Self-induction. Self-induced EMF. Inductance
Self-induction is the appearance of an induced EMF in a circuit itself when the current in it changes, because the circuit's own magnetic flux changes. The flux through the circuit is proportional to the current: Φ = L·I, where L is the inductance, which depends on the size of the circuit, the number of turns and the magnetic properties of the medium. The self-induced EMF is Eₛ = –L·ΔI/Δt: the faster the current changes, the larger the EMF. The unit is the henry (H): 1 H = 1 V·s/A, the inductance of a circuit in which a current changing at 1 A/s induces 1 V. For a long coil (solenoid) of length l, cross-section S and N turns, L = μ₀μN²S/l; a ferromagnetic core multiplies μ, and hence L, many times. Self-induction obeys Lenz's rule: when the circuit is closed the current rises slowly, and when it is opened it falls slowly; inductance gives the current «inertia» in the same way as mass does in mechanics.
Only with the teacher, using a low-voltage battery and small bulbs: connect two identical bulbs, one in series with an iron-cored coil and one with a resistor, through a switch and see which lights later. Explain which bulb lights slowly using inductance. Connecting a coil to the mains is strictly forbidden.