Learn With Examples
Why did one dead bulb kill a whole string of old fairy lights, while one dead lamp in your house changes nothing? Same electricity, two different wiring choices — and that single difference explains most of the electrical world around you.
Somewhere in a box in your attic there may still be a string of old Christmas lights with one job: to work. Half the time they didn’t. One bulb would fail and the entire string went dark, and you’d be left testing forty bulbs one at a time to find the culprit.
Meanwhile, in the same house, a bulb in the hallway could blow and nothing else noticed. The kitchen stayed lit. The fridge kept running.
That difference isn’t about the quality of the bulbs. It’s about how the wires connect them — in series, one after another in a single chain, or in parallel, each with its own path. Understanding that one distinction lets you explain fairy lights, house wiring, car headlights, torches, phone chargers, and why your kettle trips the breaker when the microwave is already running.
We’ll get there properly: first what electricity actually is, then what a circuit is, then the two ways of wiring one.
Series and parallel, side by side
Series — one single path. The same current flows through everything, the voltage gets divided up between components, and if any one component fails, the path breaks and everything stops.
Parallel — multiple paths. Every branch gets the full voltage, the current splits between branches, and if one branch fails, the others carry on.
Your home is wired in parallel. That’s why every socket delivers the same voltage, why appliances work independently — and why adding more of them adds up current until a breaker steps in.
What electricity actually is
Inside a copper wire, the outer electrons of the copper atoms are only loosely attached and drift about randomly. Connect that wire to a battery and something changes: the random drifting gains a direction. All those electrons begin edging the same way, and that organised movement of charge is what we call an electric current.
Now for the fact that surprises almost everyone. Those electrons move remarkably slowly — a fraction of a millimetre per second in a typical household wire. Slower than a snail. An individual electron leaving your fuse box might take hours to reach the ceiling light.
So why does the light come on instantly? Because you’re not waiting for a particular electron to arrive. The wire is already full of them. Flipping the switch sends an electromagnetic push through the wire at close to the speed of light, and every electron along the way starts moving at once — the way water comes out of a tap immediately even though that particular water has been sitting in the pipe for hours.
The water analogy, and where it breaks
Electricity is invisible, which is why almost every explanation reaches for plumbing. It’s a genuinely good analogy, as long as you know its limits.
| Electrical term | Unit | Water equivalent | What it really means |
|---|---|---|---|
| Voltage (V) | volts | Water pressure | The push. How hard the source shoves charge around the loop |
| Current (I) | amps | Litres per second | The flow. How much charge passes a point each second |
| Resistance (R) | ohms (Ω) | Pipe narrowness | The obstruction. How hard the material makes it to flow |
| Power (P) | watts | Work the water does | Energy used per second — light, heat, motion |
Where the analogy fails. Water can pour out of an open pipe; electricity cannot. Charge needs a complete loop back to the source, which is why a broken wire stops everything and why a battery with one terminal connected does precisely nothing. Also: a burst pipe wastes water, but a short circuit doesn’t waste electricity — it dumps enormous current through a near-zero resistance and generates enough heat to start a fire.
Ohm’s law: the one equation that matters
Those three quantities aren’t independent. They’re locked together by a relationship discovered by Georg Ohm in the 1820s, and it’s the single most useful equation in electronics.
In plain language: for a given push, more resistance means less flow. Double the voltage and you double the current; double the resistance and you halve it.
Worked example 1 — how much current through a bulb?
A small bulb with 24Ω of resistance is connected to a 12 V car battery.
I = V / R = 12 / 24 = 0.5 amps
Half an amp flows. Swap in a 6Ω bulb and the current jumps to 2 amps — four times as much, because resistance dropped to a quarter.
Worked example 2 — why thin wires get hot
Power dissipated as heat is P = I² × R. Note the current is squared — doubling the current quadruples the heat.
A thin extension cable might have 0.5Ω of resistance. Run a 10 A heater through it: P = 10² × 0.5 = 50 watts of heat produced in the cable itself. That’s a small heater’s worth of energy, inside a coiled cable, under a rug. This is exactly how extension-lead fires start.
Worked example 3 — what your bulbs actually cost
Power is P = V × I, and your bill is in kilowatt-hours: power × time.
An old 60 W incandescent bulb running 3 hours a day uses 60 × 3 × 365 = 65,700 watt-hours = 65.7 kWh a year. An equivalent 9 W LED uses about 9.9 kWh.
