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Inductance
Storing energy in a magnetic field.
Storing energy in a magnetic field.
A coil of wire stores energy in the magnetic field around it. Inductance is how strongly the coil resists any change in its current. The coil is the mirror image of a capacitor: where a capacitor resists changes in voltage, an inductor resists changes in current.
Current through a wire makes a magnetic field. Coil the wire and the fields of all the turns add up inside the coil. Now change the current. The field has to change with it, and a changing field induces a voltage in the wire, in a direction that opposes the change (Lenz's law). The coil pushes back against a rise and props up a fall.
That is why a steady current produces no voltage across an ideal coil, and why a very fast change produces a very large one. Switch off current through a relay coil or motor and the collapsing field can spike the voltage to many times the supply. Diodes across such coils absorb that kick.
Energy is stored in the field while the current flows: ½ × L × I². It cannot vanish instantly, which is what makes the current resist sudden changes.
Inductance grows with the square of the number of turns. More turns make more field for each amp, and every turn also links more of the field made by all the others. Inductance also rises with a fatter coil (more area), and falls as the coil is stretched out (the field spreads over a longer path).
A magnetic core makes a large difference. Iron or ferrite concentrates the field, multiplying the inductance by the core's permeability, which can be many times that of air. The cost: cores saturate and lose energy at RF. See Inductors, toroids and ferrites.
For a single-layer air-core coil, Wheeler's formula (above) is good to within about 1% when the coil is at least about as long as its radius. For example, 20 turns on a 1-inch-diameter form 1 inch long gives about 6.9 µH.