Difficulty: Introductory to Intermediate | Prerequisites: Magnetic fields, bar magnets, electromagnets (see Part 1 notes)
This topic answers the question Faraday spent a decade working on: if electricity can create magnetism, can magnetism create electricity? The answer is yes, but only when something is changing. A static magnetic field near a wire does nothing. Move the magnet, change the field, or move the wire, and a current appears. This principle powers every generator on Earth and underlies transformers, induction cooktops, and wireless chargers. You need to be comfortable with magnetic field direction and the behaviour of bar magnets and electromagnets before tackling this material.
A changing magnetic field through a coil induces an electric current in that coil. Moving a magnet toward or away from a pickup coil lights a bulb; the faster the motion and the greater the change in field, the brighter the bulb. Transformers use this principle with an AC-driven electromagnet and a nearby pickup coil, and generators convert the kinetic energy of a spinning magnet into electrical energy via the same mechanism.
Electromagnetic induction
The process by which a changing magnetic field produces (induces) an electric current in a conductor. Think of it as magnetism creating electricity, the reverse of what Ørsted discovered.
Magnetic flux
A measure of how much magnetic field passes through a given area. It depends on the field strength, the area of the loop, and the angle between the field and the loop's surface. In simple terms, it is "how many field lines thread through your coil."
Faraday's law of induction
The induced voltage (EMF) in a coil is proportional to the rate of change of the magnetic flux through the coil. Faster change means more voltage. No change means no voltage.
Pickup coil
A coil of wire designed to detect changes in magnetic flux. When the flux through it changes, a current flows and can power a load such as a light bulb.
Induced current
The electric current that appears in a conductor when the magnetic flux through it changes. It is not driven by a battery; it is driven by the changing field.
Transformer
A device that transfers electrical energy from one coil (primary) to another (secondary) through a shared changing magnetic field. It works only with AC, because DC produces a steady field and no flux change in the secondary.
Generator
A device that converts mechanical energy (e.g. a spinning magnet or a spinning coil) into electrical energy via electromagnetic induction. In simple terms, it is a motor run in reverse.
Transverse motion
Movement of the magnet perpendicular to the coil's axis (side to side). Generally less effective at changing flux through the coil than longitudinal motion.
Longitudinal motion
Movement of the magnet along the coil's axis (straight through the coil). This is the most effective orientation for induction because it maximises the rate of flux change through the coil.
EMF (electromotive force)
The voltage induced in the coil by the changing magnetic flux. Despite the name, it is not a force; it is a voltage. Think of it as the "push" that drives the induced current around the circuit.
A stationary magnet next to a stationary coil produces no current. The magnetic field is present, but it is not changing through the coil, so there is no induction.
Move the magnet (or the coil) and a current appears. The key variable is the rate of change of magnetic flux through the coil.
Faster motion produces more current and a brighter bulb. Slower motion produces less.
If the coil cannot move: push the bar magnet back and forth through the coil along its axis (longitudinal internal motion). This maximises the rate of flux change.
If the magnet cannot move: slide the coil back and forth over the magnet along its axis. Same physics, same result.
From most to least effective at lighting the bulb:
C. Longitudinal Internal (magnet moves along the coil axis, through the coil): most effective. The magnet passes directly through the coil, producing the largest and fastest flux change.
B. Transverse Internal (magnet moves perpendicular to the coil axis, inside/near the coil): second. The magnet is close, so the field is strong, but transverse motion changes flux less efficiently than longitudinal.
A. Transverse External and D. Longitudinal External are less effective because the magnet is far from the coil, so the field at the coil is weaker and the flux change is smaller.
When you move a bar magnet through the coil, the direction of electron flow in the forward arc of the coil depends on which pole approaches and from which side:
North pole approaches from the left → electrons move downward.
Magnet departs to the right (south end trailing) → electrons move upward.
South pole approaches from the right → electrons move downward (same as north-from-left, because the flux change through the coil is equivalent).
Magnet departs to the left (north end trailing) → electrons move upward.
The pattern: what matters is whether the flux through the coil is increasing or decreasing. Approaching always drives current one way; departing drives it the other way. Reversing the magnet's orientation (leading with the opposite pole from the same side) reverses the electron direction for that approach.
South pole approaches from the left → electrons move upward.
Magnet departs to the right (north end trailing) → electrons move downward.
North pole approaches from the right → electrons move upward.
Magnet departs to the left (south end trailing) → electrons move downward.
