The 2022-2026 Technician License exam pool has a couple of questions about inductors. We'll elaborate on a question here about the inductor's energy storage:
T6A06: What type of electrical component stores energy in a magnetic field?
In this question we’ll cut right to the chase, and then explore a little more about the inductor.
Reviewing the response options… A varistor opposes the flow of direct current in an electrical circuit, and its resistance varies with the applied voltage level. It stores no energy, only converts some to heat, much like a standard resistor that is non-variable with voltage. Using a water analogy of electrical current, it would be like a constriction or an obstacle in the plumbing pipe, restricting flow to variable degrees depending upon how much water pressure is applied. Option B, capacitor, does stores energy, but it stores energy in an electric field created between two separated plates or surfaces on which opposite electrical charge builds on the opposing surfaces. Option D, diode, allows current to flow in only one direction, like a one-way valve in plumbing, but it stores no energy. None of those responses is quite correct.
Option C, inductor, is a component that stores energy in a magnetic field. An inductor is usually composed of a coil of wire (see question T6A07), frequently wound about an iron or ferrite core. The schematic symbol of the inductor resembles the coiled wire.
Just a tiny bit of physics here… 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. 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.
Because energy is expended over time to “inflate” the magnetic field (to get the waterwheel turning), the inductor offers resistance initially to the current. Once the steady state magnetic field is achieved (waterwheel at stream speed), the resistance is eliminated.
If the electrical current reverses direction 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, 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 electric circuits. In fact, that little coil that you see in some dual band VHF/UHF antennas is an inductor acting as a filter: For instance, the antenna choke does not allow higher frequency 70cm band signals to pass further up the antenna length and so it effectively shortens the antenna to a resonant length for the 70cm band. But the lower frequency 2m signals are allowed to pass along relatively unimpeded by the coil so they resonate along the entire length of the antenna. Amazing, huh?
The inductor can serve similar purposes inside 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 answer to Technician question T6A06, “What type of electrical component stores energy in a magnetic field?” is C: Inductor.
-- Stu WØSTU