(T6A07) Coil of Wire Component

T6A07 from the Technician License Course Section 9.1, Electronic Basics:
What electrical component is usually composed of a coil of wire?

A. Switch
B. Capacitor
C. Diode
D. Inductor

In this question we’ll cut right to the chase, and then explore a little more about the correct component.

Reviewing the response options… A switch is used to connect or disconnect an electrical circuit, and it does not resemble a coil of wire. A capacitor stores energy in an electric field created between two separated plates or surfaces as opposite charge builds on each surface. While some capacitors are packaged as coiled surfaces with an insulating layer in between, they are not coiled wire. A diode allows current to flow in only one direction, like a one-way valve in plumbing. A diode is constructed from layers of different semiconductor materials and not from coiled wire.

An inductor is a component that stores energy in a magnetic field. An inductor is usually composed of a coil of wire, frequently wound about an iron or ferrite core. The schematic inductor symbolsymbol of the inductor resembles the coiled wire.

A common physical principle is at work in the inductor: Inductance. Any time electrical current flows through a conductor like a wire, magnetic flux (magnetic lines of force) is created around the conductor. The current flow induces a magnetic field. Inversely, any time a magnetic field moves past a conductor it induces electrical current in the conductor. This is why electromagnets and electrical generators work – In an electromagnet the electrical current creates a dense magnetic field, while in a generator magnets are swept past coils of wire in which currents are induced by the magnetic field changes.

With a coil of wire the magnetic field lines may overlap or build with one another to create a dense magnetic field, just as with an electromagnet. As current flows through the coil this magnetic field builds up around the coils over a brief time to a stable magnitude or field strength, as determined by the amount of current flowing through the inductor’s coils.


Magnetic field lines build with one another along the length of the inductor coil, looping through the coils and around the outside of them. Note: Polarization of the magnetic field (N-S or direction of field lines) reverses when electric current direction reverses.

The inductor’s magnetic field is stored energy, magnetic energy that has been created from electrical energy. If the current stops flowing through the inductor, the magnetic field will begin to collapse. As the collapsing magnetic field lines move past the inductor’s coiled wire an electrical current is induced in the wire, providing a current that continues to flow in the wire until the magnetic field is depleted. In this way the magnetic energy is transformed back into electrical energy.

Using a water analogy may help grasp this a bit better. Imagine the electric current is flowing water, as in a stream. Imagine the inductor is a very massive waterwheel in the stream’s path. If the stream begins to flow suddenly from a dry gulch it will require some time and force to get the heavy wheel rotating with the speed of the stream. The inertia of the wheel resists the stream’s current until it gets up to rotational speed with the stream. Energy of the current is now stored as kinetic energy of the rotating wheel, analogous to electrical energy stored as the magnetic field.


Waterwheel Analogy for Inductors: In AC circuits, the current reverses direction regularly, analogous to a waterwheel in a stream that can’t decide which way to flow. If the frequency of current reversal is low enough, the waterwheel will store energy in its motion in one direction. If the frequency of current reversal is too high, the wheel will never begin turning sufficiently to store up energy, and it will oppose current flow in either direction.

If the stream’s current suddenly stopped flowing, leaving a static pool, the heavy waterwheel would continue to rotate for a while, pushing the water on until the rotation wound down to a stop. Like the inductor, the waterwheel has stored some of the stream’s energy in another form — rotational kinetic energy in this case rather than magnetism. As the waterwheel slows to a stop, its kinetic energy of rotation is converted back into water current, just as the magnetic field is converted back into electrical current in the inductor. Once the wheel’s stored energy is depleted and it stops turning, the current flow stops.

If the electrical current suddenly reverses direction of flow, the magnetic lines of force are induced in the opposite direction also. Thus, the magnetic field must collapse and rebuild with opposite polarity of the lines of force; i.e. the waterwheel would resist the reversed water current, but ultimately it will grind to a halt and gradually pick up rotational speed in the opposite direction with the stream’s current. During this process resistance to the flow would be offered until the new steady state is achieved in the opposite direction.

So, an inductor opposes alternating current due to the necessity to repetitiously build-collapse-build the magnetic field. But the inductor allows direct current to flow freely after the initial build up of the magnetic field by current flow when the circuit is powered.

As the frequency of current alternation increases, such as in an AC electrical circuit, the inductor’s opposition also increases. You can imagine the waterwheel just becoming a static blockage in the stream’s current if the water flow reversed direction on a regular basis too frequently, with insufficient time to start wheel rotation in either direction before the next reversal! This is why an inductor may be used as a frequency filter, or a “choke,” in AC electric circuits. The value of the frequency at which the inductor completely opposes AC current flow may be adjusted by varying the number and diameter of wire coils, and also by adjusting any material about which the coils may be wound, and thereby adjusting its value of inductance units of henries.

But you may ask, “The electrical opposition is not exactly like a stream and waterwheel, so what is creating the opposition to current flow in the inductor?” Great question and here’s the advanced topic answer for the overachievers.

Inductor Forms

Inductor common form factors. Inductors often have iron or ferrite cores that enhance the inductance value, or the ability to store energy in a magnetic field. The toroid wrapped inductor helps keep the magnetic field close about the inductor, avoiding potential magnetic coupling with other electronic components nearby.

Because energy is expended over time to “inflate” the magnetic field (to get the waterwheel turning), the inductor offers opposition initially to the current. Once the steady state magnetic field is achieved (waterwheel at stream speed), the resistance is eliminated. This initial opposition to electrical current is due to an effect called back EMF.

Electromotive force (EMF), measured as voltage, is the driving force or pressure behind current flow. As the EMF causes current to flow through an inductor, the magnetic field builds. Note: this is a changing magnetic field that will induce its own EMF in the inductor that is unique from the applied EMF that caused current flow in the first place (i.e., different from the voltage from an AC power source applied to the circuit in which the inductor resides). Due to the physical principle of induction discussed earlier and the particular polarity of the magnetic flux, the voltage induced by the inflating magnetic field will be of opposite polarity (opposite direction) as compared to the applied voltage. Hence, the name back EMF. The back EMF opposes the applied EMF.

It is the back EMF against which the applied voltage must perform work in order to inflate the magnetic field about the inductor. The back EMF must be overcome by the applied EMF in order to keep the magnetic field building with increasing current flow. Interesting effects, huh?

The inductor serves numerous purposes in electronic circuits, and coupled with a capacitor it can help create an oscillator circuit for producing RF alternating currents. Inductors are simple coils of wire, but they can perform incredible electronic tasks. The simple inductor deserves much respect!

The answer to Technician question T6A07, “What electrical component is usually composed of a coil of wire?” is D: Inductor.

Related Questions: T5C03, T5C04, T6A06, T6C10, T6D08