Circuits

An electric circuit is a complete loop that allows electric current to flow. Every circuit needs three things: a power source (battery, generator or solar cell), a load (the device that uses the electricity, like a bulb or motor) and wires connecting everything in an unbroken path. If any part of the loop is broken (a switch open, a wire cut), the circuit is incomplete and no current flows. Circuits are the basis of all electrical and electronic devices. Even the most complex computer is just an enormous collection of tiny circuits working together.

  • What it isComplete loop for currentNo loop, no current
  • NeedsSource, load, wiresPlus a switch usually
  • Two main typesSeries and parallelDifferent layouts of components
  • SeriesOne path, all in lineBreak one, all fail
  • ParallelMultiple paths side by sideBreak one, others keep working
  • Inside electronicsMillions of tiny circuitsOn microchips

The basic circuit

The simplest possible circuit has just three parts:

  • A battery (or other power source).
  • A bulb (or other load).
  • Two wires, one going from each battery terminal to each end of the bulb, forming a complete loop.

Current flows from the battery, through the wire, through the bulb (which lights up because of resistance in the filament), through the second wire, and back to the battery. The journey makes a complete circle: a "circuit".

Switches

A switch is a way of breaking and reconnecting the circuit. When the switch is open, there is a gap in the loop, no current can flow, and the device is off. When the switch is closed, the loop is complete and current flows.

Almost every electrical device has switches. Your bedroom light has a wall switch. Your kettle has a switch on the handle. Your laptop has a power button that closes an internal circuit. Even your fridge has a switch (the door switch turns the light on when you open it).

Series circuits

In a series circuit, all the components are arranged one after another in a single loop. The same current flows through every component.

Properties of series circuits:

  • If one bulb breaks, the loop is broken and ALL bulbs go out (the classic problem with old Christmas tree lights).
  • The voltage of the battery is shared between the components.
  • More components mean more resistance, so less current flows.

Parallel circuits

In a parallel circuit, components are connected on separate side-by-side branches between the same two points.

Properties of parallel circuits:

  • If one bulb breaks, the others stay lit, because each has its own complete path. Modern Christmas lights mostly use parallel branches.
  • Each component gets the full battery voltage.
  • Adding more parallel branches actually lowers the total resistance (more lanes for current).

Most household wiring uses parallel circuits. That way, plugging in extra devices does not dim the others.

Fact Your home is wired as a giant parallel circuit. Each socket, light and appliance is on its own branch off the main wiring. That is why turning on the kettle does not make the lights dim, and why one broken bulb does not affect any other.

Other circuit components

  • Resistors: components that deliberately resist current flow, controlling how much flows in a circuit.
  • Capacitors: store small amounts of electrical charge briefly. Used to smooth voltage and in many electronic timing circuits.
  • Diodes: let current flow in only one direction, like one-way valves for electricity. LEDs (light-emitting diodes) are special diodes that glow when current flows.
  • Transistors: tiny electronic switches and amplifiers. Modern microchips contain billions of them, each smaller than a virus.
  • Inductors: coils of wire that resist sudden changes in current.
  • Fuses: thin wires that melt and break the circuit if too much current flows, protecting against fire.
  • Transformers: change the voltage of AC electricity, either up or down.

Circuit diagrams

Engineers draw circuits using standard symbols. A few common ones:

  • Battery: two parallel lines of different lengths.
  • Bulb: a circle with an X or cross inside.
  • Switch: a short line that can pivot to break the connection.
  • Wire: a straight line.
  • Resistor: a zigzag (USA) or rectangle (UK and Europe).
  • Diode: a triangle pointing into a flat line.

Anyone can read a circuit diagram in any language: the symbols are international.

Did you know? A modern smartphone contains around 10 to 20 billion transistors, all packed into a few square centimetres of silicon. Each transistor is a tiny switch in a microscopic circuit. The complexity is so high that no human could draw the full circuit diagram, but every individual switch follows the same simple rules invented in the 19th century.

Short circuits

A short circuit is when a low-resistance path accidentally connects the two terminals of a power source, bypassing the load. The current shoots up to dangerous levels, and the wires can melt or set things on fire.

To protect against this:

  • Fuses: a fuse is a thin wire that melts at a specified current, breaking the circuit before damage spreads.
  • Circuit breakers: switches that automatically open when too much current flows. Can be reset after the fault is fixed.
  • RCDs (residual current devices): detect tiny imbalances in current (a sign of leaking to earth, possibly through a person) and cut the power within milliseconds.
Try this Get a simple battery (1.5 V AA), a small bulb (the kind used in old torches, around 1.5 V) and two short wires. Tape the bulbs metal base to one end of the battery, and connect the other side of the bulb to the battery using the wires. The bulb lights up. Now interrupt either wire with your finger: the bulb goes off. Reconnect: it lights again. You have just built and operated your first electric circuit.
Deeper dive: how the National Grid keeps the UK lit

The UKs entire electricity system is one giant interconnected circuit, with thousands of power stations and tens of millions of homes and businesses all linked together. The network is called the National Grid, and keeping it stable is one of the most impressive engineering feats in the country.

At any moment, the Grid has to balance how much electricity is being generated with how much is being used. If generation falls short, voltage drops and lights dim. If generation is too high, voltage spikes and equipment can burn out. Both must match within seconds, all day every day.

The Grid has several layers:

  • Generation: gas power stations, wind farms, nuclear plants, solar farms and others.
  • Transmission: high-voltage lines (up to 400,000 V) that carry electricity long distances with minimum losses.
  • Distribution: lower-voltage lines (11,000 V down to 230 V) that bring electricity to your neighbourhood.
  • Connection: the actual wires entering your home.

One of the most famous moments in Grid history was during the 2012 London Olympics. The Grid carefully predicted and managed huge surges of demand when millions of people switched on kettles and TVs at the same time (during a TV ad break or after a major event). The biggest spike in history was for the World Cup match between England and Germany in 1990: a 2,800 MW surge when the game ended, equivalent to suddenly turning on a small power station.

Today the Grid is going through huge change. The proportion of electricity from wind and solar (which vary with the weather) is increasing. Batteries, electric vehicles and smart grids are starting to balance supply and demand minute by minute. The basic idea (a giant parallel circuit) is the same as your home wiring, just scaled up to a country.

For more, see conductors and insulators and what is electricity.