At around 16 cents per kWh, that’s roughly $10.50 a year versus $1.60 — per bulb. Across twenty bulbs in a house, the difference is real money, and none of it required changing a single habit.
What counts as a circuit
A circuit is a complete loop that charge can travel around. Break the loop anywhere and everything stops — which is, incidentally, all a light switch does. It doesn’t “turn on electricity”. It closes a gap in a loop that was otherwise complete.
The load is the point of the whole exercise. It’s where electrical energy converts into something you want — light in a bulb, heat in a kettle, motion in a motor, sound in a speaker. A circuit with no load is a short circuit, and that’s a problem rather than a design.
Series vs parallel: the two ways to wire anything
Once you have more than one component, you face a choice. Chain them one after another, or give each its own branch. Here is what that looks like:
That caption contains the entire trade-off, so it’s worth restating: series shares the voltage out, parallel shares the current out. Everything else follows from those two facts.
Series vs parallel, compared properly
tap to switchSeries — components in a single chain
Current: identical through every component. There’s only one path, so whatever leaves the battery must pass through all of them. If 0.5 A flows through the first bulb, 0.5 A flows through the last.
Voltage: divides between components in proportion to their resistance. Three identical bulbs on 12 V get 4 V each.
Resistance: simply adds. Rtotal = R1 + R2 + R3. Three 10Ω bulbs give 30Ω, so less current flows overall.
If one fails: everything stops. The single path is broken and there’s no alternative route.
Where you actually find it: switches and fuses (deliberately in series so they can break the whole circuit), batteries stacked in a torch to add voltage, LED strips with a current-limiting resistor, and old-style Christmas lights.
Parallel — components on separate branches
Voltage: identical across every branch. Each one connects directly to both sides of the source, so each gets the full 12 V — or the full 120 V or 230 V in your walls.
Current: splits between branches according to what each one draws. A 2 A branch and a 1 A branch means the source supplies 3 A.
Resistance: goes down as you add branches, because you’re adding more routes. 1/Rtotal = 1/R1 + 1/R2. Two 10Ω branches give just 5Ω.
If one fails: the others carry on. The remaining paths are untouched.
Where you actually find it: every socket and light fitting in your home, car headlights, USB ports on a hub, power strips, and the cells in an electric vehicle battery pack.
Do the maths once and it sticks
Take two 6Ω bulbs and a 12 V battery, and wire them both ways.
| Quantity | In series | In parallel |
|---|---|---|
| Total resistance | 6 + 6 = 12Ω | 1/(1/6+1/6) = 3Ω |
| Current from battery | 12/12 = 1 A | 12/3 = 4 A |
| Voltage across each bulb | 6 V | 12 V |
| Current through each bulb | 1 A | 2 A |
| Power per bulb | 6 W | 24 W |
| Result | Both dim | Both bright, battery drains 4× faster |
Look at the last row. The parallel pair produces eight times the total light output of the series pair — and drains the battery four times as fast. Neither wiring is “better”. They’re different bargains, and which one you want depends entirely on what you’re building.
Series divides the push. Parallel divides the flow. Every other difference is a consequence.
Five real things, and how they’re wired
This is where the theory earns its keep. Each of these is a wiring decision someone made deliberately.
Real-world circuits
tap an exampleChristmas lights — the classic series circuit
Old string lights wired fifty small bulbs in series across mains voltage. Each bulb only had to handle a fiftieth of it, which meant cheap, low-voltage bulbs and almost no wiring cost. That was the point.
The price was the failure mode everyone remembers: one filament breaks, the single path is severed, and all fifty go dark with no clue which one failed.
How modern strings fixed it: each bulb contains a tiny shunt — a backup conductor that bridges the gap when the filament burns out, keeping the path intact. The dead bulb goes dark, the rest stay lit. Still a series circuit; just one with a built-in escape hatch.
The catch: with one bulb shunted out, the remaining bulbs share the same voltage between fewer of them, so each runs slightly hotter. Let enough of them fail and the survivors burn out faster and faster.
House wiring — parallel, for four good reasons
Every socket and light in your home sits on its own branch across the same supply. That choice buys four things at once:
- Full voltage everywhere. Every appliance gets 120 V or 230 V regardless of what else is plugged in.
- Independence. A blown bulb, or a switched-off lamp, changes nothing elsewhere.
