Physics-SchoolUK.com

The UK site for KS3 and KS4 Physics

KS4 Magnetism and electromagnetism: Permanent and Induced Magnetism
We start this section looking at the basics of magnetism and magnetic fields.

The 2 Poles of a Magnet

The first "magnets" were pieces of found material known as lodestones.
In the ancient world it was known that these materials could attract pieces of iron and, more importantly, that when allowed to turn freely, they would come to rest always in the same direction (eventually to be called "North" and "South"). This latter behviour enabled them to be used for navigation.
The word "lodestone" means "leading stone", ie they literally led the way. If you have seen the Disney movie "Pocahontas" you will remember how she uses a piece of lodestone to navigate out of a forest, resting a shard of lodestone on a leaf on some water; it rotated until it came to rest pointing in the way of "North-South".

In 1600 William Gilbert, in England, developed the process of making magnets from iron, doing away with the need to find the relatively rare lodestone for the purposes of navigation. Furthermore, he performed experiments and concluded that the Earth itelf was magnetic and that this was why the lodestones or his man made magnets always lined up in the same 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

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.

Finally, about the 2 poles, if you were ever in need of finding the direction of North (or South) and you had a magnet with its Poles labelled and a piece of string, all you have to do is tie the string around the magnet and hold the string such that the magnet can freely pivot and wait for it to come to rest. If allowed to freely pivot and come to rest, it will point in the North-South direction as shown below; in other words, you would have made a basic compass.

Last of all, the above "experiment" can also be used as a test for a bar magnet!
If somebody gave you a few identical bars of metal and said that only one was a magnet and you had to identify it, you could do so by suspending each bar a number of times from some string; only the magnet bar would consistently come to rest in the same direction; the other bars would come to rest in different directions on each attempt.

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.)

Permanent and Induced Magnets

The lodestones described earlier or the man made magnets of William Gilbert which are like those drawn above, all of which will align in a North-South direction if allowed to rotate freely, and will exhibit the two forces of Attraction and Repulsion if brought near to another magnet, are all Permanent Magnets. These are objects which produce their own magnetic field (more on this later).

If, however, we bring an iron nail up to a permanent magnet we will find that the iron nail is attracted to the magnet (even though the nail is not a magnet it is magnetic; more on this later).

If we take another nail and position it at the end of the first nail, we find that it is attracted to the first nail!!

What is going on here?
It seems like the first nail has become a magnet.
Well, that is actually what has happened. However the first nail has not become a permanent magnet like the blue and red bar magnet.
It has become an induced magnet, meaning that magnetism has been induced into it (or "put into it") for as long as it is attached to or very close to the original blue and red permanent magnet.
And whilst it is in this position, since it has become a magnet, it will have the same two poles as any other magnet; the end of the nail nearest to the North pole of the permanent magnet will become a South pole since it is attracted. Consequently, the other end of the nail must become a ...... North pole.
Let's re-draw the first diagram showing the induced poles on the first nail.

So, since the first nail is now an induced magnet with poles it will attract the second nail, inducing magnetic poles into it. The end of the second nail nearest to the N pole of the first nail will become a South pole since we see attraction; the other end will become a N pole.

Two things to notice:
1. Induced magnetism, as shown above, always causes the force of attraction. A third nail will be attracted to the end of the second nail etc.
2. The induced magnets (the nails above) are getting their magnetism from the original permanent magnet so there is a limit to how many nails will join end to end; 2 or 3 might be the limit, depending on the magnetic strength of the permanent magnet and the size of the nails.

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

Magnetic Fields

Before we can understand Magnetic Fields we should learn a little about magnetic and non-magnetic materials and try to answer the question - how do we make a permanent magnet?

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: iron, steel, cobalt and nickel.
(In fact some people would say there are only 3 magnetic materials, iron, cobalt and nickel, because steel is made from iron, but due to the difference in the way steel and iron behave, it is better to stick to the list of 4.)

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

Testing for Magnetic and non-magnetic materials

The easiest way to test materials to see if they are magnetic or non-magnetic is to test if they are attracted by a magnet.
In fact the definitions of magnetic and non-magnetic materials is based on this:

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

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

What makes our 4 magnetic materials "magnetic"?

