## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

#### KS3 3.2.4 Magnetism

In this section we explore the magnetism of permanent magnets, of the compass and of the Earth!

The Poles of a Magnet

The first "magnets" were pieces of found material known as lodestones and it was discovered that, when allowed to turn freely, they would always come to rest pointing in a north-south direction. So they were used for navigation.

Then in 1600 William Gilbert, in England, developed the process of making magnets from iron, doing away with the need to find the relatively rare lodestones. Furthermore, he performed experiments and concluded that the Earth itelf was magnetic and that this was why the lodestones or his iron magnets always lined up in the north-south direction.

Gilbert's work enabled the ends of the magnet to be named:
The end that always pointed in the direction of North was called the North seeking pole,
The end that always pointed in the direction of South was called the South seeking pole
because these were literally what they did!

The diagram below shows a magnet and its 2 poles seeking out (or pointing to) their named Earth poles.

For quickness we generally shorten the names of the poles from:
North seeking pole to just North Pole, and
South seeking pole to just South Pole.

But try to remember their full names so that you will always understand which way they point.

Magnet Poles and Magnetic Forces

When 2 magnets are brought close together, they exert a force on each other; the direction of the force depends on which poles are brought close together.

And:

We can now write a summary of this as the "Law of Magnetism":
"LIKE poles Repel
UNLIKE poles Attract."

You can see from the diagrams above that the force felt between magnets is an example of a non-contact force, like gravity or electrostatic forces.
(Pay attention to how the force arrows are placed on the magnets; make sure they start on the magnets and do not draw them such that they touch each other.)

Magnetic and Non-Magnetic Materials

Most materials that you can think of are not magnetic.
For example: paper, plastic, rubber, glass, wood, vinyl, wool, alumininium, brass, copper, silver, gold, bronze, cotton, nylon, perspex, silk, cellophane etc are all none magnetic materials. They will never have magnetic poles, they will never produce magnetic forces and they will never have a magnetic field.

There are only 4 materials that are magnetic. These are the metals: iron, steel, cobalt and nickel.

So, all non-metals are non-magnetic
and only 4 metals are magnetic.

A magnetic material is one that is attracted by a magnet.

A non-magnetic material is one that is not attracted by a magnet.

Before we leave this short section, make sure you get clear in your head that:
A magnet will always attract a magnetic material; it will never repel it!

You will only see a repelling force if you have two magnets with LIKE poles near each other.

Magnets

We mentioned at the start of this section that magnets can be made rather than found.
Not surprisingly, magnets can be made from one of, or a mixture of, the magnetic materials, iron, steel, cobalt or nickel.

Comparing iron and steel, for example.
If we chose to make a magnet from steel we would find that it would be hard to magnetise but the resulting magnet would retain its magnetism quite well (it would be hard to de-magnetise).
Iron, however would be easy to magnetise but the resulting magnet would lose its magnetism quickly (it would be easy to de-magnetise).

Of the two, steel would be the better choice for a permanent magnet.

Steel is known as a hard magnetic material because it is hard to magnetise but also hard to demagnetise.
Iron is the opposite - it is known as a soft magnetic material; it is easy to magnetise but also easy to demagnetise.

Notice - when a magnetic material has become a magnet we say that it has been magnetised.

Magnetic Fields and Field Strength

We already know that a magnetic material brought near a magnet will feel a force (eg it is attracted), so we define a magnetic field as:

The magnetic field around a magnet is invisible to our eyes but if we could see it, this is what it would look like:

We call the lines (shown in red) field lines and you can see that they have a direction shown by the arrows;
the direction is always - out from the North and into the South.

The strength of the magnet varies with distance from the magnet, so the closer a magnetic material is to the magnet the stronger the force it will feel
ie decrease distance, increase strength or size of force felt. Pretty obvious!

Another way to consider this is to look at the magnetic field strength and the spread of the magnetic field lines around the magnet.
What we find is - where the field lines are close together, at the Poles, the magnetic field strength is greatest, and where the field lines are spaced apart, the magnetic field strength is reduced.

Where would the absolute greatest field strength be felt?
Well, look at the diagram.
Where are the field lines most closely spaced?

Answer: inside the magnet! Which isn't much use is it, but it is important to point out that the field lines do not start and stop at the poles; they run all the way through the magnet. This also explains why if you cut a magnet in half, you get two magnets with N and S poles at their new ends.

However, for practical purposes, the field is strongest at the Poles.

The Compass and The Earth's Magnetic Field

A compass is simply a small permanent bar magnet set on a pivot so it can rotate freely and usually it is enclosed in a plastic or other non-magnetic container with a transparent cover.
The small permanent bar magnet is generally shaped into a pointer so that the user can see which end is its North seeking end (pole).

But why does the compass needle (or any suspended bar magnet) rotate and line up in a North-South direction?

Before we jump straight to the answer, here is an interesting clue (if you need one).
Look how the compass needle aligns along a magnetic field line near a bar magnet:

As you can see, the N pole of the compass needle follows the direction of the magnetic field line.
If we show more field lines then we can show compass needles in a few positions:

This is what we find with the Earth. There is a magnetic field around the Earth which makes compass needles line up such that they always point with their N pole seeking out the Earth's geographical north pole, as shown below:

So the magnetic field around the Earth is as though there was a permanent bar magnet at the centre of the planet; but notice that to generate the correct field direction, the North pole of this bar magnet has to be aligned as shown, with its N pole towards the Earth's geographical South pole! This might seem strange, but it makes sense.

The Earth's magnetic field makes navigation with compasses possible and it provides protection from a lot of solar activity, as well as helping to produce the spectacular Northern (and Southern) Lights.

The Magnetic Field Between Two Magnets

What does the magnetic field look like between two magnets?

1. Let's consider first the field between a N pole and a S pole:
We know that the force between these two will be an attractive force, so the field lines should suggest this.

Surely the field lines should link the poles like ropes thrown across two boats being used to pull the boats towards each other.
Well that is what we find:

This field pattern explains why UNLIKE poles attract each other.

2. Now let's conside the field between two of the same poles:
This time we know that the force will be a repulsive force, so the field lines should suggest this.

The field lines won't link the poles as above, instead they must suggest a pushing away:

This field pattern explains why LIKE poles repel each other.

Notice that the field lines never touch each other; they literally push each other away.

Also notice - there will never be any field lines right in the centre between these two LIKE poles; this means that there is no magnetic field strength at all in the centre; we call this a neutral point.

### Keywords

Magnetic force: Non-contact force from a magnet on a magnetic material.

Permanent magnet: An object that is magnetic all of the time.

Magnetic poles: The ends of a magnetic feld, called north-seeking (N) and south-seeking poles (S)

Now its time for you to have a go at a few questions.