# What is an inductor?

Photo by oskay

Question: What is an inductor? What can you use it for?

Answer: An inductor is a device that resists a change in current. For another question, I explained how a capacitor resists a change in voltage. If you put the two together in parallel, you create a resonant circuit. Energy stored as voltage in a capacitor will want to flow through the inductor as current, but the inductor resists this. An initial current will be low, but will then grow with time. The voltage meanwhile falls to zero. At this point the inductor has stored energy in a current. So continuing, this current flows into the capacitor, with the voltage starting at zero and eventually growing to be the reverse/negative voltage that we started at. Except for losses to resistance in the components, the process continues in reverse.

This is an oscillation in a resonant LC circuit. The size of the inductor L and capacitor C effect the natural frequency of the oscillation. If you put an external oscillating signal to an LC circuit, like pushing on a swing, the closer the natural oscillation frequency is to your external signal frequency, the larger the voltage/current signals can flow through it. When you go farther away in frequency, the signal is lower. By adjusting the L or C, you can tune to new resonant frequencies.

A tunable LC circuit is useful for selecting a particular frequency radio station with a radio receiver circuit.

So how does an inductor resist a change in current? A current in a wire creates a magnetic field. The strength of the magnet is proportional to the current.The magnetic field from one wire and a battery tends to be small, so it is hard to notice. But if you make the wire longer, and loop it many times wire next to wire, you are overlapping the magnetic fields in a way that reinforces the strength of the magnet. If you have one wire with 1 amp next to another wire with 1 amp, you will get a magnetic field equivalent to one wire with 2 amps. So when you loop the wire, you increase the magnet strength by the number times you have loops.

When a magnetic field changes near a wire, it induces a current in that wire. An inductor is a device made up of wire wound in overlapping loops. When a current changes in an inductor, the magnetic field from that current changes too. That changing magnetic field multiplied by all those loops creates a current of its own reinforcing the current and holding it to the pre-change current. That is what it means to resist a change in current. It does change, but it is slowed by the act of changing. Try to change it fast, and the resistance to change is stronger. The more loops in the inductor, the larger the inductance there is, and the more resistance to change in current.

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