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

Condensed notes for the final weeks, aligned to the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), topic P4.1, statements P4.1a to P4.1g. All of these are for both tiers. For full explanations, use the study guide, then try the practice questions. See the OCR GCSE Physics hub, the checklist and the free diagnostics.

Key definitions

Term	Meaning
Magnetic field	Space around a magnet or current in which magnetic materials and other magnets experience a force
Permanent magnet	Keeps its magnetism and always has two poles
Induced magnet	Magnetic material that is a magnet only while in another magnet’s field
Field line	Shows direction a free north pole would move; spacing shows strength
Neutral point	Place where two fields cancel, so the resultant field is zero
Solenoid	Long coil of wire; field is like a bar magnet’s
Electromagnet	Solenoid with an iron core; can be switched on and off
Uniform field	Parallel, equally spaced field lines

Poles (P4.1a)

• Unlike poles attract. Like poles repel.
• Force pairs are equal and opposite and act on both magnets.
• Test with a suspended magnet: repulsion or attraction shows the force direction.
• Field picture: between unlike poles the lines run across the gap; between like poles they bend away, with a neutral point halfway between equal poles.

Permanent and induced (P4.1b)

• Magnetic materials: iron, steel, cobalt, nickel.
• Induced magnet: nearest end gets the opposite pole, so it is always attracted, never repelled.
• Repulsion is the only sure test for a magnet. Attraction could be a magnet or just a magnetic material.
• Soft iron loses magnetism quickly. Steel keeps it.

Field lines (P4.1c)

Method in steps to plot a field with a compass:

1. Put the magnet on paper and a plotting compass beside a pole.
2. Mark the paper at both ends of the needle.
3. Shift the compass along until the tail of the needle sits over the last dot you made.
4. Repeat, then join the dots.
5. Repeat from other starting points.

Rules:

• Outside the magnet, lines go north to south.
• Close lines mean strong field. Strongest at the poles.
• Lines never cross.
• Field weakens with distance.
• Iron filings show the pattern but not the direction.

Earth’s field and the dipping compass (P4.1d)

• A compass needle is a small magnet that aligns with the field it is in.
• Away from magnets, it points along the Earth’s field.
• A dipping compass turns in a vertical plane. Needle nearly horizontal near the equator, nearly vertical near a magnetic pole, tilted down in the UK.
• That pattern matches a bar magnet’s field, so the Earth’s core must be magnetic.
• Geographic poles and magnetic poles are in different places.
• A compass north end points to geographic north, so the Earth’s pole nearby acts as a magnetic south pole.

Current and field (P4.1e, P4.1f)

• A current produces a magnetic field.
• Around a straight wire: concentric circles in planes at right angles to the wire.
• Right-hand grip rule: thumb along conventional current, fingers curl along the field.
• Reverse the current and the field direction reverses.
• A bigger current gives a stronger field, while greater distance gives a weaker one.
• Circles are closer together near the wire.

Change	Effect on field
Larger current	Stronger
Further from wire	Weaker
Current reversed	Same strength, opposite direction

Solenoids (P4.1g)

• A solenoid’s field has a bar magnet’s shape: strong, nearly uniform inside, spreading outside.
• Each turn adds to the field of the others.
• Ends act as poles. Reverse the current to swap them.
• To strengthen: more current, more turns, turns closer together (shorter coil for the same turns), iron core.
• Iron core becomes an induced magnet and adds to the field.

Worked reminder: 240 turns in 12 cm is 240 ÷ 12 = 20 turns per cm. The same turns in 8 cm is 240 ÷ 8 = 30 turns per cm, which is 50% more per centimetre ((30 − 20) ÷ 20 = 0.5).

Must-know distinctions

Compare	Point
Permanent v induced	Permanent can repel. Induced only attracts.
Magnet field v wire field	Magnet: lines from pole to pole. Wire: circles round the wire.
Solenoid v electromagnet	Electromagnet is a solenoid with an iron core.
Soft iron v steel	Soft iron for a core that must switch off. Steel for a magnet that must stay.
Geographic v magnetic pole	Different places.

