Induced potential (HT, physics only) — AQA GCSE Physics

Module 7 · Lesson 7.05 · Spec 4.7.3.1

Induced Potential

Higher Tier only

Physics only

The generator effect — flip the motor effect upside down. Move a wire through a magnetic field, get a potential difference for free. Every power station on Earth runs on this idea.

From Motor → Generator

In Lesson 7.04 you saw the motor effect: pass a current through a wire in a magnetic field → the wire feels a force. The current is the input, motion is the output.

Now we run that backwards. The generator effect takes motion as the input and gives a potential difference as the output.

Motor effect

Current + field → force / motion

Electrical → kinetic

Generator effect

Motion + field → induced pd

Kinetic → electrical

By end of lesson

Define the generator effect · list the three factors that increase induced pd · state the direction rule (opposes the change) · explain why a stationary wire has no induced pd · apply all of this to a written exam answer.

Definition · Generator Effect

Spec definition · 4.7.3.1

If an electrical conductor moves relative to a magnetic field, or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced. This is called the generator effect.

Two equivalent ways to describe the same thing:

Conductor moves through field — e.g. wire pulled past a magnet, coil rotating between magnet poles

Field changes around conductor — e.g. magnet pushed into a stationary coil, AC current in a nearby coil

Mark-scheme phrasing to memorise: "the wire cuts through magnetic field lines". That word — cuts — is what examiners look for.

The Classic Demo

Move a straight wire through the field between the poles of a magnet. The wire is connected to a sensitive ammeter (a galvanometer). Watch what happens.

Active wire moves through field → cuts field lines → pd induced → induced current flows around the complete circuit → galvanometer deflects

Observation: while the wire moves, the galvanometer needle deflects. Stop moving the wire — needle returns to zero. Reverse the direction of motion — needle deflects the other way.

What You Need to Induce a PD

The generator effect needs every one of these together:

IngredientWhy it matters

A conductorYou need somewhere for charges to be pushed — usually a wire or a coil

A magnetic fieldField lines are the "lines being cut" — no field, nothing to cut

Relative motion or a changing fieldEither the conductor moves, or the field changes — there must be change

(For a current:) a complete circuitpd is always induced if motion happens; current only flows if there's a closed loop

Subtle but examined

A pd is induced even with an open circuit. A current only flows when the wire is part of a complete circuit. The spec is explicit about this distinction.

The Crucial Corollary

If nothing changes — no pd.

No change → no induced pd

A wire sitting stationary in a steady magnetic field has zero induced pd. The field is there, the wire is there — but nothing is changing. No field lines are being cut. No pd is induced.

This trips students up constantly. Examiners give a diagram showing a wire in a field and ask "what is the induced current?" — the trick answer is zero if the wire isn't moving.

Induces pd

Wire moves through field · magnet pushed into coil · field strength changing · coil rotates in field

No pd

Wire stationary in steady field · magnet resting inside coil · coil at rest between poles

Factors that Increase Induced PD

Spec requires you to recall the factors affecting the size of the induced pd. Make any of them bigger → induced pd gets bigger.

Faster Motion

Move the wire (or magnet) faster → field lines are cut more rapidly → larger induced pd

Stronger Field

Use a stronger magnet → field lines are closer together → more lines cut per second → larger induced pd

More / Closer Turns

A coil with more turns of wire — or turns packed closer together — gives a larger total induced pd

Exam trap · Be specific

Mark schemes (Jun 2018 Q3.9) explicitly state "bigger magnet" is insufficient and "bigger coil" is insufficient. You must say stronger magnet (or heavier magnet), and more turns (or turns closer together). "More coils of wire" is also not accepted — say "more turns".

Stock 2-mark question

"Give two ways to increase the reading on the voltmeter." Pick any two of: faster motion, stronger magnet, more turns on the coil, turns closer together. Don't write "use a bigger wire", "use a thicker wire", or "increase the battery voltage" — there's no battery, and wire size doesn't matter.

Direction Rule · The Induced Current Opposes the Change

Spec also requires the factors affecting the direction of the induced pd/current. There's one rule:

Direction rule · 4.7.3.1

An induced current generates a magnetic field that opposes the original change — either the motion of the conductor or the change in the magnetic field.

This is Lenz's law — the name isn't required at GCSE, but the principle is. Three consequences worth knowing:

Reverse the direction of motion → reverse the direction of the induced pd / current. Pull the wire up instead of down → galvanometer deflects the other way.

Reverse the field (swap N and S) → reverse the direction of the induced pd / current.

Push and pull a magnet in and out of a coil → induced current reverses every time — this is the foundation of AC.

Worked Example A · Magnet Pushed Into a Coil

A bar magnet's north pole is pushed into one end of a coil connected to a galvanometer.

Magnet N pole pushed into coil → near end of coil becomes induced N → repels the incoming magnet (opposes motion)

Step 1

Identify the change: the magnet's N pole moves towards the coil → the magnetic field through the coil is increasing.

Step 2

Apply the direction rule: the induced current must oppose this change. It creates a magnetic field in the coil that pushes back on the magnet.

Step 3

Conclude: the near end of the coil becomes an induced N pole (same as the incoming pole) → repels the magnet → opposes the motion.

Step 4

Reverse case: if you pull the magnet out, the field is decreasing → induced current reverses → near end of coil becomes an S pole → attracts the magnet → still opposes the motion.

Why Generators Produce AC

Put the direction rule on a coil that rotates between magnet poles instead of one that translates. Watch what happens to the induced pd in each half-turn.

