Definition

An electromagnet is a magnet that can be switched on and off, made by wrapping wire around a piece of iron and running electricity through it, and junkyard cranes use giant electromagnets to lift whole cars, since turning the current off drops the car right where it is wanted. It is a temporary magnet created by passing current through a coil of wire, usually wound around an iron core that concentrates and strengthens the resulting field, disappearing when the current stops and reversing its poles if the current reverses, with strength increasing with more current, more turns of wire, and a better core material, powering motors, speakers, doorbells, MRI machines, and scrapyard cranes. A long solenoid with $n$ turns per meter carrying current $I$ produces a nearly uniform interior field $B = \mu_0 n I$, and inserting a ferromagnetic core of relative permeability $\mu_r$ multiplies this by hundreds to thousands for soft iron until the core saturates near $1.5$ to $2\ \text{T}$; the coil stores energy $U = \tfrac{1}{2}LI^2$ in its inductance, its self-induced emf resists changes in current, producing the spark seen when an inductive load is switched off, and superconducting electromagnets carry persistent currents with no resistive loss, reaching fields above $10\ \text{T}$ in MRI scanners and particle accelerators.