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Bring two bar magnets slowly together and you can feel something invisible at work — sometimes a gentle pull that snaps them together, sometimes a stubborn push that no amount of squeezing will overcome. That invisible influence is magnetism, and the region around a magnet where it acts is the magnetic field. Magnetism shapes the modern world: it steers the needle of a compass, holds notes to a fridge door, sorts steel from aluminium at a recycling plant, and lies at the heart of every electric motor and generator you will meet later in this topic. This lesson opens Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A by sorting out permanent and induced magnets, naming the magnetic materials, stating the rules of attraction and repulsion, and mapping the magnetic field of a bar magnet — including the Earth's own field that a compass responds to.
By the end of this lesson you should be able to distinguish permanent magnets from induced magnets, name the magnetic materials, state the rules for attraction and repulsion between poles, describe and draw the magnetic field around a bar magnet, explain how a plotting compass reveals the field, and explain how the Earth's magnetic field acts on a compass.
A permanent magnet is an object that produces its own magnetic field all the time, without any help. A bar magnet, a horseshoe magnet and a fridge magnet are all permanent magnets. Every magnet has two poles, called the north pole (strictly the "north-seeking" pole) and the south pole. The poles are where the magnetic field is strongest — which is why iron filings cluster most thickly at the ends of a bar magnet.
A single magnet always has both a north and a south pole; you cannot have a magnet with only one pole. If you snap a bar magnet in half, you do not get a separate north piece and a separate south piece — instead each half becomes a complete magnet with its own north and south pole.
Exam Tip: Every magnet has two poles, north and south. Cutting a magnet in two gives two smaller magnets, each with a full pair of poles — never a single isolated pole.
The way two magnets behave when brought together depends entirely on which poles face one another. The rule is short and must be known exactly:
This is a non-contact force: the magnets push or pull on each other without touching, through the magnetic field in the gap between them. The force gets stronger as the magnets are brought closer together, because the field is more concentrated near the poles, and weaker as they are moved apart.
| Poles facing each other | Force | Effect |
|---|---|---|
| North – North | Repulsion | Push apart |
| South – South | Repulsion | Push apart |
| North – South | Attraction | Pull together |
| South – North | Attraction | Pull together |
Exam Tip: "Like repel, unlike attract." A test of whether something is a magnet is repulsion: a magnet can attract an unmagnetised piece of iron, but only another magnet facing it the wrong way will repel it. Repulsion is the sure sign that both objects are magnets.
Not everything is magnetic. A magnetic material is one that is attracted to a magnet. The magnetic materials you must know are the metals iron, steel (which is mostly iron), cobalt and nickel. Most other materials — aluminium, copper, plastic, wood, glass — are not magnetic and are not attracted to a magnet at all.
When a magnetic material such as a piece of iron is placed in a magnetic field (for example, next to a permanent magnet), it becomes a magnet itself. This is called an induced magnet. The induced magnetism is what lets a magnet pick up a chain of paper clips: the first clip becomes an induced magnet, which then induces magnetism in the next clip, and so on.
The crucial difference between a permanent and an induced magnet is what happens when the field is taken away:
Induced magnetism always causes attraction: the induced pole nearest the permanent magnet is always the opposite pole, so the two are pulled together. An induced magnet can never repel the magnet that is inducing it.
| Feature | Permanent magnet | Induced magnet |
|---|---|---|
| Produces its own field? | Always | Only while in another magnetic field |
| When field removed | Stays magnetic | Loses most/all magnetism |
| Example | Bar magnet, fridge magnet | Iron nail held near a magnet, paper clips in a chain |
| Force produced | Attraction or repulsion | Attraction only |
Exam Tip: The give-away of an induced magnet is that it loses its magnetism when removed from the field and can only attract, never repel. A material that stays magnetic after the field is taken away is a permanent magnet.
A magnetic field is the region around a magnet where a magnetic material or another magnet feels a force. We picture the field using magnetic field lines (lines of force), which have a set of fixed rules:
The diagram below shows the classic looping pattern of a bar magnet's field.
The closely-spaced lines bunched at each end show that the field is strongest at the poles, while the widely-spaced lines further out show a weaker field. This idea — line spacing tells you field strength — applies to every field diagram in this topic.
Exam Tip: When you draw a bar magnet's field, you must put arrows pointing from N to S on the lines outside the magnet, and make the lines closer together at the poles. Lines without arrows, or lines that cross, lose marks.
How do we know the field has this shape? We can map it using a small plotting compass, because a compass needle is itself a tiny magnet that lines up with the field. The needle always points along the field line at whatever point it is placed, with its north end pointing in the direction of the field (the N → S direction).
Method to plot a bar magnet's field:
An alternative quick method is to sprinkle iron filings over a card placed on the magnet and tap it gently: the filings turn into tiny induced magnets and line up along the field lines, revealing the pattern instantly — though, unlike the compass, the filings do not show the direction (the arrows) of the field.
