Three building blocks of electronics
Open up almost any electronic device, from a radio to a phone charger, and you’ll find three basic passive components: resistors, capacitors and inductors. They don’t need their own power, but they control how current and voltage behave.
| Component | What it does | Stores energy? | Unit | Symbol |
|---|---|---|---|---|
| Resistor | Limits current | No (turns energy into heat) | Ohm (Ω) | R |
| Capacitor | Stores charge | Yes, in an electric field | Farad (F) | C |
| Inductor | Resists changes in current | Yes, in a magnetic field | Henry (H) | L |
Resistors
A resistor limits the current in a circuit. It follows Ohm’s law: V = I × R.
Uses:
- protecting LEDs and chips from too much current
- dividing voltage into smaller amounts
- making heat (toasters, kettles and electric heaters use high-resistance wires)
Some resistors change with conditions:
| Type | Changes with | Use |
|---|---|---|
| Variable resistor (potentiometer) | Turning a knob | Volume controls, dimmers |
| Thermistor | Temperature | Thermostats, fire alarms |
| Light-dependent resistor (LDR) | Light | Streetlights that switch on at dusk |
Capacitors
A capacitor stores electric charge. The simplest is two metal plates separated by an insulator.
How it works:
- Connect it to a battery. Electrons pile up on one plate and leave the other.
- The plates become oppositely charged. Energy is stored in the electric field between them.
- Disconnect the battery and connect the capacitor to a bulb. It releases its charge in a burst.
Capacitance measures how much charge a capacitor stores per volt. One farad is very large; most capacitors are measured in microfarads (µF) or picofarads (pF).
Capacitance increases with larger plates, plates closer together, and better insulating material between them.
Uses of capacitors
| Use | How it helps |
|---|---|
| Camera flash | Stores energy and releases it in an instant |
| Defibrillator | Delivers a strong pulse to restart a heart |
| Smoothing power | Evens out bumps in the voltage from a power supply |
| Touchscreens | Your finger changes the capacitance at a spot on the screen |
| Timing circuits | Charging and discharging sets the timing, like in blinking lights |
Safety: large capacitors can hold a dangerous charge even after a device is unplugged. That’s why you should never open old TVs, microwaves or power supplies.
Inductors
An inductor is a coil of wire, often wound around an iron core. When current flows through it, it creates a magnetic field and stores energy there.
Key property: an inductor resists changes in current. When current tries to rise, the inductor pushes back. When current tries to fall, the inductor tries to keep it going. This is due to electromagnetic induction.
Uses of inductors
| Use | How it helps |
|---|---|
| Transformers | Two coils change voltage up or down |
| Filters | Block high-frequency noise |
| Wireless charging | A coil in the pad induces current in a coil in the phone |
| Traffic light sensors | Loops of wire in the road detect metal cars |
| Electric motors and generators | Coils interact with magnetic fields |
Working together
Filters
Capacitors and inductors respond differently to signals of different frequencies, so they’re used to make filters.
- A capacitor blocks steady DC but lets fast-changing AC signals through.
- An inductor lets steady DC through but resists fast-changing AC.
Speakers use filters to send low bass notes to the woofer and high notes to the tweeter.
Tuning circuits
An inductor and capacitor together form a circuit that "rings" at one particular frequency. Radios use these to tune in a station.
Common mistakes
- Thinking capacitors are batteries. Capacitors release energy quickly and hold less; batteries store more and release it slowly through chemical reactions.
- Thinking resistors are useless because they waste energy. They’re essential for control and protection.
- Thinking inductors work with steady DC. Their main effects show up when current changes.
Try it yourself
- Look at an old circuit board (from a broken toy, with an adult). Try to identify resistors (small with coloured bands), capacitors (often cylinders) and inductors (coils of wire).
- Use an online circuit simulator to charge a capacitor through a resistor and watch the voltage rise.
- Explain why a camera flash takes a few seconds to recharge.
Quick recap
- Resistors limit current and turn energy into heat.
- Capacitors store charge in an electric field and release it quickly.
- Inductors store energy in a magnetic field and resist changes in current.
- Together they make filters and tuning circuits.
- These three components are the foundation of electronics.