A transformer consists of two coils (primary and secondary) that are not physically connected but share a changing magnetic field.
The primary coil is an electromagnet driven by an AC source. Because the current alternates, the magnetic field around the primary coil is continuously changing.
The secondary coil (pickup coil) sits in this changing field. The changing flux through it induces a current, which can power a load such as a light bulb.
Position the pickup coil as close as possible to the electromagnet, ideally aligned along the same axis so that the maximum flux passes through it.
Use the AC source (not DC) on the electromagnet. DC creates a steady field, which means no flux change in the secondary coil and therefore no induced current.
Set the AC amplitude (vertical slider) to maximum for the strongest possible field swings.
Set the AC frequency (horizontal slider) to maximum. Higher frequency means the field changes faster, which increases the induced EMF in the pickup coil.
A transformer requires a changing magnetic field. A DC electromagnet produces a constant field once the current stabilises. Constant flux through the secondary coil means zero induced EMF. This is one of the most important practical facts about transformers: they are AC-only devices.
In the PhET simulation, a faucet releases water onto a paddlewheel. The paddlewheel spins a bar magnet. The spinning magnet sits near a pickup coil.
As the magnet rotates, the magnetic field through the pickup coil changes continuously, inducing an alternating current in the coil. This current lights the bulb.
Open the faucet fully for maximum water flow, which spins the paddlewheel (and therefore the magnet) as fast as possible.
Position the pickup coil close to the spinning magnet for maximum flux change.
More coil loops (if adjustable) increase the total induced EMF.
The full story of light production in the generator, in order:
Kinetic energy of the water drives the paddlewheel.
The paddlewheel drives the motion of the bar magnet (rotation).
The rotating magnet creates a changing magnetic field through the pickup coil.
The changing magnetic field produces an induced electric current in the coil (Faraday's law).
The induced current flows through the bulb's filament, and resistance converts electrical energy into heat in the filament.
The hot filament emits light radiated from the bulb.
Note the missing element the lab asks you to add: the resistance of the filament (or equivalently, the conversion of electrical energy to thermal energy) is the step between the induced current and the light. The lab's "plot elements" leave this implied but it is worth stating explicitly.
This is how virtually all grid electricity is generated. Replace "faucet and paddlewheel" with a steam turbine (powered by coal, gas, nuclear, or geothermal heat), a wind turbine, or a hydroelectric dam, and the rest of the chain is identical: mechanical rotation → changing magnetic field → induced current → useful electrical energy.
Faraday's law (qualitative form)
Induced EMF = rate of change of magnetic flux through the coil. More flux change per second means more voltage. At the conceptual level, you can state this as: the faster the field through the coil changes, the larger the induced voltage.
Faraday's law (mathematical form, for reference)
EMF = −N × (ΔΦ / Δt)
Where N is the number of coil loops, Φ is the magnetic flux, and t is time. The minus sign indicates that the induced EMF opposes the change that caused it (Lenz's law). You may or may not be expected to use this formula depending on the course level.
Lenz's law (qualitative)
The induced current flows in a direction that opposes the change in flux that produced it. If the flux through the coil is increasing, the induced current creates a field that pushes back against the increase. If the flux is decreasing, the induced current tries to maintain it.
Energy conversion chain for a generator
Kinetic energy (water/steam/wind) → Mechanical rotation of magnet → Changing magnetic flux through coil → Induced EMF and current → Electrical energy delivered to load → Heat and light in the filament.
Every power station on the grid is, at its core, a generator: something spins a magnet inside a coil (or a coil inside a magnet), and the changing flux induces the current that reaches your wall outlet. Transformers step that voltage up for long-distance transmission (reducing current and therefore resistive losses in the wires) and step it back down for household use. Wireless phone chargers work on the same principle: an AC coil in the charging pad creates a changing magnetic field, and a pickup coil inside the phone converts it back to current. Guitar pickups, induction cooktops, and anti-theft tags in shops all exploit electromagnetic induction.
Students often think a magnetic field near a wire is enough to produce a current. It is not. The field must be changing, or the wire must be moving through the field. A static field next to a static wire induces nothing.
Students frequently confuse the direction of the induced current with the direction of the magnetic field. They are related but not the same thing. The induced current direction is determined by Lenz's law (it opposes the change in flux), not by the field direction itself.
A common error is thinking that transformers work with DC. They do not. DC produces a steady field, and a steady field means zero flux change in the secondary coil. Transformers require AC.
