Guided lesson

Electromagnetic Induction

6.15

3 guided lessons · 27 checks · 6. Magnetism and Electromagnetism → Electromagnetic Induction

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Making voltage from motion

Induction is the motor effect run backwards: move a conductor through a magnetic field — or move the field past the conductor — and a voltage appears across the conductor. No battery is needed; the motion itself is the source, and the faster or stronger the arrangement, the bigger the induced voltage.

What you know already

Motion makes electricity: sweep a wire across field lines or thread a changing field through a coil, and a voltage appears without any battery. This note covers both induction triggers, the meter-kick diagrams, and the three size factors — speed, field strength and turns.

Key points of understanding

A voltage is induced in a conductor when it moves through a magnetic field.
A voltage is also induced when the magnetic field through a conductor changes.
The induced voltage grows with the speed of the movement.
A stronger magnetic field induces a larger voltage.
No relative change between conductor and field means no induced voltage.

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1.Making voltage from motion

6.15

Key points

A voltage is induced in a conductor when it moves through a magnetic field.
A voltage is also induced when the magnetic field through a conductor changes.
The induced voltage grows with the speed of the movement.
A stronger magnetic field induces a larger voltage.
No relative change between conductor and field means no induced voltage.

First pass

Wave a wire across a magnet's field and the wire becomes a tiny battery — that is induction. Hold it still with the field steady and the battery disappears.

Analogy — The field is a river and the wire a fishing net: sweep the net and it catches voltage; leave it parked in a steady current and it catches nothing new.

Exam pointer — Both trigger phrases score: 'moves through a magnetic field' OR 'the field through it changes' — quote one precisely.

Going deeper

Induction is the cutting of field lines: a conductor sweeping across the lines, or field lines sweeping across a stationary conductor, generates the induced voltage, whose size rises with cutting speed, field strength and (for coils) the number of turns. Relative motion is the essence — a conductor carried along WITH the field lines, cutting none, induces nothing.

Analogy — Induction counts crossings: every field line the wire slices adds to the harvest, and riding parallel to the lines slices none.

Exam pointer — For 'factors' questions, list speed, field strength and turns — omitting turns is the common half-answer on coil versions.

Checks

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1.When is a voltage induced in a straight conductor?
2.A wire is held still in a steady, unchanging magnetic field. The induced voltage is:
3.The field through a stationary conductor is changed by pushing a magnet towards it. What happens?
4.The same wire is swept through the same field twice as fast. The induced voltage:
5.Which change DECREASES the induced voltage in a moving wire?
6.State the two ways a conductor or coil can have a voltage induced in it.
Written response · recall
Your answer
7.State three factors that affect the size of an induced voltage.
Written response · recall
Your answer
8.Explain why a wire carried ALONG the field lines, never cutting them, induces no voltage.
Written response · explain why
Your answer
9.A bar magnet is held still inside a large coil while the coil is slid rapidly sideways past it. State whether a voltage appears and justify the answer.
Written response · application
Your answer

2.Reading the induction diagrams

6.15

The diagram of a wire sweeping through a horseshoe field tells the whole story: the wire cuts the lines, the meter needle kicks, and the kick's direction follows the sweep direction. Reverse the sweep and the needle kicks the other way — polarity tracks the motion.

Electromagnetic induction diagram: a wire conductor sweeping through the magnetic field between two magnet poles, with an induced voltage shown across it — Moving an electrical conductor in a magnetic field to induce a potential difference

Key points

The moving wire cuts across the field lines between the poles.
The connected meter shows the induced voltage as a needle kick.
Reversing the sweep direction reverses the kick direction.
Holding the wire still between the poles gives a zero reading.
The kick lasts only while the wire is actually moving.

First pass

Wire sweeps left: needle kicks one way. Wire sweeps right: the kick flips. Wire stops: the needle falls back to zero.

Analogy — The meter is a lie detector for motion: any crossing gets flagged, the flag flips with direction, and stopping erases it.

