Electromagnetic induction. Induced electromotive force. Faraday's law
In 1831 Faraday found that a current appears in a closed circuit when the magnetic flux through it changes; this phenomenon is electromagnetic induction and the current is the induced current. The current exists only while the flux is changing: if the magnet rests inside the coil the galvanometer reads zero; it does not matter how the flux is changed (moving the magnet, moving the coil, changing a current, rotating the circuit). The Faraday–Maxwell law: the induced EMF in a circuit equals the rate of change of the magnetic flux, Eᵢ = –ΔΦ/Δt, and for a coil of N turns Eᵢ = –N·ΔΦ/Δt (unit: volt, 1 V = 1 Wb/s). If the circuit has resistance R, the induced current is I = |Eᵢ|/R. The minus sign expresses Lenz's rule: the induced current has such a direction that its own magnetic field opposes the change of the flux that produced it (the field opposes the flux if it grows and supports it if it decreases). In an open (cut) ring an EMF may appear but no current flows, so the ring does not react to the magnet.
A demonstration shown only by the teacher: in a circuit of a coil and a sensitive galvanometer (or voltmeter) watch the pointer as a magnet is pushed into and pulled out of the coil. Explain how the deflection differs for fast and slow movement and why a resting magnet gives no current. Strong (neodymium) magnets can pinch fingers and damage devices and cards, so handle them carefully.