Series and Parallel Circuits
One path or many: how connecting the same parts two different ways changes everything.
The question
Why does one burnt-out bulb on an old string of Christmas lights make the whole string go dark, while newer strings keep glowing?
What you’ll discover: how series and parallel circuits behave, and how to predict and measure the voltage and current in each.
In simple terms
BeginnerOne road or several
Series: parts are connected one after another, so there’s only one path. If any part breaks, the path is broken and everything stops. Many older Christmas light strings were wired this way, so one bad bulb could darken the whole string. (Many newer strings add tricks to keep the rest lit.)
Parallel: each part has its own path back to the battery. If one part fails, the others keep working. The outlets and lights in your home are wired in parallel, so turning off the kitchen light doesn’t turn off the fridge.
The rules in a table
| Series | Parallel | |
|---|---|---|
| Current | The same through every part | Splits between the branches, then adds back up |
| Voltage | Shared among the parts | The same across every branch |
| Resistance | Adds up: R1 + R2 | Less than the smallest branch |
| If one part fails | Everything stops | The other branches keep working |
Go deeper
IntermediateLEDs in series and parallel
Two LEDs in parallel, each with its own 220 Ω resistor, on a 4.5 V pack: each branch gets the full 4.5 V, so each LED gets about (4.5 − 2.0) ÷ 220 ≈ 11 mA, and the battery supplies the total, about 22 mA.
Why does each LED need its own resistor? LEDs vary slightly. If two share one resistor, the one with the slightly lower forward voltage can hog most of the current, glowing brighter and wearing out sooner.
Two red LEDs in series need about 2.0 + 2.0 = 4.0 V just to light, leaving only 0.5 V for the resistor on a 4.5 V pack. That small margin makes the current very sensitive to battery voltage. Series LED strings work well when the supply has plenty of voltage to spare.
AdvancedKirchhoff in practice
Kirchhoff’s current law (Lesson 3) says the currents into a junction equal the currents out: in the parallel example, 11 mA + 11 mA = 22 mA leaves the battery and 22 mA returns. Kirchhoff’s voltage law says each loop’s voltage drops add up to the supply: in each branch, VLED + Vresistor = 4.5 V.
Real batteries have internal resistance, so drawing more current (adding branches) lowers the battery’s terminal voltage slightly. That’s why adding many LEDs in parallel to a small battery can make each one a little dimmer.
Try it
Build two LEDs in parallel, each with its own 220 Ω resistor, between the rails of your breadboard. Then pull one LED out.
Quick check
In a series circuit, if one part breaks, what happens?
The only path is broken, so everything stops.
Two branches each carry 11 mA. How much current leaves the battery?
22 mA: branch currents add up.
Why give each parallel LED its own resistor?
LEDs vary slightly, so a shared resistor lets one hog the current. Separate resistors keep each LED’s current set.
Words to know
- Series
- Parts connected one after another in a single path.
- Parallel
- Parts connected side by side, each with its own path.
- Branch
- One of the separate paths in a parallel circuit.
- Internal resistance
- The small resistance inside a battery itself.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Stage 3 challenge: the Fix-It Challenge
- Next: Ohm’s law in practice
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.