Electromagnetic Induction

Bring a magnet toward a coil or pull it out and the galvanometer needle moves: that is electromagnetic induction. Induced current appears only while the magnet moves, vanishes when it stops, and grows with speed and number of turns. This principle runs generators, and Lenz's law says the induced current opposes the change in flux. Here you can insert and remove a magnet and see how induced current forms.

A Moving Magnet Makes Current?

🧲 Faraday's Discovery

①Bring a magnet near a coil → the galvanometer needle moves!

②Stop moving → current vanishes

③Pull the magnet out the other way → current reverses direction

Coil-and-Magnet Experiment

Magnet position30%

Speed5

💡 Conditions for Induced Current

①Current flows only while the magnet approaches or recedes

②Faster magnet → larger induced current

③More turns of coil → larger induced current

How Generators Work

Coil angle90°

Law of Induction

Changing flux through a coil → induced EMF

Larger and faster flux changes → larger EMF

💡 Alternating Current (AC)

①A rotating coil constantly cuts a different amount of magnetic flux

②Induced EMF varies as a sine wave → AC

③Household electricity (220 V, 60 Hz) works on this principle

Lenz's Law

Induced current flows to oppose the change in flux

Magnet approaching → repels; magnet leaving → attracts

Summary

Electromagnetic Induction (Core)

Flux change → EMF → induced current

Faraday's law: changes in magnetic flux through a coil produce current

🎯 Exam Key Points

①Magnet must move to induce current (still magnet → no current)

②Faster speed, stronger magnet, more turns → larger current

③Generator: rotating coil → AC

④Lenz's law: induced current opposes the cause

⑤Motor is opposite: current → rotation; generator: rotation → current

Examples and Unit Test

Example 1

If a magnet is left still inside a coil, does an induced current flow? And how should you move the magnet to make the induced current larger?

An induced current flows only when the number of magnetic field lines through the coil changes (when the magnet moves).

If the magnet is still there is no change in field lines, so no current flows; the faster you move the magnet, the larger the induced current.

▸ No current when still / it gets larger the faster you move the magnet

An induced current flows only when the magnetic field changes, and it is larger when the change is faster.

Example 2

When a coil rotates in a magnetic field in a generator, what kind of current is produced, and by what principle?

As the coil rotates, the number of magnetic field lines through it keeps changing, producing an induced current.

This induced electromotive force forms a sine wave that periodically reverses direction, producing alternating current (AC).

▸ Alternating current (AC) / because the rotating coil keeps changing the magnetic field lines

A generator uses electromagnetic induction to turn rotational motion into AC electricity.

Unit-test style

Which statement about electromagnetic induction is correct?

①An induced current flows even when the magnet is still inside the coil

②An induced current flows when the number of field lines through the coil changes

③The slower you move the magnet, the larger the induced current

④A generator is a device that turns electrical energy into a magnet

⑤The induced current flows in a direction that aids the change in the magnetic field

▸ ② An induced current flows when the number of field lines through the coil changes

Electromagnetic induction is the phenomenon where an induced current flows when the number of field lines through the coil changes.

There is no current when the magnet is still (①), the faster it moves the larger the current (③), a generator turns motion into electricity (④), and the induced current flows so as to oppose the change in the magnetic field (⑤, Lenz law).

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