Press the button on a torch, and it lights up. Press it again, and it goes dark. Inside, the button opens and closes a path for electricity. That path is called an electric circuit.

Why does electricity need a loop?

Electricity in a wire is a flow of tiny particles called electrons. We call this flow an electric current. A battery pushes the electrons along. But they can only flow along a complete path: out of the battery, all the way around and back in.

A complete path like this is called a circuit. The word means “a way around”, because a circuit always leads back to where it started.

What are the parts of a circuit?

A simple circuit has four kinds of parts:

  • A source, such as a battery. It gives the push, called voltage.
  • Wires, which carry the electricity.
  • A load, such as a bulb, a buzzer or a motor. It turns the energy brought by the electricity into light, sound or movement.
  • A switch, which opens or closes the loop.

The wires are metal inside, because metal is a good conductor: it lets electrons through easily. Their plastic covering is an insulator, which keeps the electricity inside.

Open or closed?

A circuit with no gaps is closed. Electrons flow all the way around, and the bulb lights up.

A circuit with a gap is open. The electrons stop moving everywhere in the loop — not only at the gap. The bulb goes dark.

An open circuit and a closed circuit Two copies of the same circuit: a battery, a switch and a bulb joined by wires into a loop. Top: the switch is open, so there is a gap in the loop. The electrons in the wires stay still and the bulb is dark. Bottom: the switch is closed, the electrons flow around the loop and the bulb shines. open circuit: the bulb is off − + closed circuit: the bulb shines − + bulb battery switch
Top: the switch is open, so there is a gap and the bulb stays dark. The electrons are still in the wires, but they don’t move. Bottom: the switch is closed, the electrons flow around the loop and the bulb shines.

Watch out for the words! For a circuit, “closed” means the electricity flows, and “open” means it stops. For a door or a tap, it is the other way around.

What about circuits at home?

The lights in your home are on circuits too. Their loops are hidden inside the walls. When you press a light switch, you close a gap in the loop that leads to the lamp.

Every plug has at least two metal pins, because electricity needs a way to the lamp and a way back.

Your body can become part of a circuit, too. If electricity from a socket or a power line flows through you into the ground or into another wire, it can kill. A bird sitting on one power line is safe, because both its feet touch the same wire. Read more in why birds on wires are safe.

How do you draw a circuit?

Drawing real batteries and bulbs takes a long time, so people use simple symbols instead. A drawing made of these symbols is called a circuit diagram.

  • A long thin line next to a short thick line is a battery. The long line is the plus end.
  • A circle with a cross inside is a bulb.
  • A line with a lifted lever is a switch.
  • Straight lines are wires.
Circuit symbols The symbols for a battery (a long thin line next to a short thick line, with the long line as plus), a bulb (a circle with a cross), a switch (a line with a lifted lever) and a wire (a straight line). Below them, a whole circuit with a battery, a switch and a bulb is drawn with these symbols. + − battery bulb switch wire
Each part has its own symbol. At the bottom, a whole circuit with a battery, a switch and a bulb is drawn with them.

One after another, or side by side?

A circuit can have more than one bulb. There are two ways to connect them:

  • One after another, also called in series. The bulbs sit in the same loop, like beads on a string. If one bulb breaks, the loop is broken, and all of them go dark.
  • Side by side, also called in parallel. Each bulb has its own path. If one bulb breaks, the others keep shining.
Bulbs one after another and side by side Left: a battery and two shining bulbs in one single loop, one after another. Right: a battery and two shining bulbs side by side, each bulb on its own path between the same two wires. one after another (in series) side by side (in parallel)
Left: two bulbs one after another, in a single loop. Right: two bulbs side by side, each with its own path.

The lamps in your home are connected side by side. That is why you can switch one off while the others stay on. How the current divides between the bulbs is explained in how electric current behaves.

What is a short circuit?

What if a wire joins the two ends of a battery directly, with no bulb in between? Then the electrons rush around far too easily. The wire and the battery get hot, and the battery can be damaged. This is called a short circuit.

At home, fuses and circuit breakers watch for this. If far too much electricity flows, they open the circuit.

Go deeper: Which way does the current flow? for grown-ups and the extra curious

Electrons flow out of the battery’s minus end, around the circuit and back into its plus end. But arrows for current in circuit diagrams usually point the other way, from plus to minus. This “conventional current” goes back to the 1700s, when Benjamin Franklin named the two kinds of charge — long before anyone knew about electrons. Both ways of describing it give the same answers, as long as you stick to one of them.

Similar symbols are used all over the world, so people can read a circuit diagram whatever language they speak. The ones in the picture above follow the international standard (IEC 60617), but some countries draw a few of them differently. In the United States, for example, a bulb is often drawn as a circle with a little loop inside.

Go deeper: Where the water-pipe picture goes wrong for grown-ups and the extra curious
  • Cut a water pipe, and the water pours out. Cut a wire, and no electrons pour out: the flow stops everywhere in the loop.
  • A closed tap stops water, but a closed switch lets electricity flow. The words work the other way around.
  • A battery does not run out of electrons: they are already in the wires. What runs out is the battery’s chemical energy.
  • At home, the current is alternating current: the electrons swing back and forth 50 times a second (60 in some countries) instead of going around. The circuit still has to be a closed loop, through the lamp and back.

Check yourself

Why does the bulb go out when you open the switch?

Show the answer

The loop has a gap, so the electrons stop flowing — Electricity needs a complete loop. The open switch makes a gap, so the flow stops all around the circuit.

Check yourself

One lamp in your home breaks. Why do the other lamps keep shining?

Show the answer

They are connected side by side, each with its own path — The lamps in a home are connected side by side. Each lamp has its own path, so one broken lamp does not break the loops of the others.