Permanent and Induced Magnetism, Magnetic Forces and Fields

A magnet is an object that produces a magnetic field in the space around it. The two regions where the field is strongest are called the poles of the magnet.

By convention the two poles are named:

• The north-seeking pole (usually called the north pole, written N) — the end of a freely suspended bar magnet that points towards the geographic north of the Earth.
• The south-seeking pole (the south pole, written S) — the end that points towards the geographic south.

This is why a compass works: the magnetised needle is itself a tiny bar magnet, and its north-seeking end aligns with the Earth's magnetic field. AQA expects you to use the names "north pole" and "south pole" in your answers; some textbooks shorten to "N" and "S" but write the full term at least once in a written response.

Forces between poles follow a single rule that you must learn for the exam:

Pole 1	Pole 2	Force
N	N	Repel (push apart)
S	S	Repel
N	S	Attract (pull together)
S	N	Attract

In short: like poles repel, unlike poles attract. This is identical in form to the rule for electric charges, but the underlying cause is different — electric charges produce electric fields; magnets produce magnetic fields.

Because magnetic forces act through empty space, magnetism is classed as a non-contact force alongside gravity and electrostatic force (spec section 4.5.1.2).

AQA distinguishes between two kinds of magnetism, and the difference comes up almost every series:

Permanent magnets keep their magnetic field whether or not another magnet is nearby. They have two clear poles and can both attract and repel another magnet. Everyday examples include:

• Bar magnets and horseshoe magnets used in school labs.
• Fridge magnets (a flexible ferrite strip with built-in N and S strips).
• The magnets in motors, headphones, hard drives and electric vehicle drive units.

Induced magnets are pieces of magnetic material (iron, steel, cobalt or nickel) that become magnetic only when placed in another magnet's field. The key features the AQA mark scheme expects you to mention:

1. The induced magnet's poles arrange themselves so that the face nearest the permanent magnet's N pole behaves as an S pole. So the force is always one of attraction — an induced magnet cannot repel.
2. When the external field is removed, the induced magnetism disappears (quickly in soft iron, more slowly in steel).
3. The strength of an induced magnet depends on how close it is to the inducing magnet and how easily the material magnetises.

Why this matters. This is the standard AQA exam question:

"A student places an unknown iron bar near a magnet. The bar is attracted to the magnet. Can the student be sure the bar is itself a magnet? Explain your answer."

The mark scheme awards full credit for: "No. A magnetic material (such as iron) is always attracted to a magnet because of induced magnetism. To prove the bar is itself a magnet the student must show that the bar can repel the other magnet — only a magnet can do this."

This is sometimes called the 'repulsion test' and it appears almost every other series.

Worked example — paper clips. A steel paper clip is held next to the N pole of a strong bar magnet. The end of the clip facing the magnet becomes a temporary S pole (by induction). The clip is attracted, sticks, and can itself attract a second clip below it. Remove the bar magnet and the chain slowly loses its magnetism (steel retains a little — that is why magnetised screwdrivers exist).

Only a small number of materials are attracted by a magnet. AQA's named list for GCSE Physics 8463 is:

Material	Notes
Iron	The classic magnetic material. Soft iron magnetises and demagnetises easily — used for electromagnet cores (see spec 4.7.2.1).
Steel	Iron alloyed with carbon. Holds its magnetism well — used for permanent magnets and magnetised screwdrivers.
Cobalt	Magnetic; used in alloys (Alnico = aluminium + nickel + cobalt + iron) to make strong permanent magnets.
Nickel	Magnetic; the basis of older 'magnetic' coins (modern UK 1p and 2p have a steel core that is also magnetic).

Many other materials — copper, aluminium, plastic, wood, glass, water — are not magnetic. Holding a magnet to a copper pipe shows no force at all.

Two everyday GCSE applications:

• Magnetic separation of scrap metal. A large electromagnet over a conveyor belt picks up iron and steel pieces while non-magnetic aluminium drinks cans, copper wire and plastic fall through. The metals are then sorted and recycled.
• Maglev trains. Powerful magnets (some permanent, some electromagnetic) repel the train above the track. Friction is reduced to almost zero, allowing speeds of 400+ km/h on the Shanghai Maglev.

A note on terminology. Some textbooks talk about "ferromagnetic" materials. AQA does not require this word at GCSE — saying "magnetic material" is enough.

The standard AQA-style practical. You are given three identical-looking iron bars: one is a strong permanent magnet, one is a weakly magnetised steel bar, and one is unmagnetised soft iron. How do you tell them apart?