Model explanations to learn

These show how to build a clear answer. Adapt the wording to the question.

Why does a compass show the Earth has a magnetic core? The needle is a tiny magnet that turns to follow the field around it, so it shows the direction of the Earth’s field. A dipping compass tilts steeply near the poles and lies nearly flat near the equator, as it would above a bar magnet. A magnetic field must come from a magnet or a current, so the core must be magnetic.

Why does an iron nail stick to a magnet? The magnet’s field makes the nail an induced magnet. The end nearest the magnet becomes the opposite pole, so there is an attractive force between them. Remove the magnet and the nail loses most of its magnetism.

Why is the field stronger in a solenoid than round a single straight wire? Each turn produces its own field. Inside the coil these fields point the same way and add together, and an iron core adds more.

Data and graph reminders

• A table of clips lifted against current or turns is a test of your description. Say the direction of the trend, quote two values with units, and state the conclusion about field strength.
• Clips lifted is an indirect measure of strength. The magnet can pick up more only if the field is stronger, and the total depends on the size of the clips as well.
• A fair test changes one variable only. For turns, keep the current, the core and the length of the coil the same.
• Repeat readings and use the mean. Very large currents heat the wire, so switch off between readings.
• Percentage change = change ÷ original value × 100. Check: 1.0 A giving 9 clips and 2.0 A giving 18 clips is (18 − 9) ÷ 9 × 100 = 100%.

Diagram checklist

• Bar magnet: lines leave N, enter S, never cross, and are densest at the poles. Add at least one arrow on each line.
• Two unlike poles facing: lines pass across the gap between them.
• Two like poles facing: lines bend away from each other, with a gap where the field is zero.
• Wire: draw a circle or ring with arrows, closer together near the wire.
• Solenoid: parallel lines inside, bar-magnet loops outside, with an N end and an S end labelled.

Quick self-test

1. What happens when two south poles are brought together?
2. Why is an iron nail attracted to either pole of a magnet?
3. Which test proves that a metal bar is a magnet?
4. In which direction do field lines outside a magnet run?
5. What does it mean when field lines are close together?
6. What shape is the field around a straight current-carrying wire?
7. A dipping compass needle is almost vertical. What does this say about where it is?
8. Name two ways to make an electromagnet stronger.
9. An electromagnet with 20 turns lifts 5 clips and with 80 turns lifts 22. By what percentage does the number of clips increase?
10. A solenoid has 150 turns over 6.0 cm. How many turns per cm is that?
11. Why is the core of an electromagnet made of soft iron rather than steel?

Answers

1. They repel.
2. The magnet induces an opposite pole at the end nearest it, so the nail is attracted.
3. Repulsion with a known magnet.
4. From north pole to south pole.
5. The field is stronger there.
6. Concentric circles centred on the wire.
7. It is near one of the Earth’s magnetic poles.
8. Any two of: larger current, more turns, closer turns, iron core.
9. (22 − 5) ÷ 5 × 100 = 340%.
10. 150 ÷ 6.0 = 25 turns per cm.
11. Soft iron stops being magnetic soon after the current stops, so the electromagnet releases its load. Steel would stay magnetised.

Where marks are usually lost

• Saying “attract” when the answer needs the repulsion test for a magnet.
• Drawing field lines that cross or have no arrows.
• Putting arrows on lines from south to north outside the magnet.
• Drawing straight field lines round a wire instead of circles.
• Forgetting to say the compass needle is a magnet that aligns with the field.
• Naming only “a bigger magnet” for a stronger electromagnet instead of current, turns or core.
• Using “stronger” without saying what is stronger: the field, the force, or the number of clips lifted.
• Stating that the geographic and magnetic poles are the same place.
• Giving the percentage change as a ratio, or dividing by the new value rather than the original.

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.