ProcirelPassive Components: Resistors, Capacitors and Inductors Compared
Resistors, capacitors and inductors are the passive trio — none can add energy to a signal, yet between them they set bias points, filter noise, time events and store energy. Every active circuit is mostly these three parts arranged with intent.
> At a glance: 9 minute guide · part of the electronics fundamentals complete guide track · worked example, quick-reference table and field notes included.
## Resistors: the law-keepers
Resistance converts current to a predictable voltage drop — Ohm’s law made physical. In series they divide voltage; in parallel they share current. Power rating matters as much as value. A quarter-watt part in a half-watt role discolours quietly before it fails loudly.
## Capacitors: electric field stores
A capacitor stores charge in an electric field between plates. It blocks DC, passes AC, and its impedance falls with frequency. That single behaviour creates decoupling, coupling, filtering and timing — the four verbs of analog design.
## Inductors: magnetic field stores
An inductor stores energy in a magnetic field and opposes changes in current. Its impedance rises with frequency — the mirror image of the capacitor. Coils, chokes and transformers all follow. And whenever an inductor’s current is interrupted, its flyback voltage must be respected.
| Property | Resistor | Capacitor | Inductor |
| — | — | — | — |
| Opposes | Current (equally) | Voltage change | Current change |
| Stores | Nothing | E field | M field |
| DC behaviour | Conducts | Blocks | Conducts |
| AC behaviour | Conducts | Passes more at f↑ | Blocks more at f↑ |
| Key formula | V = IR | Q = CV | V = L·dI/dt |
## How to apply this in your build
Work through the sequence below. Each step assumes the previous one passed. The numbers that need arithmetic are covered by the linked tools at the end of this guide.
1. Identify which passive role each part plays in the schematic
2. Check power ratings on resistors and ripple ratings on capacitors
3. For time constants, pair R with C or L deliberately
4. Verify markings with the LCR or multimeter when values matter
### Worked example
An RC pair — 10 kΩ with 100 nF — has τ = 1 ms and a cutoff near 1. 6 kHz. The same two components serve as a debounce filter at DC or an audio rolloff, purely by context. Cross-check with the Resistor Color Code and the result should agree to within rounding.
> Practical note from the bench. Bench exercise worth an hour: measure one of each passive on the LCR meter at two frequencies and watch the story change — datasheets suddenly make sense.
## Who this guide is for
First-time readers get a single focused topic instead of a textbook chapter, with every term defined where it first appears. Returning readers use it as a reference. The table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
## Prerequisites and preparation
Before starting: identify which passive role each part plays in the schematic and check power ratings on resistors and ripple ratings on capacitors. Keep the [Resistor Color Code](/tools/resistor-color-code) and [Capacitor Code (3-Digit)](/tools/capacitor-code) open, every number in the worked example is reproducible. Total time including the bench steps: about 7 to 9 minutes.
## Common mistakes to avoid
Each of these has cost real hardware on someone’s bench, usually ours:
– Treating all capacitors as equal — dielectric and ESR change jobs
– Ignoring inductor saturation current in switching circuits
– Reading in-circuit resistance and trusting the value
## Key takeaways
– Resistors: the law-keepers — the foundation of this guide; revisit it if any measurement here surprises you.
– Capacitors: electric field stores — the foundation of this guide; revisit it if any measurement here surprises you.
– Inductors: magnetic field stores — the foundation of this guide; revisit it if any measurement here surprises you.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Resistors: the law-keepers |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |
## How this fits the electronics fundamentals complete guide track
This guide is one stop in a structured path. Start from the [electronics fundamentals complete guide](/tutorial/electronics-fundamentals-complete-guide) pillar page for the full map, or continue with [schematic symbols](/tutorial/schematic-symbols-reference) and [decibels in electronics](/tutorial/decibels-in-electronics). For the arithmetic, open the [Resistor Color Code](/tools/resistor-color-code) or [Capacitor Code (3-Digit)](/tools/capacitor-code).
## Frequently asked questions
Why “passive”?
No external power supply and no signal gain — they shape what is there rather than amplify it.
Which fails most?
Electrolytic capacitors — drying out with heat and age; most vintage equipment repairs start and end with them.
Is there a calculator for this?
Yes, the [Resistor Color Code](/tools/resistor-color-code) and [Capacitor Code (3-Digit)](/tools/capacitor-code) run the formulas from this guide instantly, client-side, no signup.
## Keep going with this track
– The complete electronics fundamentals guide: [Electronics Fundamentals complete guide](/tutorial/electronics-fundamentals-complete-guide)
– Read next: [diodes and transistors explained: the two semiconductor families](/tutorial/diodes-and-transistors-explained)
– Also in this track: [the pn junction diode: physics, curves and applications](/tutorial/pn-junction-diode)
– Continue with: [ohm’s law tutorial: the one formula that runs everything](/tutorial/ohms-law)
– Calculate as you go: [Ohm’s law calculator](/tools/ohms-law) · [resistor colour code decoder](/tools/resistor-color-code) · [RC time constant tool](/tools/rc-time-constant)
– Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.
## Working method notes
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
## Formulas and checks from this guide
Verification checklist for this track: measure before and after every change, confirm polarity before power-up. Log each value beside the guide’s stated number. If a reading differs by more than rounding, find out why before moving on, tolerance, wiring or an untested assumption is always the reason.
Bookmark this page against your next build in the track. The checklist above is the same one used across 29 guides in this series.
## What the bench taught us
Component tolerance and meter accuracy stack. A 5% resistor, a 2% reference and lead resistance easily explain small gaps. Compare direction and magnitude before suspecting the guide.
A resistor and capacitor kit, common diodes and transistors, a breadboard and jumpers. Add modules as tracks demand them.