OCR GCSE Physics A: Magnets, magnetic fields and electromagnets (J249)

This guide is aligned to the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018. It covers topic P4.1, Magnets and magnetic fields (statements P4.1a to P4.1g). None of these statements is marked as Higher Tier only, so everything here is for both tiers. Use it with the revision notes and the practice questions. The course hub is at OCR GCSE Physics, the printable checklist lists every statement, and the exam preparation page covers general technique. If you want to test yourself first, try the free diagnostics.

What this unit covers

Statement	What you must be able to do
P4.1a	Describe attraction and repulsion between unlike and like poles of permanent magnets, including field diagrams
P4.1b	Describe the difference between permanent and induced magnets
P4.1c	Describe the magnetic field of a magnet, showing how strength and direction change from point to point
P4.1d	Explain how a magnetic (dipping) compass relates to evidence that the Earth’s core is magnetic
P4.1e	Describe how to show that a current has a magnetic effect, and the field directions round a wire
P4.1f	Recall that field strength depends on the current and the distance from the conductor
P4.1g	Explain how solenoids enhance the magnetic effect

P4.1a: Poles, attraction and repulsion

A magnet has two poles, a north-seeking pole (north pole) and a south-seeking pole (south pole). The force between two magnets acts without contact. It is a non-contact force.

• Unlike poles (north and south) attract.
• Like poles (north and north, or south and south) repel.

Each force acts on both magnets, equal in size and opposite in direction. Hang one bar magnet from a thread, bring the pole of a second magnet near, and the hanging magnet swings towards it or away from it. This is the suspended-magnet method for showing attraction and repulsion.

The field patterns show the same thing. Between unlike poles the field lines run straight across the gap from one magnet to the other. Between like poles the lines bend away from each other and squash apart. Halfway between two equal like poles there is a point where the two fields cancel, called a neutral point. A small compass placed there does not settle in any one direction.

P4.1b: Permanent and induced magnets

A permanent magnet keeps its magnetism and has its own north and south poles all the time. An induced magnet is a piece of magnetic material that becomes a magnet only while it is in another magnetic field. When the field is removed it loses most or all of its magnetism.

Iron, steel, cobalt and nickel are magnetic materials. Copper, aluminium and plastic are not.

Why does a magnet pick up an iron nail? The magnet induces poles in the nail. The end of the nail nearest the magnet becomes the opposite pole, so the nail is attracted. Induced magnets are always attracted to the magnet that induces them. They are never repelled.

Worked example 1. A student has a steel rod. It is attracted to both ends of a bar magnet. Another rod repels one end of the same bar magnet. Which rod is definitely a magnet?

The second rod. Repulsion happens only between two magnets with like poles facing, so it is the reliable test. Attraction alone could be a permanent magnet or an unmagnetised piece of iron or steel that has become an induced magnet. The first rod could be either.

P4.1c: Magnetic fields

A magnetic field is the space around a magnet or current in which magnets and magnetic materials experience forces. The field is shown with field lines.

Rules for field lines:

1. They run from the north pole to the south pole outside the magnet. The arrow shows the direction a free north pole would move.
2. Where lines crowd together the field is strong, and where they spread out it is weak. The poles are where the lines are most crowded.
3. Lines never cross.
4. Direction changes from point to point, so lines curve. The direction at any point is along the tangent to the line.

Field strength falls as you move away from a magnet. Do not think a bigger magnet must be a stronger one. Strength depends on the material and how it was magnetised, not just the size.

A uniform field has parallel, equally spaced lines. You get one in the gap between the flat faces of two opposite poles placed close together, away from the edges, where the lines bulge.

Investigating a field with a plotting compass. Place a bar magnet on paper. Put a small plotting compass near one pole and mark the paper at each end of the needle. Move the compass so that its tail lies on the head mark, and repeat. Joining the points gives one field line. Repeat from different starting points. Where the dots bunch up the field is strong. A second method scatters iron filings on card over the magnet. The filings become induced magnets and line up along the field, showing the pattern, but not its direction.

P4.1d: The Earth’s field and the dipping compass

A compass needle is a small, light permanent magnet that is free to turn. Away from other magnets it settles along the Earth’s magnetic field, with its north-seeking end pointing roughly to geographic north.

A dipping compass (dip circle) has its needle free to turn in a vertical plane. It shows that the field is not simply horizontal. In the UK the needle tilts downwards towards the ground. Near the equator it lies close to horizontal. Near the magnetic poles it points almost vertically.

The observations match the field of a bar magnet inside the Earth: field lines leaving near one pole, curving round, and re-entering near the other. A magnet’s field can only be produced by a magnet or a current, so the compass behaviour is evidence that the Earth’s core must be magnetic.

