OxfordAQA International GCSE Physics: Magnets, magnetic fields and electromagnets (9203)

This guide follows the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, section 3.5.2 (Magnetism and electromagnetism), points a to g. The qualification is untiered. It also covers the required practical on the factors that determine the strength of an electromagnet.

Related pages: revision notes, practice questions, course hub, printable checklist, exam preparation and the 10-minute diagnostics.

What you need to know

Point	You must be able to
3.5.2 a	State where magnetic forces are strongest, predict how two magnets interact, and say why this is a non-contact force
3.5.2 b	Define a magnetic field and recognise the pattern for one or two bar magnets and for a uniform field
3.5.2 c	Explain induced magnetism and why it always gives attraction
3.5.2 d	Describe the Earth’s field and explain how a plotting compass detects it and helps navigation
3.5.2 e	Describe the field around a straight current-carrying wire
3.5.2 f	Explain how a solenoid strengthens the field and describe the field inside it
3.5.2 g	Explain the solenoid’s bar-magnet shape, the effect of an iron core, the electromagnet, its uses, and the required practical

Poles and forces between magnets (a)

Magnetic forces are strongest at the poles of a magnet. Every magnet has a north-seeking pole and a south-seeking pole, usually called north and south.

• Two poles of the same kind, such as north and north, repel.
• Two poles of different kinds, north and south, attract.

These are non-contact forces. The magnets push or pull on each other across a gap, with nothing touching. A magnet can also exert a force on a piece of iron, steel, cobalt or nickel. Those four are the magnetic materials. Copper, aluminium, glass and plastic are not.

Worked example 1. A bar magnet hangs from a thread with its north pole on the left. A student holds the north pole of a second magnet near the left end. What happens?

The left end of the hanging magnet is a north pole. The student’s pole is also north. Like poles repel, so the hanging magnet swings away from the student’s magnet. If the student turns the magnet round and offers the south pole, the hanging magnet swings towards it.

Repulsion is the only sure test for a magnet. A magnet attracts both a magnet and a plain piece of iron, so attraction alone proves nothing. If an object is repelled by a known pole, it must be a magnet.

Magnetic fields and field patterns (b)

A magnetic field is the region around a magnet in which another magnet, or a piece of iron, steel, cobalt or nickel, feels a force. Its strength and direction change from one point to another.

Field lines show this. The rules are:

1. Outside the magnet, lines run from the north pole to the south pole. Each arrow points the way a lone north pole would be pushed.
2. Lines are closest together where the field is strongest, which is at the poles.
3. Lines never cross.

One bar magnet. Lines leave the north pole, curve round and enter the south pole. They are crowded at both ends and spread out in the middle and at the sides.

Two bar magnets, unlike poles facing. Lines run straight across the gap from north to south. They are close together and nearly parallel in the middle of the gap, so the attraction is strong.

Two bar magnets, like poles facing. Lines curve away from each other and do not cross. Midway between the poles there is a point where the field cancels to zero. This is the repulsion pattern.

A uniform field. The field lines are parallel, equally spaced and point the same way, so the field has the same strength and direction everywhere. You can get one in the small gap between two flat, opposite poles placed close together.

Iron filings sprinkled on card over a magnet show the shape of a pattern, because each filing becomes a tiny induced magnet. They do not show direction. Use a plotting compass for that.

Induced magnets (c)

An induced magnet is a material that turns into a magnet only while it sits in a magnetic field. It loses most or all of its magnetism quickly when it is taken out of the field.

Worked example 2. Explain why a bar magnet picks up an iron nail.

1. The north pole of the magnet is brought close to the nail.
2. The magnet’s field turns the nail into an induced magnet.
3. The end of the nail nearest the magnet becomes a south pole, and the far end a north pole.
4. The nearer pole is the opposite of the magnet’s pole, so the force on the nail is attraction.
5. When the nail is pulled away from the magnet it loses most of its magnetism, so it no longer attracts a second nail.

Induced magnetism always gives attraction, whichever pole you use. The nearer end of the nail always takes the opposite pole to the magnet. This is why you never see a plain piece of iron being repelled by a magnet.

The Earth’s magnetic field (d)

The Earth has a magnetic field. It is most concentrated at the magnetic north and south poles, and it looks like the field of a giant bar magnet.

A plotting compass is a small magnet on a pivot, so it can turn freely. Away from other magnets, the needle lines up with the Earth’s field. Its north-seeking end points towards the Earth’s north. That shows the Earth has a magnetic field, and it gives you a direction to navigate by. With a map, you turn the map until north on the map matches the needle, then read off your route.

Keep the compass away from magnets, iron and steel objects and current-carrying wires, or the needle will follow those fields instead.

Plotting the field of a magnet with a compass.

