Section Overview
The capacitor stored energy in an electric field and fought changes in voltage. Its mirror twin, the inductor, stores energy in a magnetic field and fights changes in current. Physically it is the simplest component in this chapter — just a coil of wire, sometimes wound around a core — but its behavior is the exact opposite of the capacitor's at every turn: it passes steady DC freely while opposing AC, where a capacitor blocks DC and passes AC. This section introduces what an inductor is, the property of inductance and its unit the henry, the "opposes changes in current" behavior that defines it, and a hazard the capacitor's mirror brings with it — the voltage spike that appears when you suddenly interrupt an inductor's current.
Why This Matters
Inductors are less common than capacitors on a typical small circuit board, but where they appear they are usually doing something important: filtering and storing energy in switch-mode power supplies, blocking noise as chokes, tuning radios, and — as coupled inductors — forming the transformers that convert voltages. Understanding the inductor completes your picture of the two energy-storing components, which together underlie filters, oscillators, and resonance (the next sections). And the inductor introduces a specific, real hazard every technician must respect: an inductor carrying current does not want to stop, and forcing it to — by opening a switch — can produce a voltage spike many times the supply, enough to arc across contacts, destroy a transistor, or deliver a shock. Knowing why that happens is part of working safely around relays, motors, and coils.
Required Prerequisites
- Current — The Flow of Electrons — current is what creates an inductor's magnetic field, and what the inductor resists changing.
- Power and Energy in DC Circuits — an inductor stores energy rather than dissipating it, so the energy ideas from Chapter 1 apply.
- What Is a Capacitor? — the inductor is the capacitor's mirror; every idea here has a capacitor counterpart, so read that section first.
Recommended Consumables
No consumables required. The exercises are pen-and-paper plus examining any inductors or coils you may have on hand.
Recommended Practice Hardware
- A few inductors or coils if you have them — a small ferrite-core choke, an air-core coil, or even the coil inside a relay or a small transformer shows the range
- A magnifier or good light to read the markings on small inductors (values are often coded, like resistors)
- No powered circuit is needed for this section; you are only looking at and identifying inductors, not energizing them
You do NOT need to power or switch any coil in this section — that is where the inductive-spike hazard lives, and it is saved for the later hands-on chapter that teaches it safely.
Real-World Applications
Open a switch-mode power supply and the bulky coil beside the main chip is an inductor, storing and releasing energy many thousands of times a second to convert one voltage to another efficiently. A choke in series with a power line blocks high-frequency noise while passing the DC or low-frequency power — the mirror of a capacitor's noise shunt to ground. Radios use inductors with capacitors to tune to a station. And a transformer — two coils sharing a magnetic field — steps voltages up or down and is really a pair of coupled inductors. Inductors show up wherever a circuit needs to store energy magnetically, oppose a changing current, or couple energy through a magnetic field.
Common Challenges
- Getting the mirror backwards. The capacitor blocks DC and passes AC; the inductor does the opposite — it passes DC and impedes AC. It's easy to mix these up; anchoring each to "what it resists" (capacitor resists voltage change, inductor resists current change) keeps them straight.
- Thinking a coil is "just a wire." To steady DC, an ideal inductor really is just a low-resistance wire — but the instant the current tries to change, the coil fights back with an induced voltage. Its whole personality only shows up when things change.
- Underestimating the switch-off spike. A charged capacitor's hazard is obvious (it holds voltage). An inductor's is sneakier: it looks harmless carrying steady current, but interrupt that current and it can produce a startlingly high voltage for an instant.
Safety Notes
Risk Level: Low. This section is conceptual — you examine and identify inductors, you don't energize or switch them. But the inductor brings a real hazard you must understand from the outset, so read the callouts.
Professional Tips Before Starting
- Anchor the mirror in one phrase: capacitor resists voltage change, inductor resists current change. Everything else — which passes DC, which passes AC, which stores energy in which field — follows from that one contrast.
- When you see a coil in a circuit that switches current (a relay, a motor driver), immediately look for its protection component — usually a diode across the coil. Its absence, or its failure, is a common cause of damaged switches and drivers.
- Read an inductor's value and its current rating together, just as you did voltage rating for capacitors and power rating for resistors. An inductor pushed past its rated current can saturate its core and stop behaving like an inductor.
