Index / Maps / Anatomy / Electrical circuits
What electricity is, the 3 numbers that describe every circuit, a live circuit to experiment with, and the 15 parts you will meet most. No prior knowledge needed.
Step 1
Everything is made of atoms, and atoms contain tiny charged particles called electrons. In a metal like copper, some electrons are free to move. Push them all in 1 direction and you have an electric current.
A circuit is a complete loop for that current: from the battery, through wires and a device such as a bulb, and back to the battery. Break the loop anywhere and the current stops everywhere. That is all a switch does.
The electrons themselves crawl along at less than a millimetre a second. The light still comes on instantly because the wire is already full of electrons: push at 1 end and they all start moving at once, like water in a full hose.
By convention, current is drawn flowing from + to −. The electrons actually move the other way: the convention was set in the 1700s, before electrons were discovered, and it stuck.
Step 2
The push. How hard the battery or supply drives charge round the circuit.
The flow. How much charge passes a point each second.
The narrowing. How hard it is for current to get through.
Ohm's law. Voltage equals current times resistance. Any 2 give you the 3rd: I = V ÷ R. A 9 V battery across a 330 Ω resistor drives 9 ÷ 330 = 0.027 A, or 27 mA.
Power, in watts, is how fast energy is used: voltage times current. A kettle on 230 V drawing 10 A uses 230 × 10 = 2,300 W. Your bill counts kilowatt-hours: 1,000 W for 1 hour.
Step 3
Everything below is calculated with Ohm's law as you change it. The dots show the current: the faster they move, the bigger the current. Click a switch on the drawing to close it.
Try this, in order
The models are simplified: bulbs are treated as a fixed 24 Ω (a real filament's resistance rises as it heats), the LED as needing exactly 2 V, and each battery has a small internal resistance (AA 0.15 Ω, 9 V 1.5 Ω, 12 V 0.02 Ω).
Step 4
| Series | Parallel | |
|---|---|---|
| Paths | 1 path through everything | Each part has its own path |
| Current | The same everywhere | Splits between the paths, then adds back up |
| Voltage | Shared out between the parts | Every part gets the full voltage |
| Resistance | Adds up: 24 + 24 = 48 Ω | Falls: 2 × 24 Ω in parallel = 12 Ω |
| If 1 part fails | Everything stops | The others keep working |
| Everyday example | Old-style fairy lights: 1 dead bulb and the whole string went out | Every socket and light in your home |
Step 5
Each part with the symbol used on circuit diagrams. Learn the symbols and you can read most simple diagrams.
Pushes charge round the circuit. The voltage printed on it is the push. The longer line in the symbol is the + end.
For exampleAn AA cell gives 1.5 V; a car or boat battery 12 V.
What goes wrongShorted, a battery can deliver a huge current. A 12 V boat battery shorted by a spanner can weld it in place and start a fire.
A path of copper, which lets charge through very easily. A dot where wires meet means they are joined.
For exampleThe copper inside every cable in your home.
What goes wrongToo thin a wire for the current gets hot. That is how overloaded extension leads melt.
Opens or closes a gap in the circuit. Open: no complete loop, so no current anywhere in that loop.
For exampleThe light switch by your door.
What goes wrongA worn switch can spark as it opens, burning its contacts.
Resists the flow, so less current passes for the same voltage. Measured in ohms. The US symbol is a zigzag; this is the European box.
For exampleAlmost every LED in a gadget has a small resistor next to it.
What goes wrongRunning too much power through it makes it hot enough to discolour and fail.
A light-emitting diode: lets current through 1 way only and glows. It needs about 2 V before it starts, and only a little current, around 20 mA.
For exampleYour phone's flash, your TV remote, modern bulbs.
What goes wrongConnected straight to a battery with no resistor, it takes far too much current and burns out in a flash. Try it in the live circuit.
A thin filament that gets so hot it glows. It turns most of its energy into heat, not light.
For exampleThe old incandescent bulb, or the navigation lights on older boats.
What goes wrongThe filament thins with use and finally snaps, usually as it is switched on.
A thin wire designed to melt if the current goes over its rating, breaking the circuit before anything else overheats.
For exampleThe fuse in a UK plug, or the blade fuses in a car.
What goes wrongReplacing a fuse with a bigger one removes the protection: the wiring becomes the weakest part instead.
Stores a little charge and releases it quickly. It smooths out bumps in voltage.
For exampleThe charge that fires a camera flash.
What goes wrongLarge capacitors can hold a dangerous charge long after a device is unplugged.
A 1-way valve for current: it passes current in the direction of the arrow and blocks it the other way.
For exampleChargers use diodes to turn the mains' back-and-forth current into 1-way current.
What goes wrongConnected backwards, it simply blocks, and the circuit does nothing.
Turns electrical energy into rotation, using magnets.
For exampleA boat's bilge pump, a fan, an electric car.
What goes wrongIf it is jammed and cannot turn, it draws a large current and overheats.
A resistor you can adjust by turning a knob or moving a slider.
For exampleAn old volume knob, or a light dimmer.
What goes wrongWorn tracks crackle as you turn them.
The reference point all voltages are measured from. In a house it is literally connected to the earth.
For exampleThe 3rd pin on a plug.
What goes wrongA broken earth connection removes a key safety path, so a fault can leave a metal case live.
A switch that trips open by itself when the current is too high. Unlike a fuse, you can reset it.
For exampleThe row of switches in your fuse box.
What goes wrongIf it trips again straight after resetting, there is a real fault: find it before resetting again.
A switch worked by an electromagnet, so a small current can switch a big one.
For exampleA car's starter: the key sends a small current, the relay switches the large current to the starter motor.
What goes wrongContacts can weld together after switching a big load, leaving the circuit stuck on.
An electronic switch with no moving parts: a small current into 1 leg controls a big current through the other 2.
For exampleA modern phone chip contains billions of them.
What goes wrongToo much heat or voltage destroys it, usually without any visible sign.
Step 6
Cheat sheet
| V = I × R | Ohm's law. Also I = V ÷ R and R = V ÷ I. |
| P = V × I | Power in watts. Also P = I² × R: double the current, 4 times the heat. |
| R = R₁ + R₂ | Resistors in series add up. |
| 1/R = 1/R₁ + 1/R₂ | Resistors in parallel: the total is less than the smallest. |
| R = (V − 2) ÷ I | The resistor for an LED: battery voltage minus about 2 V, divided by about 0.02 A. For 9 V: 7 ÷ 0.02 = 350 Ω, so use 330 or 390 Ω. |
| kWh = kW × hours | Energy on your bill. A 2 kW heater for 3 hours uses 6 kWh. |
About this page
Physics from standard introductory references. Shock thresholds and the 30 mA RCD rating follow IEC 60479 and European wiring practice; figures are typical, and real effects depend on the path and duration of the current. Circuit symbols follow the IEC style used in Europe; American diagrams draw resistors as zigzags.