Students sometimes believe that the induced current depends on the total amount of flux through the coil. What matters is the rate of change of flux. A coil sitting in a very strong but perfectly steady field produces no current at all.
⚠️ "What conditions are required for electromagnetic induction?" is one of the most common exam questions. The answer: a changing magnetic flux through a conducting loop. State it precisely.
⚠️ Be ready to rank different magnet-coil arrangements by effectiveness. Longitudinal internal motion (magnet passing through the coil along its axis) always wins.
⚠️ Know the full energy conversion chain for a generator, from kinetic energy of the driving medium all the way to light from the bulb. Exams love asking you to fill in missing steps.
⚠️ Lenz's law questions are very common. If asked which way the induced current flows, identify whether the flux through the coil is increasing or decreasing, then state that the induced current opposes that change.
⚠️ Transformers only work with AC. If an exam question describes a transformer connected to a battery, the answer is that no current is induced in the secondary coil (except for a brief transient when the circuit is first connected or disconnected).
True or false: A bar magnet held perfectly still inside a coil will produce a steady current.
Answer: False. No flux change means no induced current.
Fill in the blank: The induced EMF in a coil is proportional to the ______ of magnetic flux through the coil.
Answer: rate of change.
True or false: A transformer works with both AC and DC.
Answer: False. A transformer requires AC. DC produces a steady field and no induction in the secondary.
Fill in the blank: In a generator, ______ energy is converted to electrical energy.
Answer: mechanical (or kinetic).
True or false: Moving a magnet faster through a coil produces a larger induced current.
Answer: True. Faster motion means a greater rate of flux change.
Q: A bar magnet is pushed through a coil connected to a light bulb. The bulb glows briefly. Explain why it glows and why it stops glowing once the magnet is stationary inside the coil.
A: While the magnet is moving, the magnetic flux through the coil is changing, which induces an EMF and drives a current through the bulb. Once the magnet stops, the flux through the coil is constant (even though it is large), so the rate of change is zero and no current flows. The bulb goes dark.
Q: A student claims that moving a magnet sideways past the outside of a coil (transverse external) is just as effective as pushing it straight through the coil (longitudinal internal). Is this correct? Explain.
A: This is not correct. Longitudinal internal motion pushes the magnet directly through the coil, maximising the change in flux through the loop area. Transverse external motion changes the flux much less because the magnet stays outside the coil and the field lines do not thread through the coil as efficiently. The longitudinal internal arrangement produces a significantly brighter bulb.
Q: In a generator, water from a faucet spins a paddlewheel that rotates a bar magnet near a pickup coil. Trace the complete energy conversion from the water to the light emitted by the bulb.
A: Kinetic energy of the water turns the paddlewheel (mechanical energy transfer). The paddlewheel rotates the bar magnet. The rotating magnet creates a continuously changing magnetic field through the pickup coil. By Faraday's law, this changing flux induces an alternating EMF and current in the coil. The current flows through the bulb's filament, where electrical resistance converts it to heat. The hot filament radiates light.
Q: Why does a transformer not work when connected to a DC battery?
A: A DC battery drives a steady current through the primary coil, which produces a constant magnetic field. A constant field means zero rate of change of flux through the secondary coil. By Faraday's law, zero flux change means zero induced EMF. No current flows in the secondary, and the bulb remains dark.
Q: A magnet approaches a coil from the left, north pole first, and the electrons in the forward arc of the coil move downward. Predict the direction of electron motion if the magnet instead approaches from the left, south pole first.
A: The electrons move upward. Reversing the pole that leads the approach reverses the sign of the flux change through the coil, which reverses the direction of the induced current.
This material builds directly on the magnetic fields topic (bar magnets and electromagnets). It also connects forward to AC circuits, impedance, and resonance if your course covers those. The energy conversion chain in the generator section ties into thermodynamics and conservation of energy: the electrical energy in the circuit is not created from nothing; it comes from the kinetic energy of the water (or steam, or wind) that spins the magnet.
electromagnetic induction, Faraday's law, Lenz's law, magnetic flux, pickup coil, induced current, induced EMF, transformer, generator, alternating current, AC, DC, primary coil, secondary coil, energy conversion, kinetic energy, paddlewheel generator, PhET simulation, Faraday's electromagnetic lab, longitudinal motion, transverse motion, flux change, rate of change, solenoid, Michael Faraday, Hans Christian Ørsted, electromotive force