A Coil That Stores Energy in a Magnetic Field
What an Inductor Physically Is
An inductor is a coil of wire, often wound around a core of magnetic material such as iron or ferrite. When current flows through the coil, it creates a magnetic field around and through it — the same effect that makes an electromagnet. That magnetic field is where the inductor stores energy: as long as current flows, energy is held in the field, and when the current changes, the field changes with it. This is the direct mirror of the capacitor, which stored energy in the electric field between its plates. Two components, two fields, two ways to store energy without dissipating it as heat.
The stored energy depends on the inductance and the current:
E = ½ × L × I²
which is the exact mirror of the capacitor's E = ½ × C × V² — just swap inductance for capacitance and current for voltage. A 10 mH inductor carrying 2 A, for instance, stores E = ½ × 0.01 × 2² = ½ × 0.01 × 4 = 0.02 joules (20 mJ).
Inductance and the Henry
Inductance measures how strongly a coil opposes a change in the current through it — equivalently, how much magnetic field it produces per amp of current. More inductance means a stronger reaction to any change in current. Its unit is the henry (H), named for Joseph Henry. Like the farad, the henry is a fairly large unit in practice: many inductors are measured in millihenries (mH) or microhenries (µH), where:
1 H = 1000 mH = 1,000,000 µH
What makes one coil more inductive than another is its physical construction: more turns of wire increase inductance — and turns count especially strongly, since inductance rises with roughly the square of the number of turns — a core of magnetic material concentrates and strengthens the field so it dramatically increases inductance compared to an air core, and the coil's geometry (how tightly and over what area it's wound) matters too. More turns, a magnetic core, tightly wound: more inductance — the physical mirror of "bigger plates, closer together, better dielectric" for a capacitor.
The Defining Behavior: Opposing Changes in Current
Here is the heart of the inductor, and the exact mirror of the capacitor. A capacitor opposes changes in voltage: its voltage can't jump instantly, because charge takes time to move. An inductor opposes changes in current: the current through it can't jump instantly, because the magnetic field takes time to build or collapse. When the current through an inductor tries to change, the coil induces a voltage that fights that change — a rising current is held back, a falling current is propped up. (This is Lenz's law: an induced voltage always opposes the change that caused it.)
Follow that through to the two cases that matter:
- Steady DC: once a steady current is established and no longer changing, the inductor has nothing to oppose — it's just a coil of wire with low resistance, and current flows freely. An inductor passes steady DC.
- AC: an alternating current is always changing, so the inductor is always opposing it, impeding the flow. An inductor impedes AC — and the faster the change (the higher the frequency), the more it opposes, the exact mirror of a capacitor.
So the mirror is complete: a capacitor blocks DC and passes AC; an inductor passes DC and impedes AC. A capacitor resists voltage change; an inductor resists current change. Same idea of storing energy and reacting to change, pointed in opposite directions.
Common Types and How They Fail
You'll meet a few inductor families:
- Air-core — a coil with no magnetic core, giving small, stable inductance; common in high-frequency and radio work.
- Iron-core and ferrite-core — a coil around a magnetic core for much greater inductance; iron for low frequencies (power), ferrite for higher frequencies (switch-mode supplies, noise filtering). Chokes — inductors whose job is to block AC/noise — are usually cored.
- Transformers — two or more coils sharing a core so their magnetic fields couple; really coupled inductors, used to step voltages up or down.
Inductors are fairly robust — a coil of wire has little to go wrong — but they do fail. A winding can break and go open (no current path at all), or turns can short together and reduce the inductance, or a core can crack. These failure modes, and how to spot them, are covered with the capacitor failures in this chapter's final section.
Common Mistakes
- Reversing the DC/AC behavior. Inductor passes DC, impedes AC — the opposite of a capacitor. Mixing them up is the most common inductor error.
- Confusing the units. Henries, millihenries, and microhenries differ by factors of a thousand; 1 mH is 1000 µH, not the other way around.
- Ignoring the current rating. An inductor has a maximum current before its core saturates and its inductance collapses. A replacement must handle the current the circuit delivers.
- Forgetting the switch-off spike. Treating a coil as harmless because it carries only a low steady current ignores the large voltage it makes the instant that current is interrupted.
