Electromagnetism

Currents make magnetism. From a single wire to a solenoid to an electromagnet — the switchable magnet behind every scrapyard crane, MRI scanner, and electric motor.

SPEC 4.7.2.1 • MAGNETISM & ELECTROMAGNETISM

The Big Idea — Three Steps

Every electromagnet, motor and transformer rests on a single discovery: moving charge produces a magnetic field. The lesson builds in three stages.

Straight Wire

A current in a wire creates a weak circular field around it.

Solenoid

Coil the wire — fields from every turn add together to make a strong, uniform field.

Electromagnet

Add an iron core — the field gets dramatically stronger, and switches on/off with the current.

Why It Matters

This lesson sets up everything for the rest of Module 7 — Fleming's left-hand rule, electric motors, generators and transformers all depend on currents producing magnetic fields.

Field Around a Current-Carrying Wire

Pass a current through a wire and a magnetic field appears around it. The field lines form concentric circles centred on the wire.

Spec Statement

When a current flows through a conducting wire, a magnetic field is produced around the wire. The strength of the field depends on the current through the wire and the distance from the wire.

Notation

Dot in a circle (⊙) = current coming out of the page (like the tip of an arrow). Cross in a circle (⊗) = current going into the page (like the tail feathers of an arrow flying away).

How To Demonstrate The Magnetic Effect

Three classic lab demonstrations show that a current produces a field:

Compass deflection (Oersted's experiment): place a compass next to a wire. With no current, the needle aligns with Earth's field. Switch the current on, and the needle deflects — proving the wire produces its own field. Reverse the current → deflection reverses.

Iron filings: push a vertical wire through a horizontal sheet of card; sprinkle iron filings on the card; pass a current — the filings line up in concentric circles around the wire.

Plotting compasses: place several small compasses around a current-carrying wire; each needle aligns tangentially with the field at its position — tracing out the circular field pattern.

The Right-Hand Grip Rule

How do we know which way the field circles point? Use the right-hand grip rule.

The Rule

Grip the wire with your right hand so that your thumb points in the direction of the conventional current.

Your fingers curl in the direction of the magnetic field around the wire.

Applying the Rule

Current going out of the page (⊙) → fingers curl anticlockwise as seen from the dot end

Current going into the page (⊗) → fingers curl clockwise as seen from the cross end

Reverse the current direction → field direction reverses

Right Hand, Not Left

Always the right hand. The left hand is for Fleming's left-hand rule (motor effect — different topic, different rule, comes in 7.03). Mixing them up is a common exam mistake.

What Affects the Field Strength?

Two factors control how strong the field is at any point around a current-carrying wire.

Factor 1 — Current

Larger current ⇒ stronger field.

Increase the current through the wire and the field at every point around it gets stronger.

Factor 2 — Distance

Greater distance from the wire ⇒ weaker field.

The closer you are to the wire, the stronger the field. Move away, and the field falls off quickly.

Why The Field Is So Weak

The field from a single wire is normally too weak to see useful effects with a compass or paperclip — typical lab currents produce fields of just a few μT. That's the same order as Earth's field. The fix is to shape the wire into a coil.

What is a Solenoid?

The trick to making a useful field is to coil the wire — and lots of it.

Solenoid

A long coil of wire. Shaping a wire into a solenoid increases the strength of the magnetic field created by the current.

Why It Works

Each loop of the solenoid is its own little wire producing a field. When the loops are stacked side-by-side, the fields between loops cancel out, but the fields along the central axis add together. The result: a strong, focused field through the middle of the coil.

Two Key Properties Inside a Solenoid

The field is strong — much stronger than a single wire carrying the same current

The field is uniform — same strength, same direction, throughout the inside

Solenoid Field Pattern

The field around a solenoid has a similar shape to a bar magnet's. One end behaves as a N pole, the other as a S pole — and which is which depends on the current direction.