Method.

1. Suspend bar A on a length of thread so it can rotate freely.
2. Bring one end of bar B towards each end of bar A. Note whether you see attraction only, or a mixture of attraction at one end and repulsion at the other.
3. Repeat with bar C.

Interpretation.

• If both ends of bar A are attracted to bar B, then bar A must be a magnetic material (not a magnet) — induction is causing the attraction every time.
• If one end of bar A is attracted and the other end is repelled, then bar A is itself a magnet — the repulsion at one end is conclusive proof.
• A weakly magnetised steel bar will show small but real repulsion if you turn it round.

AQA marks awarded. The mark scheme for this kind of question (Paper 2, 2023 Foundation) rewards:

• Naming "repulsion" as the only certain test (1 mark).
• Stating that attraction alone is inconclusive because of induced magnetism in magnetic materials (1 mark).
• A clear method: bring each end of the test object near each pole of a known magnet (1 mark).

A magnetic field is the region of space around a magnet in which another magnet (or a piece of magnetic material such as iron) experiences a force. The field exists in three dimensions and gets weaker with distance from the magnet.

You cannot see a magnetic field directly. To visualise it, we draw field lines. These follow three rules in AQA GCSE Physics:

1. Direction: outside the magnet, field lines run from the N pole to the S pole. The arrows on the lines show the direction in which the N pole of a small compass would point.
2. Spacing: where field lines are close together the field is strong; where they are far apart the field is weak.
3. No crossing: field lines never cross. They also never split or merge.

The lines you see in textbook diagrams of a bar magnet are not real — they are a way of representing the field. Iron filings and small compasses both line up along the field lines because each filing becomes an induced magnet (see 4.7.1.1) and rotates to align with the field.

Field strength (also called magnetic flux density) is measured in tesla (T) — though AQA does not use the word until spec 4.7.2.2 (Fleming's left-hand rule). For comparison:

Where	Approximate field strength
Earth's surface	~50 μT (0.000 05 T)
Fridge magnet (close up)	~5 mT
Strong bar magnet (pole)	~0.1 T
MRI scanner	1.5–3 T
Strongest lab magnets	45+ T

The tesla is a large unit — you will almost always meet milli-tesla (mT) or micro-tesla (μT) values in GCSE problems.

A plotting compass is a small magnetised needle balanced on a low-friction pivot inside a transparent case. Its N pole aligns with the magnetic field at its position. Step-by-step it can be used to map the field of a bar magnet — this is the AQA-style practical.

Apparatus: white paper, pencil, bar magnet, plotting compass.

Method.

1. Lay the bar magnet flat on the paper and draw round it so you can replace it accurately if it moves.
2. Place the plotting compass near one end of the magnet (say the N pole).
3. Mark the position of the compass needle's N tip with a pencil dot.
4. Move the compass so its S end is now at the dot you just made. Mark a new dot at the new N position.
5. Repeat until the dots trace a path round to the magnet's S pole.
6. Join the dots with a smooth curve and add an arrow showing direction (N → S).
7. Start again from a different point on the bar magnet to plot another field line. Aim for about 6–8 lines around the magnet.

Common results. The dots will reveal the familiar pattern: loops running out of the N pole, around the magnet, and back into the S pole. The lines are tightest at the poles (strongest field) and far apart at the sides (weak field). Two neutral points appear directly above and below the magnet where the Earth's field cancels the bar magnet's field — at these points the compass needle is undisturbed.

Iron filings alternative. Sprinkling iron filings on paper over a magnet and gently tapping shows the same pattern. Each filing becomes a tiny induced magnet and rotates to lie along the field. Iron filings show shape but not direction — you still need a compass to add the arrows.

AQA exam questions in this area typically ask you to:

• Draw the field of a bar magnet, including arrows on the lines.
• Describe how to use a compass to plot the field — full marks need the dot-to-dot method, not just "use a compass to follow the field".
• Explain why iron filings line up — induced magnetism makes each filing a small magnet that aligns with the field.

When a compass is allowed to swing freely in a place free of other magnets, it always settles roughly along a north–south line. The N pole of the compass needle points towards the geographic North of the Earth.

What this tells us. A magnetised needle aligns with the magnetic field at its location, so a north-pointing compass tells us that the Earth has a magnetic field at its surface — and that this field runs (approximately) from the geographic South to the geographic North in the region around the UK.