1. Lay the magnet on a sheet of paper and trace its outline.
2. Place the compass near the north pole. Mark a dot at each end of the needle.
3. Move the compass so that the south-pointing end sits on the first dot. Mark the new dot at the north-pointing end.
4. Repeat until you reach the other pole, then join the dots with a smooth line. Add an arrow pointing the way the needle’s north end points.
5. Start at a different place to draw more lines.

The field around a current-carrying wire (e)

A magnetic field is produced when an electric current flows through a wire. For a long straight wire, the lines are concentric circles in a plane perpendicular to the wire, centred on the wire.

• The field is stronger closer to the wire. The circles are more crowded near it.
• Increasing the current makes the field stronger.
• Reversing the current reverses the direction of the field lines.

You can show this with a wire passing vertically through a card with plotting compasses placed on it. With the current off, all needles point the same way, along the Earth’s field. With the current on, the needles turn to follow circles round the wire. Reverse the current and they all turn the opposite way. A larger current gives a larger turn, and a compass further from the wire turns less.

A common memory aid for direction is the right-hand grip rule. Point your right thumb along the current and your curled fingers show the field direction. The specification only requires that you know reversing the current reverses the field.

Worked example 3. A compass is placed 2 cm from a wire carrying a steady current. The needle turns through 40 degrees from its starting direction. At 4 cm it turns through 22 degrees. Explain these readings and calculate the percentage decrease in turn.

The field is weaker further from the wire, so it deflects the needle less. The Earth’s field also acts on the needle, and the wire’s field has to compete with it.

Percentage decrease = (40 − 22) / 40 × 100 = 45%.

Solenoids (f)

A solenoid is a wire wound into a long coil of many turns. Shaping a wire into a solenoid increases the strength of the field produced by the current. Each turn adds its own circular field, and inside the coil these add up in the same direction.

Inside a solenoid the field is strong and uniform: parallel lines running along the axis. Outside, the lines spread out and return from one end to the other.

Electromagnets (g)

The field around a solenoid has a similar shape to that of a bar magnet. One end behaves as a north pole and the other as a south pole, and reversing the current swaps them over.

Adding an iron core increases the field strength. The core becomes an induced magnet inside the solenoid, and its field adds to that of the coil. An electromagnet consists of a solenoid with an iron core.

Soft iron is used because, as an induced magnet, it loses most or all of its magnetism quickly when the current is switched off. Switching the current on and off therefore switches the magnet on and off. A permanent magnet cannot be switched off.

To make an electromagnet stronger, you can:

• increase the current,
• increase the number of turns,
• add an iron core (or use a bigger one).

Uses of electromagnets

• Scrapyard crane. A large electromagnet lifts iron and steel. Switching off the current drops the load.
• Electric bell. Current in the coil makes an iron armature move to hit the gong. The movement breaks the circuit, the current stops, a spring pulls the armature back, the circuit remakes and the cycle repeats.
• Relay. A small current in a coil attracts an iron armature, which closes a second circuit that can carry a much larger current.

Required practical: factors affecting electromagnet strength

Aim. Investigate how the number of turns, the current and the core affect the strength of an electromagnet.

Method. Wind insulated wire round an iron nail. Connect it to a low-voltage dc supply with a variable resistor, an ammeter and a switch. Hold the electromagnet over a tray of steel paper clips and count how many it lifts. The count is the measure of strength.

• Independent variable: the one you change (turns, current or core).
• Dependent variable: the number of clips lifted.
• Control variables: the other two factors, the same nail and clips, and the same way of lowering the nail onto the clips.

Safety. The coil warms up with a large current, so switch off between readings and do not hold the coil.

Worked example 4. The current is 1.0 A. Three trials at each number of turns give the clips lifted below.

Turns	Trial 1	Trial 2	Trial 3	Mean
10	2	3	2	2.33
20	5	5	6	5.33
30	8	7	8	7.67
40	10	11	10	10.33
50	13	12	13	12.67

The mean for 10 turns is (2 + 3 + 2) / 3 = 2.33. The mean rises steadily as the number of turns rises, so more turns give a stronger electromagnet. Clips per 10 turns is about 2.3 to 2.7 throughout, so the relationship is close to directly proportional. The clip count is a whole number, so the resolution is coarse, which is why repeats and a mean help.

Worked example 5. With 30 turns, the clips lifted at 1.0 A and 2.0 A are 7 and 16. Percentage increase = (16 − 7) / 7 × 100 = 128.6%, or 129% to 3 significant figures. Doubling the current more than doubles the clips lifted, so the effect is strong.

Common errors

• Writing that a magnet “attracts” a magnet without saying the poles must be unlike.
• Drawing field lines that cross, or with no arrows.
• Saying a magnet attracts all metals. Only iron, steel, cobalt and nickel are magnetic.
• Saying induced magnetism can repel.
• Saying the field around a wire is straight lines along the wire. It forms circles.

Official syllabus

OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, published by OxfordAQA. This page covers section 3.5.2.