What is Inductance?
Inductance is the property of an electrical circuit that resists changes in current flow by storing energy in a magnetic field. It plays a key role in AC circuits, transformers, motors, and inductors.
What is Inductance?
Inductance is a fundamental electrical property that describes how circuits resist changes in current flow by generating a magnetic field.
✅ Opposes changes in current by storing energy in a magnetic field
✅ Measured in henrys (H), used in inductors and transformers
✅ Essential in AC power systems, filters, and motor design
Inductance occurs when a change in current flow is utilized to prevent signals with a higher frequency component from passing, while allowing signals with lower frequency components to pass. This is why inductors are sometimes referred to as "chokes," as they effectively block higher frequencies. A common application of a choke is in a radio amplifier biasing circuit, where the collector of a transistor needs to be supplied with a DC voltage without allowing the RF (radio frequency) signal to conduct back into the DC supply. To understand how inductance interacts with voltage changes in AC circuits, see our article on voltage.
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Imagine a wire 1,000,000 miles (about 1,600,000 kilometres) long. Imagine that we form this wire into a huge loop and then connect its ends to the terminals of a battery, as shown in Figure 1, to drive current through the wire. Inductors are often paired with capacitors in circuits, so it's helpful to review the basics of what is a capacitor and how they function.
If we used a short wire for this experiment, the current would begin to flow immediately and attain a level limited only by the resistance in the wire and the resistance in the battery. But because we have an extremely long wire, the electrons require some time to work their way from the negative battery terminal, around the loop, and back to the positive terminal. Therefore, it will take some time for the current to reach its maximum level. Since inductance is used to oppose changes in current, our guide on what is current electricity provides essential background.
The magnetic field produced by the loop will initially be small during the first few moments when current flows only in part of the loop. The field will build up as the electrons get around the loop. Once the electrons reach the positive battery terminal, allowing a steady current to flow around the entire loop, the magnetic field quantity will reach its maximum and level off, as shown in Figure 2. At that time, we'll have a certain amount of energy stored in the magnetic field. The amount of stored energy will depend on the inductance of the loop, which depends on its overall size. We symbolize inductance, as a property or as a mathematical variable, by writing an italicized, uppercase letter L. Our loop constitutes an inductor. To abbreviate "inductor," we write an upper-case, non-italicized letter L.
Fig. 1. We can use a huge, imaginary loop of wire to illustrate the principle of inductance.
Obviously, we can't make a wire loop measuring anywhere near 1,000,000 miles in circumference. But we can wind fairly long lengths of wire into compact coils. When we do that, the magnetic flux for a given length of wire increases compared with the flux produced by a single-turn loop, increasing the inductance. If we place a ferromagnetic rod, called a core, inside a coil of wire, we can increase the flux density and further enhance the inductance. High-frequency signals blocked by inductors are often explained using the concept of impedance, detailed in our page on what is impedance.
We can achieve values of L many times greater with a ferromagnetic core than with a similar-sized coil having an air core, a solid plastic core, or a solid wooden core. (Plastic and dry wood have permeability values that differ little from air or a vacuum; engineers occasionally use these materials as coil cores or "forms" to add structural rigidity to the windings without significantly changing the inductance.) The current that an inductor can handle depends on the diameter of the wire. But the value of L also depends on the number of turns in the coil, the diameter of the coil, and the overall shape of the coil. Many circuits involve a balance between resistance and inductance, so reviewing what is electrical resistance will add depth to your understanding.
If we hold all other factors constant, the inductance of a helical coil increases in direct proportion to the number of turns of wire. Inductance also increases in direct proportion to the diameter of the coil. If we "stretch out" a coil with a certain number of turns and a certain diameter while holding all other parameters constant, its inductance decreases. Conversely, if we "squash up" an elongated coil while holding all other factors constant, the inductance goes up. For insight into how inductors shape power quality, especially reactive power, read our overview of reactive power.
Under normal circumstances, the inductance of a coil (or any other type of device designed to function as an inductor) remains constant regardless of the strength of the signal we apply. In this context, "abnormal circumstances" refer to an applied signal so strong that the inductor wire melts or the core material overheats excessively. Good engineering sense demands that such conditions should never arise in a well-designed electrical or electronic system.
Fig. 2. Relative magnetic flux in and around a huge loop of wire connected to a current source, as a function of time.
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