What you are looking at
A bar magnet moves along the axis of a wire coil connected to a galvanometer (the needle dial). As the magnet
moves, watch the needle swing and the current arrows light up around the coil — then notice they
reverse when the magnet turns around. Hold the magnet still, even right inside the coil, and
the current drops to zero.
Faraday's law
What matters is not the magnetic field itself but how fast the
flux through the coil is
changing. Flux Φ is the amount of field threading the loops; it's largest when the magnet is inside
the coil. Faraday's law says the induced voltage (EMF) equals the rate of change of the total flux linkage:
EMF = − N · dΦ/dt
The N is the number of turns — more loops, more flux to change, more voltage. Because it depends on the
rate, a fast wiggle induces a big EMF and a slow one barely anything, and a stationary magnet
induces nothing at all. Turn up the wiggle speed and watch the peaks on the graph grow.
Lenz's law — the minus sign
That minus sign is
Lenz's law: the induced current always flows in the direction that
opposes the change causing it. As the magnet approaches and flux rises, the coil pushes back
(creating a field to repel the magnet); as it leaves and flux falls, the current reverses to try to hold the
flux up. This is conservation of energy in disguise — you have to do work pushing the magnet, and that work
becomes electrical energy. It's exactly why a generator gets harder to turn as you draw more current.
Things to try
Wiggle fast, then slow, and compare the EMF peaks. Add more turns and watch the whole signal scale up.
Switch to "Drag magnet" and move it yourself: push it in and out to make current, or park it inside the coil
and see the current vanish — proof that only
change counts.