Exam pointer — Diagram questions often ask for the needle deflection given an arrow on the wire — trace the sweep arrow first, then answer.

Going deeper

The setup is the canonical induction probe: uniform gap field, cutting wire, centre-zero meter. The kick polarity follows the sweep sense because the induced voltage changes sign with the cutting direction, and its magnitude follows the sweep speed — so the meter trace is effectively a velocity readout of the cut.

Analogy — The needle is a speedometer for field-cutting: direction of travel sets the side, speed sets the size.

Exam pointer — Zero-between-kicks is a real reading: steady position gives steady zero — do not mistake it for a broken meter.

Checks

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1.What does the meter show while a wire sweeps steadily through a field?
2.The wire sweep is reversed. The meter needle:
3.The wire stops mid-gap, still inside the field. The meter reads:
4.The sweep speed is doubled with everything else fixed. The deflection:
5.Which change makes the meter kick REVERSE without touching the wire?
6.State what the wire does to the field lines in this setup.
Written response · recall
Your answer
7.State the meter reading when the wire is held motionless between the poles.
Written response · recall
Your answer
8.Explain why the needle kick is only momentary if the wire is given one short push.
Written response · explain why
Your answer
9.A student sweeps the wire left-right-left rhythmically through the gap. Describe the meter trace and connect it to the motion.
Written response · application
Your answer

3.Coils, factors, and bigger induced voltages

6.15

Replace the single wire with a coil and every turn cuts the field, so the induced voltages add — more turns, bigger voltage. The full factor list for the induced potential difference is the exam's favourite: speed of movement, magnetic field strength, and the number of turns.

coil-induction, IGCSE & GCSE Physics revision notes — When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference

Key points

A coil induces a larger voltage than a single wire because the turns add.
The magnet entering or leaving a coil changes the field through it and induces voltage.
Faster movement of the magnet or coil induces a larger voltage.
A stronger magnetic field induces a larger voltage.
More turns on the coil induce a larger voltage.

First pass

Push a magnet into a coil: the meter kicks one way. Pull it out: the kick flips. Push it faster or add more turns: the kick grows.

Analogy — The coil is a team of wire catchers standing in the same spot — every extra catcher nets more voltage from the same passing magnet.

Exam pointer — Factor answers must name all three — speed, field strength, turns — and 'relative motion' as the trigger.

Going deeper

The coil version multiplies the single-wire effect by the turn count because the same changing field threads every turn, so their induced voltages sum. Entry and exit of the magnet give opposite polarities, faster relative motion gives larger magnitude, and the strength and turn-count levers scale the result — the complete 6.15 factor set.

Analogy — The magnet's entry is a forwards wave and its exit a backwards wave through the coil's rings — same wave, opposite signs, bigger if the wave moves faster.

Exam pointer — Polarity contrast (entering vs leaving) is frequently examined — always answer with the pair, not just one case.

Checks

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1.Why does a coil induce more voltage than a single wire in the same changing field?
2.A magnet is pushed into a coil, then pulled back out at the same speed. The second kick is:
3.Which change increases the induced voltage in a coil-and-magnet setup?
4.A magnet sits motionless inside a large coil. Why does the meter read zero?
5.Two identical coils are threaded by the same changing field; one has 100 turns, the other 500. The 500-turn coil's induced voltage is:
6.State the effect on induced voltage of each of the three factors, increasing one at a time.
Written response · recall
Your answer
7.State the polarity relationship between a magnet entering and leaving a coil.
Written response · recall
Your answer
8.Explain why more turns on a coil give a larger induced voltage.
Written response · explain why
Your answer
9.A generator demo uses a hand-cranked magnet spinning near a coil. Predict the meter trace as the crank turns steadily, and predict the change when the crank is turned twice as fast.
Written response · application
Your answer

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Back to the revision note All revision notes

Making voltage from motion

1.Making voltage from motion

6.15

2.Reading the induction diagrams

6.15

3.Coils, factors, and bigger induced voltages

6.15