Coil Inductance
Inductance is the property of a coil or conductor that opposes changes in the current flowing through it, by storing energy in a magnetic field. When current changes, the changing magnetic field induces a voltage that resists the change. Inductance is measured in henries and is a key parameter in radio circuits, filters, transformers and power supplies. Calculating it accurately matters because the inductance of a coil, together with capacitance, sets the resonant frequency of a tuned circuit.
For a coil wound as a solenoid, the inductance depends on its geometry and material. More turns increase the inductance strongly, since it rises with the square of the number of turns. A larger cross-sectional area and a shorter, more tightly wound coil also raise the inductance, while a long, thin coil has less. Adding a magnetic core, such as iron or ferrite, multiplies the inductance by the core's relative permeability, which is why cored coils achieve large inductances in a small space. These dependencies are captured by standard formulas, with corrections for the practical shape of real coils.
Even a straight piece of wire has a small but measurable inductance, which becomes significant at high frequencies and in fast digital circuits, where it can cause unwanted voltage spikes and slow signal edges. Coil calculators apply the appropriate formula for the geometry in question, whether a single-layer air-cored solenoid, a multilayer coil, a flat spiral or a length of straight wire, and return the inductance from the dimensions and turn count entered. They can also work backwards, finding the number of turns needed to reach a target inductance, which saves trial-and-error winding when building radio coils, chokes and matching networks.
Frequently asked questions
- What is inductance?
- The property of a coil or wire that opposes changes in current by storing energy in a magnetic field, inducing a voltage that resists the change. It is measured in henries.
- How does the number of turns affect inductance?
- Inductance rises with the square of the number of turns, so doubling the turns of a coil roughly quadruples its inductance, all else being equal.
- Does a straight wire have inductance?
- Yes. A straight conductor has a small inductance that becomes important at high frequencies and in fast circuits, where it can cause voltage spikes and slow signal transitions.