We are going to keep this simple:
All materials contain Atoms and all Atoms contain Electrons.
Electrons spin on an axis and this spin causes each electron to have a tiny amount of magnetism, so each one acts as a tiny little magnet, known as a molecular magnet.

In most materials, the atoms are arranged in such a way that the magnetic effect of one electron cancels with another so the material exhibits no magnetic behaviour; these are, of course, our non-magnetic materials.
The simple diagram below illustrates how the tiny little magnets in these non-magnetic materials all point in a multitude of directions; an arrow pointing one way will be cancelled by one pointing in the exact opposite way. Such materials exhibit no magnetic behaviour.

However, the internal picture of one of our magnetic materials is quite different.
We still see the tiny magnets (molecular magnets) but now we find that they are grouped into areas known as domains, where each domain has all of its magnets aligned in the same direction. Have a look at the following diagram.

Now with all the tiny molecular magnets in an area ("domain") lining up, the material will exhibit some magnetic effects but only when near a permanent magnet. So we can use this idea to explain how Induced Magnetism occurs:
When a magnetic material is brought close to a permanent magnet the molecular magnets within the domains of the magnetic material begin to line up as shown below:

You can see that magnetism is induced into the magnetic material.
How perfectly the molecular magnets line up, producing the overall N and S poles, will depend on the strength of the permanent magnet and on the size of the magnetic material; you can probably guess that a second piece of magnetic material placed at the end of the first will find its molecular magnets line up even less perfectly.
When the permanent magnet is removed, the molecular magnets within their domains return to their original positions eventually and the induced magnetism is lost.

So what has to change in order for a magnetic material to become a permanent magnet?

What makes a magnetic material a MAGNET?

If the orientation or alignment of the molecular magnets within a magnetic material can change from:
1. Unmagnetised state

to:
2. Magnetised state

then whilst the molecular magnets are all lined up as shown, we have a MAGNET.

Notice that we call these two states unmagnetised and magnetised; these terms can only apply to magnetic materials; non-magnetic materials are always simply non-magnetic.

But how do we change a magnetic material from its Unmagnetised state to its Magnetised state?

Making a MAGNET

There are a number of ways to do this.
You can use an existing magnet and "wipe it" repeatedly over your magnetic material; this will gradually cause the molecular magnets to line up until you have "magnetised" your material. This, however, is a long and not always successful process, but it is the method that William Gilbert used four centuries ago.

A more reliable method is to place your magnetic material inside a current carrying solenoid (ie a coil of wire connected to a strong battery or a DC power supply). You only need to leave the material inside the coil for a number of minutes (though the longer the better) for your material to become magnetised.

But, for how long will your new MAGNET remain magnetised?

Hard and Soft Magnetic Materials

Remember there are four magnetic materials, steel, iron, cobalt or nickel.
The most common of these four are steel and iron, so let's assume you have a choice of these two.

If you chose iron then you would find that your magnetising process was quick and easy but the iron would lose its magnetised state as quickly as it gained it! So, not the best choice if you wanted a permanent magnet. Iron is known as a Soft magnetic material; these are materials that are easy to magnetise but also easy to demagnetise.

Hopefully steel will be a better choice!
If you chose steel then you would find that the magnetising process took longer but the steel would not lose its magnetism quickly, it would remain magnetised for a significant length of time. Steel is known as a hard magnetic material; these are materials that are "hard" to magnetise but also "hard" to demagnetise, making them perfect for use as permanent magnets.

NB As an alternative to steel, mixtures (or "alloys") of cobalt and nickel with some iron are used to make hard magnetic materials that can be magnetised and used to make strong modern permanent magnets.

Magnetic Fields

Now that we have an understanding of magnetic materials and the magnetised state, we can confidently tackle "magnetic fields".

The magnetic field around a magnetised bar magnet is illustrated as below:

Notice:
1. The direction of the field lines follows the direction of the molecular magnets inside the bar magnet.
2. The field lines go from pole to pole; in the direction "from North seeking pole to South seeking pole". This is also the direction in which a (hypothetical) free North pole would move if placed in the field, ie away from the N and towards the S.
3. The field lines are concentrated at the poles; this is important, it means that the field is strongest at the poles.
4. As distance from the magnet increases the field lines spread out more and more; this means that the field gets weaker with increasing distance.