- Individual control. Each branch can have its own switch.
- Mixing appliances. A 5 W phone charger and a 2,000 W kettle coexist happily, each drawing what it needs.
The cost: currents add up. On a 120 V circuit rated for 15 A, the ceiling is 1,800 W. A 1,500 W kettle and a 900 W microwave together want 2,400 W — and the breaker trips. That’s not a fault, that’s the breaker doing exactly its job before the wiring overheats.
Car headlights — parallel, for safety
Wired in series, one blown headlight would leave you driving at night with none. In parallel, a failed bulb leaves the other burning at full brightness — degraded, but survivable.
The same reasoning runs through the whole vehicle: indicators, brake lights, wipers, radio all hang in parallel off the 12 V system, each with its own fuse. That fuse, incidentally, is in series with the thing it protects, because a fuse’s job is to break the single path when current runs too high.
The wider pattern: anything safety-critical gets parallel wiring, so that a single failure degrades the system instead of killing it.
A torch — both at once
Stack two AA batteries end to end and you’ve wired them in series: 1.5 V + 1.5 V = 3 V. Series adds voltage, which is why devices needing more push want more cells in a line.
Put two AA cells side by side wired positive-to-positive and you have them in parallel: still 1.5 V, but twice the capacity, so the device runs about twice as long.
Big battery packs use both. An electric vehicle pack might wire cells in series to reach several hundred volts, then wire whole groups of those in parallel for capacity. Series for voltage, parallel for endurance.
Why torch bulbs are in series with the switch: so one switch breaks the only path. Same reason your wall switch works.
Phone chargers and USB — parallel with a limit
Plug three devices into a multi-port charger and each port delivers its own voltage independently — parallel branches. One device finishing its charge doesn’t affect the others.
But the charger has a total current budget. A 65 W charger shared across three ports cannot give all three their full fast-charge current at once, which is why charging slows down as you add devices. The parallel branches are independent in voltage but competing for the same supply.
The same logic scales up: a power strip is parallel sockets sharing one wall circuit. Six free sockets do not mean six appliances’ worth of capacity — the limit lives back at the breaker.
AC and DC, briefly
Everything above works identically for both, but the distinction is worth knowing.
Direct current
Charge flows one way, steadily. Batteries, USB, cars, phones, and everything with a chip in it. Simple, but hard to send over long distances without losses.
Alternating current
Flow reverses direction 50 or 60 times per second. What comes out of your walls, because AC voltage can be stepped up and down with transformers — which is what makes a national grid possible.
Transmission
High voltage means low current for the same power, and low current means far less heat lost in the cables. Transformers make that easy for AC and difficult for DC.
Your charger
The brick converts AC from the wall into DC for your device. Almost every electronic thing you own is quietly running on DC behind an AC socket.
Electrical safety, with the actual numbers
Here’s the fact that reframes everything: voltage doesn’t hurt you. Current does. Static electricity can hit thousands of volts and merely make you jump, because almost no current flows. A car battery is only 12 V and will not shock you through dry skin — but short its terminals with a spanner and the current can weld metal.
Read that middle row again. A tenth of an amp — one thousandth of what a modest household circuit can supply — is potentially fatal. The gap between “you can feel it” and “this could kill you” is about a hundred milliamps, which is nothing at all.
What fuses and breakers actually protect. A 15 A breaker exists to stop your wiring catching fire. It does not protect you — the current that stops a heart is roughly a hundred times below its trip point, so a person can be electrocuted without the breaker noticing anything unusual.
The device that protects people is different: an RCD or GFCI. It compares the current going out with the current coming back, and if even a few milliamps are escaping — through a person, for instance — it cuts power in a fraction of a second. That’s why they’re required near water. If your bathroom or kitchen sockets don’t have one, that’s worth asking an electrician about.
And the obvious one: mains wiring is not a learning project. Batteries, low-voltage kits and USB electronics are genuinely safe to experiment with. Anything connected to your wall is work for a qualified electrician.
Five things people get wrong
“Electricity gets used up as it flows.”
Charge isn’t consumed — exactly as many electrons return to the source as leave it. What gets used up is energy, converted into light, heat or motion by the load. The current arriving at your bulb and the current leaving it are identical.
“Higher voltage always means more dangerous.”
Not on its own. Danger depends on the current that ends up flowing through you, which depends on voltage and the resistance of the path — and skin resistance drops enormously when wet. That’s why a bathroom is more hazardous than a bedroom at exactly the same voltage.