A subtle point. Because unlike poles attract, the geographic North of the Earth must actually be a magnetic south pole. The Earth's north magnetic pole is near the geographic South Pole. AQA examiner reports flag this regularly — students mix up the labels because they assume geographic north = magnetic north.

What causes the Earth's field? Inside the Earth there is a liquid outer core of molten iron and nickel that flows in slow convection currents. The motion of charged particles in this conducting fluid generates electric currents — and these currents produce a magnetic field (the same physics as 4.7.2.1, but on a planetary scale). The field roughly resembles that of a giant bar magnet tilted by about 11° from the rotation axis.

Why this matters.

• The Earth's magnetic field deflects most of the solar wind (charged particles from the Sun), protecting the atmosphere and life at the surface. The aurora borealis (Northern Lights) seen in Scotland on rare strong-storm nights is a side-effect.
• Compasses are still used for navigation worldwide, by hikers, sailors and aircraft.
• The field is slowly drifting — the magnetic north pole has moved hundreds of kilometres in the last century, and geological records show the entire field has reversed (north ↔ south) many times over the Earth's history.

Worked exam question. "A compass on a school field always points roughly north. What does this tell us about the Earth?"

Mark scheme answer: The compass needle aligns with the magnetic field around it (1). The Earth must therefore have a magnetic field (1) which can be modelled as a giant bar magnet inside the planet (1), generated by motion in the molten iron/nickel core (1, Higher Tier extension).

The force on a small magnet (or magnetic material) placed in a magnetic field depends on:

• The strength of the field at that point (how closely packed the field lines are).
• The mass / type / amount of magnetic material the small object contains.

There is no equation you must use at this level — just describe the trend in words. Some key qualitative points the examiner expects:

• The field is strongest at the poles of a bar magnet (lines tightly packed).
• The field is weakest in the middle of the sides (lines spread out).
• The field becomes rapidly weaker as you move away from the magnet (roughly inverse-square, but AQA doesn't ask for a formula here).
• A magnetic material in a non-uniform field experiences a net force towards the stronger region of the field — this is why a paper clip is pulled towards (not away from) a magnet.

Inside a magnet. The field lines drawn outside the magnet continue inside it from S to N — they form complete loops. AQA does not ask you to draw the field inside a bar magnet, but knowing that the lines form loops explains why field lines never start or end in mid-air.

A neutral point is a point in space where two magnetic fields cancel exactly and the net field is zero. A small compass placed there will not align in any particular direction. They occur, for example, on the perpendicular line through the centre of a bar magnet, where the bar magnet's field cancels the Earth's field at certain distances.

In 1820 the Danish physicist Hans Christian Ørsted noticed that a compass needle deflected when placed near a wire carrying a current. This was the first demonstration that electricity and magnetism are connected — the foundation of all later electromagnetism.

The pattern. When a current flows in a straight wire, the magnetic field around the wire forms concentric circles in a plane at right angles to the wire. The field has no preferred direction along the wire; it curls around it.

Right-hand grip rule (Higher and Foundation Tier).

• Point your right thumb in the direction of the conventional current (from + to −).
• Your fingers then curl in the direction of the magnetic field.

Some textbooks use a different convention with conventional current vs electron flow — AQA expects you to use conventional current direction, the one drawn on circuit diagrams from + to −.

Reversing the current reverses the field. Flip the battery and every arrow on every field circle reverses. This will matter when we look at electric motors (4.7.2.3).

Strength of the field. Two factors decide how strong the field is at a point:

1. The current — bigger current, stronger field. (Roughly: $B \propto I$.)
2. The distance from the wire — field weakens as you move further away. (Roughly: $B \propto 1/r$ for a single straight wire — AQA doesn't require the formula, just the qualitative trend.)

The Earth's field is about $50\,\mu\text{T}$. The field $1\,\text{cm}$ from a wire carrying $1\,\text{A}$ is about $20\,\mu\text{T}$ — comparable to the Earth's field. Hence Ørsted's observation: at a few centimetres from a battery wire, the field is strong enough to swing a compass.

If a straight wire is bent into a single loop, the field inside the loop is reinforced — the circles from each part of the loop combine. Coil the wire into many turns close together and you get a solenoid.

Field of a solenoid.

• Inside the solenoid, the field is strong and almost uniform (parallel field lines along the axis).
• Outside the solenoid, the field looks just like that of a bar magnet — leaving one end (the N pole) and curving round to enter the other end (the S pole).
• The field is concentrated in and near the solenoid; far from the coil it dies away.

Identifying the poles. Look at the solenoid from one end:

• If the current flows anticlockwise, that end is the N pole.
• If the current flows clockwise, that end is the S pole.