Exam Tip: A plotting compass shows both the shape and the direction of the field; iron filings show only the shape. If a question asks how to find the direction of the field, you must use a compass.
The field of a single bar magnet is non-uniform — it is stronger near the poles and curves around. But if you place the north pole of one magnet opposite the south pole of another, the field in the gap between them becomes almost uniform: the field lines run straight, parallel and evenly spaced from the north pole across to the south pole. A uniform field has the same strength and direction everywhere within the gap, shown by parallel, equally-spaced lines.
This uniform field is exactly the arrangement used to make a wire experience a steady force in the motor effect, which you will meet in lesson 3.
Exam Tip: A uniform field is drawn as straight, parallel, evenly-spaced lines, all pointing the same way (north to south). "Uniform" means same strength and direction at every point.
The Earth behaves as though it has a giant bar magnet buried inside it, producing a magnetic field that stretches out into space all around the planet. It is this field that makes a compass work as a navigation tool: a compass needle is a small magnet, free to turn, so it lines up with the Earth's field and its north-seeking pole points roughly towards the Earth's North Pole.
There is a neat subtlety here. Because unlike poles attract, and the north pole of the compass needle is pulled towards the geographic North, the magnetic pole of the Earth that lies up near the geographic North Pole must actually behave like a south magnetic pole. You do not need to dwell on this for most questions, but it explains why the needle settles the way it does.
A compass is also powerful evidence that the Earth has a magnetic field: the fact that a freely-suspended magnet always swings to point the same way, even far from any other magnet, shows that there must be a magnetic field — the Earth's own — acting on it.
Exam Tip: A compass needle points towards the Earth's geographic North because the needle is a small magnet that lines up with the Earth's magnetic field. The consistent pointing of a compass is evidence that the Earth itself produces a magnetic field.
| Misconception | The correct idea |
|---|---|
| "You can have a magnet with just a north pole" | Every magnet has both a north and a south pole; cutting one in half gives two complete magnets |
| "All metals are magnetic" | Only iron, steel, cobalt and nickel are magnetic; aluminium, copper, gold and most metals are not |
| "An induced magnet stays magnetic forever" | An induced magnet loses most/all magnetism once removed from the field; only a permanent magnet keeps it |
| "Field lines go from south to north outside the magnet" | Outside the magnet the lines run north → south; the arrows must show this |
| "Field lines can cross each other" | Field lines never cross; where they bunch together the field is simply stronger |
| "Iron filings show the direction of the field" | Iron filings show only the shape; you need a plotting compass to show direction |
Question (6 marks): A student has an iron nail and a bar magnet. Explain the difference between a permanent magnet and an induced magnet, and describe how the student could show that the iron nail is only an induced magnet.
Mid-band response: "A permanent magnet is always magnetic but an induced magnet is only magnetic when it is near another magnet. The nail becomes magnetic when you put it next to the bar magnet but stops being magnetic when you take it away."
Examiner-style commentary: The core distinction is correct, but the answer does not describe a clear test, and it does not mention that an induced magnet can only attract. To climb a band, give a method (e.g. use the nail to pick up paper clips, then remove the bar magnet) and state that induced magnetism produces attraction only.
Stronger response: "A permanent magnet produces its own magnetic field all the time. An induced magnet is only magnetic while it is in a magnetic field and loses its magnetism when removed. To show the nail is only induced, hold it against the bar magnet and use the end of the nail to pick up some paper clips. Then take the bar magnet away — the paper clips fall off, showing the nail has lost its magnetism."
Examiner-style commentary: A clear definition and a valid test with a correct prediction. To reach the top band, add that an induced magnet can only attract (never repel), and explain that the clips fall because the induced magnetism disappears once the inducing field is removed.
Top-band response: "A permanent magnet produces its own magnetic field at all times, with a fixed north and south pole, whether or not another magnet is nearby. An induced magnet is a magnetic material (such as iron) that becomes a magnet only while it sits in a magnetic field; it loses most or all of its magnetism as soon as it is removed from that field, and it can only ever attract the magnet inducing it — never repel it. To show the iron nail is only an induced magnet, the student should hold one end of the nail against the bar magnet and use the other end to pick up a few paper clips — this works because the nail has become an induced magnet. They should then remove the bar magnet: the paper clips will fall off, because the nail loses its induced magnetism once the inducing field is gone. A permanent magnet would have kept hold of the clips. As a further check, the nail will only ever attract the bar magnet, whereas two permanent magnets can be made to repel."
Examiner-style commentary: Full marks. It defines both terms precisely, gives a clear and valid test with the correct prediction and reason, and adds the discriminating points (attraction-only, and the repulsion test for a permanent magnet) that examiners reward.
This content is aligned with OCR Gateway Science A GCSE Physics (J249), Topic P4 Magnetism and magnetic fields (permanent and induced magnets; magnetic materials; poles, attraction and repulsion; the magnetic field of a bar magnet; plotting compasses; the Earth's magnetic field). Refer to the official OCR specification document for the exact wording.