to magnetism and magnetic fields

You might ask the question 'what is magnetism', the answer is the same for gravity, nobody is really quite sure and the answer will lie somewhere in some deep quantum theory, well above GCSE physics level and my little grey cells!

However, the causes and effects of magnetic fields are well understood and can be described by 'working models' e.g. little atomic magnets to do with electrons in atoms like iron, magnetic fields from compass plotting and accurate mathematical models from very simple equations to the extremely complex!

Only relatively few materials display magnetic properties - that is, they are naturally magnetic, or more usually, capable of being magnetised.

Typical magnetic materials are iron and its alloys like steel, alloys of nickel and cobalt are used to produce very strong permanent magnets.

Other common structural metals like aluminium, copper and lead are not magnetic.

Jewellery metals like gold, platinum and silver are also not magnetic.

Actually most materials are NOT magnetic.

A bar or rod of magnetic material can be shown to have two poles, one at each end - a north seeking pole (north) and a south seeking pole (south).

As with electric charges, when you bring poles together, one of two things can happen.

Either the poles attract or they repel each other.

In other words any two magnetic objects exert a force on each other - attraction or repulsion.

The rule of magnetic poles

The rule is quite simply unlike poles attract (N => <= S) and like poles repel (<= N N => or <= S S =>).

You can easily demonstrate these rules with two bar magnets (of known polarity) suspend on string and bringing the various ends together (as in diagram above).

=> <=   or   <= =>   or   <= =>

Previously magnetised materials will attract other magnetic materials whether they were or were not, already magnetised.

e.g. a magnet will always and readily attract an item of iron or steel.

In other words when a magnet is placed near a magnetic material the two objects always experience an attractive force - it doesn't matter which pole of the magnet is placed nearest the object - see induction later.

A magnetic force is a non-contact force, even if the objects eventually touch.

Electrical and gravitational forces are also non-contact forces.

All, already magnetised materials are surrounded by a magnetic field, which we will look at in detail next.

 


Magnetic fields

You can map out the magnet field around a bar magnet by using a small plotting compass.

represent compass plotting positions on the diagram below (N=>S).

The magnetic field lines should indicate that the 'magnetic flux' runs from the north pole to the south pole.

Inside the plotting compass a tiny bar magnet (compass needle) aligns itself so that its north pole is attracted to the south pole of the magnet it is near.

The same thing happens with a navigation compass, which aligns itself with the Earth's magnetic field.

Your plotting compass does same thing when well away from the magnet - which shows the Earth creates a magnetic field - because the Earth has an iron core and this swirling mass of mainly molten iron generates a magnetic field.

You carefully select a starting point close to a pole and move the plotting compass around, making a dot adjacent to each end of the compass needle.

Gradually a line a force can be drawn by joining up the points.

You just repeat the procedure from a different starting point by a pole.

The results of plotting a magnetic field around a bar magnet.

By convention, the lines of force run around from the north pole to the south pole.

The lines of force show which way a force would act on a north pole if it was placed at that point in the field - see how the compass needle aligns itself along the line of force.

The closer the lines of force are together, the stronger the magnetic field.

The lines of force are closer together nearest the poles - this means the strength of the magnetic field is strongest close to the poles.

The further you get away from a magnet, the weaker the magnetic field.

 

Diagram A shows the magnetic field around a bar magnet.

How to plot this was described above with the lines of force running from north to south.

Diagram B shows the magnetic field when two bar magnets are brought close together and attracted.

When a N-S or S-N attraction happens the lines of force join up from one magnet to another and the lines of force still run from north to south.

 

Diagrams D and E show when like poles are brought close together and repulsion takes place.

In this case the lines of force do NOT join together from one magnet to the other.

 


Magnetisation and making magnets - induced and permanent magnets

There are two types of magnet we need to consider.

Permanent magnets retain their magnetism and create their own magnetic field.

Induced magnets are magnetic materials that become magnetic when placed in a magnetic field and will then produce their own magnetic field.

For example when a permanent magnet is placed in contact with another magnetic material like iron, they will always be attracted to each other AND a north and south pole are induced in the other magnetic material.

An example of an induced magnet

e.g. If you place a permanent steel magnet in contact with a piece of iron, the piece of iron becomes magnetised and the induced poles match the S-N attraction.

In fact, the nearest pole on the iron to the magnet will always be the opposite of the pole on the permanent magnet, hence the attraction.

 

 

This magnetic pole induction produces some quite interesting effects!

e.g. a bar magnet will pick up a whole chain of iron nails or paper clips as each iron/steel item becomes an induced magnet and can so attract another one.

The magnetic field effect becomes weaker down the chain and eventually the weight of the chain becomes greater than the force of the magnetic field and either no more 'stick on' or the whole lot fall off.

 

You can use the principle of induction to make a permanent magnet from another permanent magnet.

All you have to do is stroke the un-magnetised steel bar with a permanent bar magnet.

You repeatedly stroke the steel bar with the magnet repeatedly in the same direction.

All the 'atomic iron magnets' all line up to produce a permanent magnet with its own independent magnetic field.

 


Uses of permanent magnets (See other page for uses of electromagnets)

Sticking things on the refrigerator!

 

Loudspeakers use permanent magnets.

Magnetic screwdrivers help hold the screw in position in hard to reach places.

Jewellery clasps can be made of permanent magnetic materials.

 


The Earth's magnetic field

 

The Earth's magnetic field.

A compass works by the alignment of its needle with the Earth's magnetic field.

The north pole is actually the south pole of the Earth's magnetic field.

Therefore the magnetic north pole of the compass points north to the south magnetic pole.

The magnetic north is a few degrees away from the true geographic north pole based on the Earth's spin axis.

The fact that a magnetic compass always aligns itself in a particular direction is proof that the Earth has a magnetic field.

 

The trusty compass to help you navigate the countryside with your OS map.

However, these days, many walkers are using GPS systems for navigation, but I still like a 'real map' and a 'real compass'!


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Electricity and magnetism revision notes index

1. Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I2R, E = Pt, E=IVt

2. Electrical circuits and how to draw them, circuit symbols, parallel circuits, series circuits explained

3. Ohm's Law, experimental investigations of resistance, I-V graphs, calculations V = IR, Q = It, E = QV

4. Circuit devices and how are they used? (e.g. thermistor and LDR), relevant graphs gcse physics revision

5. More on series and parallel circuits, circuit diagrams, measurements and calculations gcse physics revision

6. The 'National Grid' power supply, environmental issues, use of transformers gcse physics revision notes

7. Comparison of methods of generating electricity gcse physics revision notes (energy 6)

8. Static electricity and electric fields, uses and dangers of static electricity gcse physics revision notes

9. Magnetism - magnetic materials - temporary (induced) and permanent magnets - uses gcse physics revision

10. Electromagnetism, solenoid coils, uses of electromagnets  gcse physics revision notes

11. Motor effect of an electric current, electric motor, loudspeaker, Fleming's left-hand rule, F = BIL gcse physics

12. Generator effect, applications e.g. generators generating electricity and microphone gcse physics revision


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