A handy memory aid: trace the direction of the current as N or S in capital letters with the arrows at the ends of the letters.

Field strength of a solenoid depends on:

1. The current (I) in the wire — bigger current, stronger field.
2. The number of turns per metre (n) — more turns packed in, stronger field. (Formally $B = \mu_0 n I$ inside an ideal solenoid — not required at GCSE.)
3. Whether there is an iron core inside (next section).

A coil of wire on its own makes a useful solenoid, but to make a much stronger electromagnet we insert a soft-iron core inside the coil. This is what's used in school electromagnets, electric bells, relays, scrap-yard cranes and the magnets in starter motors of cars.

Why does the core make such a difference? When the current flows, the solenoid's magnetic field magnetises the iron core by induction (4.7.1.1). The iron becomes a strong induced magnet. The total field is now the sum of:

• The solenoid's own field.
• The induced field of the magnetised iron.

The iron's field can be hundreds of times stronger than the solenoid's field alone, so an electromagnet with a soft-iron core is much more powerful than the same coil without a core.

Why specifically soft iron? Soft iron magnetises and demagnetises easily. The instant the current switches off, the iron's magnetism almost completely disappears, so:

• The electromagnet picks up a load when switched on.
• The electromagnet drops the load when switched off.

If you used steel as the core, the steel would keep its magnetism even after the current was off — the load would not be released. (This is why steel makes good permanent magnets but bad electromagnet cores.)

Three ways to make an electromagnet stronger:

1. Increase the current flowing through the coil.
2. Increase the number of turns in the coil (more turns per metre of solenoid).
3. Add (or use a stronger / better-shaped) soft-iron core.

Electromagnet vs permanent magnet — comparison for AQA exams:

Feature	Permanent magnet	Electromagnet
Field always present?	Yes	Only when current flows
Can be switched off?	No	Yes
Strength variable?	No	Yes (vary current/turns)
Direction of field reversible?	Only by physically flipping the magnet	Yes — reverse the current

The ability to switch the field on and off and to control its strength is why electromagnets are so widely used in industry.

Where AQA's exam questions go. Foundation Tier: name applications of electromagnets (scrap-yard crane, electric bell, relay, MRI machine). Higher Tier: explain why the soft-iron core works (induced magnetism + soft iron demagnetises easily), and design a comparison ('explain three improvements that would make this electromagnet stronger').

AQA expects you to be able to describe at least one application of an electromagnet — typically a scrap-yard crane, electric bell, relay or MRI scanner. Examiner reports flag candidates who say "an electromagnet has lots of uses" without giving a specific example. Pick one and explain.

Scrap-yard crane (the classic example).

• A large electromagnet is suspended from a crane.
• Current is switched on → core magnetises → strong field grips iron/steel scrap.
• The crane moves the scrap to a sorting bin.
• Current is switched off → core demagnetises → scrap drops.
• Non-magnetic metals (aluminium, copper, brass) are left behind and sorted separately.

The advantage over a permanent magnet is obvious: you can switch the magnet on and off at will.

Electric bell.

• When the bell push is pressed, current flows through the electromagnet.
• The electromagnet attracts a soft-iron armature attached to a hammer.
• The hammer hits the bell, but the movement breaks the circuit.
• With no current, the electromagnet switches off and a spring pulls the armature back.
• The circuit closes again, the electromagnet pulls the hammer, and the cycle repeats — many times per second, giving the familiar 'ringing' sound.

Relay (used in cars, central heating controllers, security systems).

• A small current in the input coil energises the electromagnet.
• The electromagnet attracts a soft-iron lever, which closes a separate, high-current circuit.
• This lets a small switch (e.g. on an ignition key) control a much larger current (the starter motor) without melting the small switch.

Loudspeaker (covered in detail at 4.7.2.4 — Physics only, Higher Tier).

• A coil sits in the field of a permanent magnet.
• An alternating audio current in the coil causes the coil and an attached paper cone to oscillate.
• The cone pushes air to produce sound.

MRI scanner.

• A superconducting solenoid produces a powerful (1.5–3 T) magnetic field.
• The field aligns hydrogen nuclei in the body; a brief radio pulse flips them and their relaxation produces detectable signals.
• The resulting image shows soft tissue (brain, muscle) far better than X-rays — no ionising radiation involved.

Maglev trains, hard-drive read-heads, particle accelerators, induction hobs, electric guitar pickups — all rely on the same principle: a controllable magnetic field generated by an electric current.