Three Things To Notice

Inside: field lines are parallel — strong and uniform

Outside: field lines curve from one end to the other, just like around a bar magnet

Ends act as poles: one end as N, one as S (depending on current direction — apply the right-hand grip rule to one loop to work out which)

Examiner Alert — Drawing the Field

Drawing the solenoid field is a classic 2-mark task. Most marks are lost the same way every year.

"Field lines going in, (through) and out of the solenoid (1 mark). Arrow(s) in correct direction (1 mark)." AQA Mark Scheme — Paper 2H, June 2019, Q04.1

Reading The Mark Scheme

The brackets around "(through)" mean it's optional — the examiner accepts answers showing only field lines visible outside the solenoid (like a bar magnet's field). What is not optional: lines must show in at one end and out at the other, with correct arrowheads.

Weak (loses marks)

Field lines only around the outside (no lines through the middle)

No arrowheads on any line

Arrowheads inconsistent (some pointing one way, some the other)

Lines crossing each other

Strong (full marks)

Field lines go in one end, through the middle, and out of the other

Arrowheads on every line, all consistent with one pole as N, one as S

Outside lines curve back like a bar magnet's

No crossing

Mark Scheme Allowance

The mark scheme accepts a bar-magnet shaped field (curves above and below the solenoid) — the key requirements are lines through the inside and correct, consistent arrows.

Three Ways to Strengthen a Solenoid

To make a solenoid's field stronger, you have three independent controls.

More Current

Push more current through the wire and the field gets stronger.

More Turns

More turns per metre means more field contributions added together — same current, stronger field.

Iron Core

Put a soft iron core inside the coil and the field strength multiplies dramatically.

Note On The Relationship

"More current ⇒ stronger field" is the safe wording for the exam. Don't claim the relationship is strictly proportional — at high currents the iron core saturates and the field stops increasing. Say "increases", not "doubles" or "is proportional to".

What is an Electromagnet?

Take option 3 from the previous slide seriously — drop a soft iron rod through the middle of a solenoid — and you've built an electromagnet.

Electromagnet — Spec Wording

An electromagnet is a solenoid with an iron core. Adding the iron core increases the strength of the magnetic field of the solenoid.

Three Parts

Coil of insulated wire (the solenoid) — provides the current path

Soft iron core — magnetises easily, multiplies the field

Power supply / battery — provides the current; switch off and the magnet turns off

Why The Iron Core Works

This is a direct call-back to induced magnetism from Lesson 7.01.

The Mechanism

The current in the solenoid produces a magnetic field.

The iron core sits in that field — so the iron becomes an induced magnet.

The induced iron now produces its own magnetic field.

The two fields add together — total field is much stronger than the solenoid's alone.

Why Soft Iron Specifically

Soft iron magnetises and demagnetises easily — exactly what we want for a switchable electromagnet. As soon as the solenoid current stops, the iron loses its induced magnetism and the magnet "turns off". A hardened steel core would retain magnetism even after the current stopped, ruining the switch-off behaviour.

Switchability & Applications

The big advantage of an electromagnet over a permanent magnet is control — turn the current on, the magnet works; turn the current off, it doesn't. Four headline applications:

ApplicationHow It Uses Switchability

Scrapyard cranesSwitch on → lift heavy iron/steel objects. Switch off → drop them in a new location. No mechanical grab needed.

Magnetic door locksPower on → door held shut by the strong field. Power cut → magnet drops, door opens automatically (fail-safe in fires).

MRI scannersUse enormous superconducting electromagnets to produce the field needed to image the body. Strong, switchable, and adjustable.

Electric motorsEvery electric motor — fans, drills, EVs — uses electromagnets to convert electrical energy into rotation. Foundation of the next lesson.

How An Electromagnet Picks An Object Up

A three-step mechanism that every electromagnetic device exam question rewards. (1) Current flows through the solenoid → produces a magnetic field. (2) The iron object placed in the field becomes an induced magnet. (3) Unlike poles between the electromagnet and the induced object attract → the object is pulled in (or held in place). Switch the current off → no field → no induced magnet → the object falls or releases.

