A path for electricity
When you flip a light switch, the light turns on almost instantly. That’s because you’ve completed an electric circuit, a closed loop that electricity can flow around.
Parts of a circuit
| Part | Job | Example |
|---|---|---|
| Energy source | Pushes current around the circuit | Battery, outlet |
| Conductors | Carry the current | Copper wires |
| Load | Uses the energy | Bulb, motor, speaker, heater |
| Switch | Opens or closes the circuit | Light switch, push button |
Open and closed circuits
| Closed circuit | Open circuit |
|---|---|
| Complete path with no gaps | Path has a gap |
| Current flows | Current stops |
| The bulb lights | The bulb is off |
A switch works by making or breaking a gap. A blown bulb filament also creates an open circuit.
Circuit diagrams
Engineers draw circuits with standard symbols so anyone can read them.
| Component | Symbol (described) |
|---|---|
| Cell (battery) | A long line and a short line side by side |
| Wire | A straight line |
| Bulb | A circle with an X inside |
| Switch (open) | A line with a raised gap |
| Resistor | A rectangle (or zigzag) |
| Ammeter | A circle with A inside |
| Voltmeter | A circle with V inside |
Series circuits
A series circuit has only one path. Current flows through each part, one after another.
| Feature | What happens |
|---|---|
| Current | The same everywhere in the circuit |
| Adding bulbs | Each bulb gets dimmer |
| One bulb breaks | The whole circuit goes off |
| Voltage | Shared among the bulbs |
Some older holiday lights were wired in series: if one bulb burned out, the whole string went dark, and you had to test each bulb to find the bad one.
Parallel circuits
A parallel circuit has two or more paths (branches). Current splits between them.
| Feature | What happens |
|---|---|
| Voltage | The same across each branch |
| Adding bulbs | Each stays bright, but the battery drains faster |
| One bulb breaks | The others stay on |
| Current | Splits among the branches, then rejoins |
Homes are wired in parallel. That’s why you can turn the kitchen light off without the fridge stopping.
Comparing series and parallel
| Series | Parallel | |
|---|---|---|
| Paths | One | Several |
| Brightness with more bulbs | Dimmer | Same |
| If one part fails | All stop | Others keep working |
| Used for | Switches, fuses, some decorations | Homes, cars, most devices |
Measuring circuits
- Current (amperes, A) is measured with an ammeter placed in series.
- Voltage (volts, V) is measured with a voltmeter placed in parallel across a part.
- Resistance (ohms, Ω) slows current down.
They’re connected by Ohm’s law: V = I × R.
Circuit safety
Short circuits
A short circuit happens when current finds an easy path that skips the load, like when two bare wires touch. Huge current flows, wires heat up, and fires can start.
Safety devices
| Device | How it protects |
|---|---|
| Fuse | A thin wire melts if current is too high, opening the circuit |
| Circuit breaker | A switch that trips automatically when current is too high; can be reset |
| GFCI outlet | Cuts power in milliseconds if current leaks, as through a person; used in bathrooms and kitchens |
| Grounding wire | Gives stray current a safe path to the ground |
Circuits all around you
- A flashlight is a simple series circuit with batteries, a switch and a bulb.
- A car has many parallel circuits for lights, radio and wipers.
- A computer contains billions of tiny circuits on chips.
Common mistakes
- Thinking current gets used up by the first bulb. Current is the same throughout a series circuit; energy is what’s transferred.
- Thinking a parallel circuit gives each bulb less voltage.
- Connecting a battery’s two ends with a bare wire. That’s a short circuit and can get hot.
Try it yourself
- Build a series circuit with two bulbs. Unscrew one. What happens?
- Build a parallel circuit with two bulbs. Unscrew one. Compare.
- Draw circuit diagrams for both.
- Design a circuit where two switches each control their own bulb.
Quick recap
- A circuit is a closed path for current, with a source, wires and a load.
- Open circuits stop current; closed circuits let it flow.
- Series circuits have one path; parallel circuits have several.
- Homes use parallel circuits so devices work independently.
- Fuses, breakers and GFCIs protect against dangerous currents.