Troubleshooting Guidance
Even before you can measure one, this section gives you two inductor instincts. First, identify what a coil is for: a bulky cored coil in a power supply is storing energy; a small choke in a signal or power line is blocking noise; two coupled coils are a transformer. Knowing the role tells you what a failure would look like. Second, respect the switch-off spike: whenever a circuit switches current through a coil — a relay, a motor, a solenoid — expect a protection component (typically a diode across the coil) and suspect it when you find damaged switches, drivers, or transistors nearby, because a missing or failed protection diode lets the inductive kick destroy them. The detailed measurement of inductors comes later; the instincts to read a coil's purpose and to fear its switch-off spike are worth having now.
Verification & Testing Methods
Check your understanding before moving on:
- [ ] Describe what an inductor is physically and explain how it stores energy in a magnetic field.
- [ ] State the capacitor–inductor mirror correctly, including which one passes DC and which passes AC.
- [ ] Use E = ½ × L × I² to find the energy stored, and convert an inductance between H, mH, and µH.
- [ ] Explain why suddenly interrupting an inductor's current produces a high-voltage spike, and why that is a hazard.
Then try the practice exercises below — reading and reasoning, no energized coils.
Practice Exercises
- Stored energy (5 minutes, pen and paper). A 47 mH inductor carries 1.5 A. Use E = ½ × L × I² to find the energy stored, in joules. (Hint: 47 mH = 0.047 H, and remember to square the current.)
- Unit conversions (5 minutes, pen and paper). Rewrite each value in the two other common prefixes: (a) 4.7 mH in H and µH; (b) 220 µH in mH and H; (c) 1.5 H in mH and µH.
- State the mirror (5 minutes, no hardware). In a small table or in your own words, line up the capacitor and the inductor on four points: what field stores the energy, what change each resists, how each treats steady DC, and how each treats AC. Make sure you have the DC/AC rows the right way round.
- Reason about the spike (10 minutes, no hardware). A relay coil carries a steady current, then a switch opens to turn it off. Explain, using "an inductor opposes changes in current," why a voltage spike appears at that instant — and why a diode is often placed across the coil to protect the circuit.
These core ideas — the coil and its magnetic field, inductance and the henry, the opposes-current-change mirror of the capacitor, and the switch-off spike — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.
Key Takeaways
- An inductor is a coil of wire (often around a core) that stores energy in a magnetic field when current flows — the mirror of a capacitor storing energy in an electric field.
- Inductance measures how strongly a coil opposes a change in its current; its unit is the henry (H), with practical values in mH and µH (1 H = 1000 mH = 1,000,000 µH).
- More turns, a magnetic core, and tight winding all increase inductance.
- The defining behavior is opposing changes in current: an inductor passes steady DC (just a low-resistance coil) and impedes AC — the exact reverse of a capacitor, which blocks DC and passes AC.
- Stored energy is E = ½ × L × I², the mirror of the capacitor's ½ × C × V².
- Interrupting an inductor's current suddenly produces a large voltage spike (the inductive kick), a real hazard around relays, motors, and coils — which is why such circuits include protection like a diode across the coil.
Skills Learned
- You can now describe what an inductor is and explain how it stores energy in a magnetic field.
- You can now state the capacitor–inductor mirror correctly, including which component passes DC and which passes AC.
- You can now use E = ½ × L × I² and convert fluently between henries, mH, and µH.
- You can now explain the inductive voltage spike produced when current is interrupted, and why it endangers switches and drivers.
- You can now recognize the common inductor types and know their basic failure modes.
Glossary Additions
- inductor — a coil of wire, often wound around a magnetic core, that stores energy in a magnetic field when current flows and opposes changes in that current.
- inductance — the measure of how strongly a coil opposes a change in the current through it (equivalently, how much magnetic field it produces per amp); measured in henries.
- henry — the unit of inductance (H); a large unit in practice, so real inductors are usually rated in millihenries (mH) or microhenries (µH), where 1 H = 1000 mH = 1,000,000 µH.
- magnetic field — the region of magnetic influence created around a coil (or any conductor) when current flows through it; it is where an inductor stores its energy, the magnetic counterpart of a capacitor's electric field.
Suggested Next Sections
Must read next:
- Inductor Behavior in Circuits — how an inductor behaves over time and with frequency: its own time constant in DC, and its frequency-dependent opposition (inductive reactance), the mirror of the capacitor's reactance.
Recommended:
- What Is a Capacitor? — read the two side by side; the inductor is the capacitor's mirror at every point.
- Power and Energy in DC Circuits — an inductor stores energy (½ × L × I²) rather than dissipating it, a useful contrast with the resistor's heat.