Move a magnet near a coil and electricity appears from nothing but motion. As the bar magnet sweeps through the coil the magnetic flux threading it rises and falls, and Faraday's law, EMF = −N·dΦ/dt, turns that change into a voltage — swinging the galvanometer needle. Watch it flip direction as the magnet passes through: coming in the current fights to push the magnet back, going out it pulls to hold it, the give-away sign of Lenz's law. Speed the magnet up or add turns of wire and the voltage climbs — the very principle behind every generator. It all runs on your device.
Watch the needle flip direction as the magnet passes through the coil
Induced EMF0.00 V
Flux through coil—
Current direction—
Coil turns N20
Peak EMF0.00 V
It's the change that counts, not the magnet sitting still. A magnet resting inside the coil induces nothing — only a changing flux drives a current. Faraday's law, EMF = −N·dΦ/dt, says the voltage grows with how fast the flux changes and with the number of turns, so a faster magnet or more coils means a bigger swing. The minus sign is Lenz's law: the induced current always opposes the change, which is why the needle deflects one way as the magnet enters and the other way as it leaves.
How It Works
A changing flux, a voltage from Faraday's law, and a direction set by Lenz's law.
1
Flux through the coil
The bar magnet's field threads the loops of the coil, and the amount that passes through — the magnetic flux — is greatest when the magnet sits right in the middle. As the magnet moves in and out, that flux rises and falls. Nothing touches the wire, yet the coil feels the magnet through the field alone.
2
Faraday's law makes a voltage
A changing flux drives a voltage around the coil equal to EMF = −N·dΦ/dt — the number of turns times the rate the flux changes. The change is fastest as the magnet sweeps past the coil's edge, so the EMF peaks there, and it drops to zero at the very centre (where the flux is largest but momentarily steady) and at the turning points (where the magnet pauses).
3
Lenz's law sets the direction
The induced current always flows so as to oppose the change that made it, the meaning of the minus sign. Approaching, the coil repels the magnet; leaving, it attracts it. So the galvanometer needle swings one way on the way in and the opposite way on the way out — reverse the motion and you reverse the current. Turn up the speed or the turns to make the whole effect stronger.
What is electromagnetic induction?
Electromagnetic induction is the production of a voltage — and, in a closed loop, a current — by a changing magnetic field. Move a magnet toward or away from a coil of wire and the magnetic flux threading the coil changes, which drives a current even though nothing touches the wire. Michael Faraday discovered it in 1831, and it is the principle behind electric generators, transformers and induction charging. This simulator shows a magnet moving through a coil and the current it induces.
What is Faraday's law of induction?
Faraday's law states that the induced EMF equals the rate at which the magnetic flux through the circuit changes, multiplied by the number of turns: EMF = −N·dΦ/dt. The faster the flux changes, the larger the voltage, and more turns of wire multiply the effect. In this tool the flux is greatest when the magnet sits in the middle of the coil and changes fastest as the magnet sweeps in and out, so the induced EMF peaks between the centre and the ends and reads zero right at the centre.
What is Lenz's law?
Lenz's law is the minus sign in Faraday's law: the induced current always flows in the direction that opposes the change producing it. As the magnet approaches, the coil pushes back by creating a field that repels it; as it leaves, the coil pulls to hold it. That is why the galvanometer needle swings one way while the magnet comes in and the opposite way as it goes out — the current reverses because the flux is now decreasing instead of increasing.
Why does moving the magnet faster give more voltage?
Because the induced EMF depends on how fast the flux changes, not on the flux itself. A magnet moved quickly sweeps the same change of flux into a shorter time, so dΦ/dt is larger and the voltage is higher; moving it slowly spreads the same change over more time and gives a smaller voltage. Raise the speed slider here and watch the galvanometer swing further, and add turns to multiply the effect — exactly how a generator produces more voltage when it spins faster.