“Adding more bulbs in parallel makes each one dimmer.”
Not from an adequate supply. Each parallel branch still gets full voltage, so each bulb is just as bright — you’re simply drawing more total current. It only dims if the supply can’t keep up, which is why a torch dims as its battery weakens.
“Birds on power lines are safe because of their feet.”
They’re safe because they’re only touching one wire. Current needs a difference in voltage to flow, and both feet are at the same potential. Touch a second wire or a grounded pole at the same time and the outcome is very different — which is why large birds are at more risk than small ones.
“Series is old-fashioned and parallel is modern.”
Neither is obsolete. Series is exactly right for switches, fuses, current-limiting resistors and stacking battery cells for voltage. Parallel is right for independent loads. Real devices use both, often within centimetres of each other.
Check yourself
Five questions. Open each to check — the correct option is marked.
1. Three identical bulbs are wired in series across a 12 V battery. What is the voltage across each?
- 12 V each
- 4 V each
- 36 V total
- It depends on the switch
Series divides voltage between components. Three identical bulbs share 12 V equally, so 4 V each — which is why they burn noticeably dim.
2. Two 10Ω resistors are wired in parallel. What is the total resistance?
- 20Ω
- 10Ω
- 5Ω
- 0.1Ω
Parallel paths reduce total resistance: 1/R = 1/10 + 1/10 = 2/10, so R = 5Ω. More routes means easier flow.
3. Why does a blown bulb in your house not affect the others?
- Modern bulbs have backup filaments
- House lights are wired in parallel, so each has its own path
- The breaker reroutes the current
- They’re wired in series with a bypass
Each fitting is its own branch across the supply. Break one branch and the rest are untouched.
4. A 24Ω bulb is connected to 12 V. How much current flows?
- 2 A
- 0.5 A
- 288 A
- 12 A
Ohm’s law: I = V/R = 12/24 = 0.5 A.
5. Which is more dangerous: 20,000 V of static, or 0.1 A through your chest?
- The static, because the voltage is huge
- The current — 0.1 A can disrupt the heart
- Both are equally harmless
- Neither can harm a person
Static carries a huge voltage but almost no current and no sustained flow. Current through the body is what causes harm, and 100 mA is in the dangerous range.
Frequently asked questions
What is the difference between series and parallel circuits?
A series circuit has one single path, so the same current flows through everything and the voltage divides between components — and if one part fails, everything stops. A parallel circuit has multiple branches, so each gets the full voltage while the current divides, and one failed branch doesn’t affect the others.
Why are houses wired in parallel?
So every socket and light gets the full supply voltage, can be switched independently, and keeps working when something else fails or is unplugged. The trade-off is that currents add up, which is why circuits have breakers.
What is Ohm’s law in simple terms?
Voltage equals current times resistance, or V = I × R. More push gives more flow; more resistance gives less flow. Rearranged as I = V/R, it tells you the current in any simple circuit from just two numbers.
Does adding more devices in parallel drain a battery faster?
Yes. Each branch draws its own current and they add together, so the battery supplies more total current and empties sooner. Each device still runs at full brightness or power until the battery itself starts to sag.
Is it the volts or the amps that kill you?
The current through your body does the damage, but voltage is what drives that current. Roughly 10 to 20 mA is enough to lock your muscles so you cannot let go, and around 100 mA can disrupt the heart. Wet skin drops your resistance dramatically, which is why the same voltage is far more dangerous near water.
The takeaway
Electricity is charge flowing around a complete loop, pushed by voltage and slowed by resistance, with those three quantities tied together by V = I × R. A circuit needs a source, conductors, a load and usually a switch — and break the loop anywhere and it all stops.
The wiring choice is the interesting part. Series puts everything on one path: same current, shared voltage, one failure takes out the lot. Parallel gives everything its own path: same voltage, split current, failures stay local. Your house is parallel so a blown bulb doesn’t darken the kitchen. Your fuse is in series so it can cut everything when it needs to.
Go and look at something nearby with that lens. Why does the switch by the door control only those lights? Why does the fridge keep running when the toaster trips a breaker? Why do two AA cells in a torch make 3 V and not 1.5 V? You now have everything you need to answer all three — and that’s the difference between having read about circuits and actually understanding them.
electricityseries circuitparallel circuitohm’s lawvoltagecurrent
