Electronics
Series vs parallel circuits
Parts are in series when they sit one after another on a single path, and in parallel when each has its own branch between the same two points. In series, the current is the same everywhere and the voltage is shared. In parallel, the voltage is the same across every branch and the currents add up.
Series: one path
Every bit of charge passes through every part, so one current flows all the way round. The resistances add:
Example: 12 V across 1 kΩ and 3 kΩ in series. The total is 4 kΩ, so 3 mA flows. The 1 kΩ drops 3 V and the 3 kΩ drops 9 V: the drops add up to the supply, and the bigger resistor takes the bigger share. Order doesn't matter.
With only one path, a break anywhere stops the current everywhere. That's why old fairy lights went dark when a single bulb failed.
Parallel: side by side
Each branch connects across the same two wires, so each gets the full voltage and sets its own current. Where the branches rejoin, their currents add, and the battery supplies the total.
Live experiment
Switch on the branches
Three branches on one 9 V battery, each with its own switch, resistor and LED, about 10 mA each. Switch them on one at a time: the ones already lit don't dim, and the battery's current bar grows branch by branch.
“Parallel circuits” is lesson 2 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Example: 9 V across 1 kΩ and 3 kΩ in parallel. The branches draw 9 mA and 3 mA, 12 mA in all, so the pair acts like 9 V ÷ 12 mA = 0.75 kΩ. For two resistors, product over sum gets there directly:
Every extra branch is another path, so a parallel combination is always less than its smallest resistor.
Side by side
| Series | Parallel | |
|---|---|---|
| Current | Same through every part | Splits; branch currents add |
| Voltage | Shared; drops add to the supply | Full supply across each branch |
| Resistance | Adds up | Below the smallest |
| One part fails | Everything stops | The others carry on |
Which to use
A house is wired in parallel: every socket sees the full mains voltage, and the kettle doesn't dim the TV. A string of LEDs is often in series, so one resistor can set the current for all of them. Parallel LEDs, though, each need their own resistor: shared, the one with the lowest forward voltage hogs the current.
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Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Parallel circuits” is lesson 2 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
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Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: both their ends connect to the same two points. Parallel means both ends share the same two nodes, however the drawing looks. Series parts are on one path, so they always share the same current. Equal currents alone don't prove either.
- Why not “the same current flows through them”? Sharing one current is what series parts do, since they sit on one path. Parallel parts share a voltage, and their currents differ unless their resistances match.
- Why not “they're drawn next to each other”? A drawing can put parallel parts anywhere on the page. What counts is the wiring: both ends joined to the same two points.
Show the answer
Answer: 9 V. Side by side, the voltage stays at one battery's worth. Only a chain adds volts.
- Why not “0 V”? You'd get close to 0 V only if one battery were reversed, which shorts the two into each other. Matched + to + and − to −, the load sees 9 V.
- Why not “4.5 V”? Parallel batteries don't split the voltage. Each one holds the load at 9 V, and they share the current between them.
- Why not “18 V”? Volts only add when batteries are chained + to −, in series. Side by side, both hold the load at the same 9 V.
Show the answer
Answer: Add their voltages. + to −: the volts stack.
- Why not “Must point different ways”? Cells pointing different ways cancel each other out. In series, each cell's + meets the next one's −, all facing the same way.
- Why not “Halve the voltage”? Nothing halves. Each cell adds its own push to the chain, so two 1.5 V cells in series give 3 V.
- Why not “Add their currents”? In a series chain one current flows through every cell, so the currents don't add. It's cells side by side, in parallel, that share the current.
“Parallel circuits” is lesson 2 of 7 in Junction City, Electronics World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Questions people ask
What is the difference between series and parallel circuits?
In series, parts sit one after another on a single path, so the same current flows through all of them and they share the voltage. In parallel, each part has its own branch between the same two points, so each gets the full voltage and the branch currents add up.
How do you add resistors in series and in parallel?
In series, resistances add: 1 kΩ and 3 kΩ make 4 kΩ. In parallel, use 1 ÷ R = 1 ÷ R1 + 1 ÷ R2, or for two resistors product over sum: (1 × 3) ÷ (1 + 3) = 0.75 kΩ. A parallel combination is always less than its smallest resistor.
Why are houses wired in parallel?
So every socket gets the full mains voltage and each appliance works on its own. Switching on a kettle doesn't dim the TV, and one blown bulb leaves the others lit.
Can parallel LEDs share one resistor?
They shouldn't. The LED with the lowest forward voltage holds the shared point near its own drop and hogs the current. A red LED (about 2 V) next to a blue one (about 3.2 V) keeps the blue one dark. Give each LED its own resistor.
Related topics
- Kirchhoff's laws explained: KCL and KVLWhat flows into a junction flows out, and the drops round a loop add up to the rise.
- Voltage divider explainedTwo resistors that split a voltage: the formula, potentiometers, and why a load drags the output down.
- What is resistance?The squeeze on the flow, in ohms: how resistors set the current and protect an LED.