We already know from our work above that a magnetic material brought near a magnet will feel a force, so we define a magnetic field as:

Before we move on, we don't usually draw the bar magnet with all its internal molecular magnets on show; we usually simplify it to:

But also note that the field does NOT start at the North pole and stop at the South pole; it runs through the magnet following the direction of the tiny molecular magnets.
Like this:

So, the field lines are actually more like circles/ovals!! Worth remembering.

The Compass and Magnetic Fields

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).
Schools usually have lots of small compasses which are known as plotting compasses. In addition to the see-through cover, these also have a transparent base so you can see right through them (we'll see why in a moment).
The illustration below shows a typical plotting compass.

The compass needle near another bar magnet

The compass needle is itself a small bar magnet, so what happens if you put it near to another bar magnet?
It will depend where you put it; if you put it near to the North pole of the magnet, as shown below:

You can justify this by saying "the North pole of the compass needle is repelled by the North pole of the other magnet", or "the South pole of the compass needle is attracted to the North pole of the other magnet". Notice, we are using the "Law of magnetism" here.

We can use the same "Law of magnetism" explanation to account for what happens if we put the compass at the South end of the other magnet:

So we now say "the North pole of the compass needle is attracted to the South pole of the other magnet" etc.

Using the "Law of magnetism" to figure out what happens to the compass needle works well in examples where the compass is placed at or near to one of the poles of the other magnet. But what if the compass is placed like this:

You could still use the "Law" but a better way is to think about the magnetic field around a bar magnet. Picture the field around the magnet, or draw it, with the compass in position:

Now its easy to see which way the compass needle will point; its North pole will follow the direction of the magnetic field.

Using your knowledge of the shape of the magnetic field will always enable you to figure out which way a compass needle will point, no matter where you put it:

How to find the magnetic field pattern around a bar magnet

From what we have just learnt, we can find the shape of the magnetic field around anything, including a bar magnet, using a plotting compass.
1. Put the magnet in the centre of a piece of plain paper and draw around it. Mark the ends "N" and "S".
2. Put the compass at a corner of the magnet and make a dot on the paper at the end of the compass needle.
3. Move the compass so that the other end of the needle lines up with your dot.
4. Then make a new dot and repeat the whole process. Keep making dots and moving the compass until you get near the edge of the paper.
5. You should now have a curve of dots which you can draw into a smooth curve.
6. Repeat at the other 3 corners and you should have a symmetrical picture.
7. Finally place your compass at the centre of one pole and make a dot. Follow the needle by making dots as before and you should see an almost perfect straight line reaching the edge of the paper. Do the same at the other pole.
8. Put direction arrow heads on your drawn field lines remembering it is "out from N, into S."

Your final picture should look something like this:

Proper use of a compass!

What we mean by "proper use of a compass" is using one to navigate, ie to find the direction of North (or South, East, West or anything in between).

We started this section describing how in the ancient world people used natural magnets called lodestones to find their way around. They discovered that the stone always lined up in what we now know is a North-South direction. We still use compasses in this exact same way today; many smartphones have tiny bar magnets built in which will allow the phone screen to be used as a basic compass.

So, the question is - why, when you hold a compass out in front of you, does its needle (a small bar magnet) always line up in a North - South direction?

Well the clue is found in what we have just been learning about magnetic fields.
We have learnt that a compass needle will follow the direction and lines of a magnetic field.
So when we hold a compass out in front of us, its needle must be aligning with a magnetic field.

The conclusion is - there is a magnetic field surrounding the Earth !

The behaviour of the compass is evidence that the core of the Earth is magnetic.

In the diagram above you can see that the North seeking pole of the compass needle points, correctly, towards the Earth's geographic North pole
and
the compass needle follows the direction of the field arrows.

Can you see the significance of this?
It means that the core of the Earth is magnetised as though it is a bar magnet with its North pole aligned downwards towards the Earth's geographic SOUTH pole!
Like this:

So, as you can see, the field lines come out from the North of the magnetised core (which is aligned towards the Geographic South pole) and curve up towards and into the South of the magnetised core (which is aligned with the Geographic North pole).

Now its time for you to have a go at the final set of questions on Permanent and Induced Magnets.