Two points to keep clear. The specification lists the first among its common misconceptions:

• The geographic poles (where the Earth’s axis of spin meets the surface) and the magnetic poles are not in the same place.
• A compass needle’s north-seeking end points towards the Earth’s geographic north. Opposite poles attract, so the magnetic pole of the Earth near geographic north behaves like a magnetic south pole.

Worked example 2. A dipping compass in one city shows the needle 66 degrees below the horizontal. At a second location the needle is 12 degrees below the horizontal. Which location is nearer a magnetic pole, and why does this support a magnetic core?

The needle is steeper at the first location (66 degrees against 12 degrees), so it is nearer a magnetic pole. A steeply dipping needle at the poles and a near-horizontal one near the equator is what you would see above a bar magnet. The pattern matches a field produced by a magnetic core.

P4.1e: The magnetic effect of a current

Any wire carrying a current is surrounded by a magnetic field. Hans Oersted’s classic demonstration is simple to reproduce. Place plotting compasses around a vertical wire passing through a card. With no current, all point the same way, along the Earth’s field. Switch on a large direct current and they turn to point round the wire.

The field lines are rings around the wire, lying in planes at right angles to it, and they are not straight. Work out the direction with the right-hand grip rule. Point the right thumb along the conventional current, and the curled fingers show the direction of the field. Reverse the current and the field direction reverses.

Compasses show the same thing. Reverse the current, and every needle turns through 180 degrees.

P4.1f: What affects the strength

Recall that the strength of the field depends on:

• the current: a larger current gives a stronger field;
• the distance from the conductor: the field is weaker further from the wire.

On a diagram, the circles are drawn closer together near the wire and further apart as you move out.

Worked example 3. Plotting compasses placed 2 cm, 4 cm, 6 cm and 8 cm from a wire turn away from the Earth’s field direction by 70, 45, 25 and 15 degrees. Describe the trend and say what it shows.

The turn falls steadily as the distance increases, from 70 degrees to 15 degrees. The wire’s field adds to a constant Earth’s field, so a smaller turn means the wire’s field is weaker. The field strength decreases with distance from the wire. Moving from 2 cm to 4 cm cuts the deflection by 25 degrees, but from 6 cm to 8 cm only by 10 degrees. The effect is largest close to the wire.

P4.1g: Solenoids and electromagnets

A solenoid is a long coil of wire. Bending a wire into a loop makes the field lines from each side point the same way through the loop, so they add up. A solenoid with many turns adds the loops together.

The field of a solenoid has the same shape as a bar magnet’s. Inside it the lines are close, nearly parallel and nearly uniform. Outside the pattern spreads from one end and curves round to the other end, like a bar magnet. One end acts as a north pole and the other as a south pole. Reversing the current swaps them. Use the right-hand grip rule again, curling the fingers in the direction of the conventional current round the coil, and the thumb points to the north pole end.

A solenoid with an iron core is an electromagnet. The core becomes an induced magnet and increases the field a lot. Soft iron is used because it magnetises and demagnetises easily, so the electromagnet switches off when the current stops. Steel would stay magnetised.

To make an electromagnet stronger:

• increase the current;
• add more turns of wire;
• pack the turns into a shorter length, so there are more turns for each centimetre;
• use an iron core.

Worked example 4. A solenoid has 150 turns spread evenly over 6.0 cm. It is stretched to 9.0 cm with the same turns. Compare the turns per centimetre.

Before: 150 ÷ 6.0 = 25 turns per cm. After: 150 ÷ 9.0 = 16.7 turns per cm (3 s.f.). The turns per centimetre fall by (25 − 16.7) ÷ 25 = 33%, so the field inside is weaker.

Worked example 5. Clips lifted by an electromagnet at 0.5, 1.0, 1.5 and 2.0 A are 4, 9, 13 and 18. Describe the pattern.

More current lifts more clips. From 1.0 A to 2.0 A the number rises from 9 to 18, an increase of (18 − 9) ÷ 9 × 100 = 100%. Doubling the current doubled the clips lifted here. The data support the idea that a stronger current gives a stronger field, but a graph of several repeats would be needed to be confident of the exact relationship.

Common errors

• Writing that field lines “go from south to north” outside the magnet.
• Saying induced magnets can be repelled.
• Describing field lines round a wire as straight or radiating out.
• Using “the magnet gets bigger” instead of the real factors: current, turns, core.
• Treating the geographic and magnetic poles as the same point.
• Confusing the field of the solenoid with the field in the wire: a solenoid field looks like a bar magnet.
• Forgetting that line spacing shows strength, so unequal spacing means a non-uniform field.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), specification Version 5.0 (July 2026), first assessment 2018, published by OCR. This page covers topic P4.1.