Each half-turn the coil cuts field lines in the opposite direction → induced pd alternates → AC output

This is the key insight that powers alternators (the next lesson). Rotating motion + the direction rule = alternating pd, automatically.

Worked Example · 3-Mark Explain Question

Question3 marks

A student pulls a straight wire downward between the poles of a permanent magnet. The wire is connected to a sensitive ammeter. Explain how a current is induced in the wire.

Mark 1

The wire moves through the magnetic field, cutting (the) magnetic field lines.

Mark 2

A potential difference is induced across the ends of the wire (generator effect).

Mark 3

Because the wire is part of a complete circuit, a current is induced in the wire.

Why this works

Every mark-scheme bullet for this style of question rewards a separate physical step: cutting, inducing, circuit-completes-the-current. Three steps → three marks. Skip any one and you cap at 2/3.

Application · Oscillating Magnet in a Coil

The recurring exam scenario: a bar magnet on a spring is set oscillating above a coil. A data logger or oscilloscope records the induced pd over time. Four classic question patterns sit on this one diagram.

Magnet oscillates → field through coil changes → induced pd alternates & decays (damped). At each turning point the magnet is momentarily stationary → induced pd = 0.

Question patternMark-scheme answer

Why does the magnet's motion induce a pd?The magnet moves relative to the coil , so the magnetic field through the coil changes → pd is induced

Why is the induced pd alternating?Because the magnet changes direction (Jun 2018 MS: 1 mark, this exact phrasing)

When the induced pd is zero, the magnet is…?Stationary — at each turning point of the oscillation, momentarily not moving → no change in field → no induced pd

Why does the pd decay over time?The oscillation is damped (energy dissipates) → magnet moves more slowly → smaller induced pd

Weak vs Strong · Same Question, Different Marks

Both answers are about a wire moving in a magnetic field. One earns 1/3, the other earns 3/3.

Weak · 1/3

"The magnet pushes the current along the wire. The faster you move the wire the bigger the current."

Why it loses marks: there is no current in the wire to start with. The wording assumes a current that doesn't exist. "Cutting field lines" is missing entirely. The factor sentence is OK but only earns the size-factor mark, not the mechanism marks.

Strong · 3/3

"The wire moves through the magnetic field, cutting magnetic field lines. This induces a potential difference across the ends of the wire. Because the wire is part of a complete circuit, an induced current flows."

Why it works: three distinct physical steps, each matching one mark-scheme bullet. Uses the verb cuts, the noun phrase induced pd, and the condition complete circuit — exactly the mark-scheme language.

Examiner Alert · Paper 2 Jun 2024 Q8.2

Real performance data

A generator-effect explanation question on Paper 2, June 2024. The mark scheme required candidates to state that the wire cut through magnetic field lines, that a pd was induced, and that a current flowed because there was a complete circuit.

8%

scored full marks (3/3)

37%

scored at least 1 mark

63%

scored zero

Many candidates failed to mention that the wire was cutting through magnetic field lines, or wrote as though a current already existed in the wire that the magnet was "pushing along". — AQA Examiner Report style summary, Jun 2024 Paper 2

Lock-in: any answer about induced pd must contain the words "cuts (the) magnetic field lines". That phrase is what flips a 1-mark response into a 3-mark response.

Examiner Alert · Motor or Generator Effect?

Another Paper 2 Jun 2024 question (Q8.1) showed a wire moving in a magnetic field and asked candidates to identify which effect was being demonstrated.

~50% correctly identified generator effect

~50%said "motor effect" or "both"

Decision rule

Read the question stem and look for the input:

"a current flows in the wire…" → motor effect (input = current, output = force/motion)

"the wire is moved…" or "a magnet is pushed…" → generator effect (input = motion, output = induced pd)

Memory aid

Motor → Movement is the output.Generator → "Get electricity" is the output. Match the question's output to the effect name.

Common Errors

Students often writeCorrect versionWhy it matters

"The magnet pushes the current along the wire"The wire has no current to begin with ; motion through the field induces a pd, which drives current in a complete circuitWrong mechanism — assumes a pre-existing current

"A stationary wire in the field has an induced pd"No motion → no induced pd. The field must be cut, or the field strength must changeMisses the central principle

"Faster motion produces a smaller induced pd"Faster motion produces a larger induced pdFactor reversed

"Use a thicker wire to increase the pd"Wire thickness doesn't matter. Increase speed , field strength , or number / density of turnsLists a non-factor — earns 0

"Use a bigger magnet"Be specific: stronger magnet (or heavier magnet). "Bigger" alone is insufficient per Jun 2018 MSWording too vague — must specify the relevant property

"Use a bigger coil" / "more coils of wire"Be specific: more turns on the coil , or turns packed closer together . "More coils of wire" is explicitly not accepted (Jun 2018 MS)Wording too vague — must use the spec term "turns"

"The induced current is in the same direction as the motion"The induced current creates a field that opposes the change causing itDirection rule reversed

✓ Checkpoint & Where Next

You can now

Define the generator effect using mark-scheme language (conductor moves relative to field OR field changes around conductor → induced pd)

State the difference between an induced pd (always when there's motion) and an induced current (only with a complete circuit)

List the three factors that increase induced pd: faster motion, stronger field, more turns

State the direction rule : induced current creates a field that opposes the original change

Explain why a stationary wire in a steady field has no induced pd

Apply the principles in context — write a 3-mark mark-scheme answer using cuts , induced pd , complete circuit

Next lesson · 7.06

Three real-world devices that use the generator effect: the alternator (AC, uses slip rings), the dynamo (DC, uses a split-ring commutator), and the microphone (sound → electrical signal). Same physics — just rotation instead of translation.