The Common Thread

All four applications above exploit the same feature: an electromagnet's strength can be controlled by the current, and it can be switched off completely. A permanent magnet can do neither.

Examiner Alert — Iron Core Misconceptions

One of the most common wrong answers in Module 7. From a flagged Q08.1, Jun 2023 Paper 2:

Wrong

"The iron core is a good conductor of electricity."

The iron core does not carry the current. The current flows through the copper wire of the coil. The core is inside the coil — it never carries the current at all.

Right

"The iron core concentrates the magnetic field."

The iron is induced to become a magnet by the solenoid's field, adding its own magnetism to the total. It strengthens the field, not the current.

The Core Vocabulary

"Conductor of electricity" describes a material that carries current — like copper. "Magnetic / magnetisable material" describes a material that responds to a magnetic field — like iron. Don't mix them up. An iron core is the second, not the first.

Practice Question 1

A straight vertical wire carries a current flowing upwards. A compass is held to the east of the wire (with the wire to the compass's west).

(a) State the direction in which the magnetic field at the compass points. 1 mark

(b) Describe one change to the experiment that would make the deflection of the compass needle larger. 1 mark

a

Apply the right-hand grip rule: point the right thumb upwards (direction of current). The fingers curl anticlockwise when viewed from above. Tracing this anticlockwise circulation around the wire — north of wire it points west, west of wire it points south, south of wire it points east — at the east of the wire, the field points NORTH. Answer: the field at the compass points north (horizontal).

b

Either: increase the current in the wire, or move the compass closer to the wire. Both increase the field strength at the compass position, giving a larger deflection.

Practice Question 2

Two short iron rods are placed side-by-side, parallel to each other, inside a horizontal solenoid. When the switch in the solenoid circuit is closed, the two iron rods are observed to move apart.

Explain why the iron rods move apart.3 marks

P

Closing the switch sends a current through the solenoid, which produces a magnetic field along the inside of the solenoid.

E

Each iron rod sits in this field, so each one becomes an induced magnet. Because both rods are aligned in the same direction along the same field, they are magnetised with the same polarity at each end (e.g. both with N at the right-hand end and S at the left-hand end).

E

Since the rods lie parallel to each other, like poles sit alongside like poles all along the length of the rods (N next to N, S next to S). Like poles repel, so the rods push apart.

Common Errors to Avoid

Three high-frequency mistakes from examiner reports on this topic.

Students Often WriteCorrect VersionWhy It Matters

"The iron core is a good conductor of electricity""The iron core concentrates the magnetic field" — current flows in the copper coil, not the ironDifferent physics — confuses conduction with magnetisation (Jun 2023 P2 Q08.1)

Drawing solenoid field lines only outside the coil, or without arrowsField lines go in, through and out of the solenoid, with consistent arrowheads on every lineLoses the standard 2 marks on solenoid-drawing questions (Jun 2019 P2 Q04.1)

"Increasing the current doubles the field" / "the field is proportional to the current""Increasing the current increases the field strength"Iron-cored electromagnets saturate at high currents — exact-proportion claims are wrong

Checkpoint

You should now be able to do all of the following without prompting.

You Can Now

State that a current in a wire produces a magnetic field — concentric circles around the wire

Apply the right-hand grip rule to find the field direction

State the two factors affecting field strength around a wire (current, distance)

Define a solenoid — long coil; field is strong and uniform inside, bar-magnet-like outside

Draw the solenoid field pattern — through the inside, looping outside, all arrows consistent

List the three ways to strengthen a solenoid — more current, more turns, iron core

Define an electromagnet — a solenoid with an iron core

Explain the role of the iron core — becomes an induced magnet, adds to the field (does not conduct current)

Name applications — scrapyard cranes, magnetic door locks, MRI scanners, electric motors

Coming Next — 7.03 (Higher Tier)

Fleming's left-hand rule and F = BIl. A current-carrying wire in a magnetic field experiences a force — the motor effect. We'll cover the rule, the equation, and direction conventions.