Physical ScienceMatter Interactions and EnergyYear 6 Science

Simple circuits: loops, switches and circuit diagrams

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What this lesson covers

The three parts every simple circuit needs
Closed circuits and open circuits
How a switch turns a circuit on and off
Why charge moves through metal but not plastic
The standard symbols for circuit parts
Drawing and reading circuit diagrams

Key idea

What a simple circuit needs

Electric charge is the tiny moving "stuff" of electricity. It can flow through some materials, such as metals.
A circuit is a path that charge can travel all the way round.
A simple circuit has three parts: a power source, such as a cell or battery, that pushes the charge; wires that carry it; and a load, such as a lamp, buzzer or motor, that uses the energy.
The parts must join up in a loop that goes from one end of the battery, through the load, and back to the other end.

Key idea

Closed and open circuits

A closed circuit is a complete loop with no gaps. Charge flows all the way round, so the lamp lights.
An open circuit has a gap somewhere. Charge stops flowing everywhere in the loop, not just at the gap, so the lamp goes out.
A gap can be a loose wire, a broken lamp, a flat battery, or a switch that is off.

Two circuit diagrams. Left: a closed circuit, a cell, a closed switch and a lamp in a complete loop, with the lamp lit. Right: an open circuit, the switch lifted to leave a gap, and the lamp off

Key idea

What a switch does

A switch is a part that opens or closes a gap in a circuit, so you can control whether charge flows.
When a switch is on, its metal parts touch and close the circuit. When it is off, they pull apart and open the circuit.
The light switch on your wall works the same way. Never open one up: the mains electricity in a house can kill.

Key idea

Try it: open and close the switch

Key idea

Conductors and insulators

Charge travels through the metal wires and through the metal parts inside each component, such as the thin wire inside a lamp.
A conductor is a material that lets charge flow through it easily. Most metals are good conductors.
An insulator is a material that does not let charge flow, such as plastic, rubber, wood and glass.
A wire has a copper conductor inside and a plastic insulator outside, so the charge stays in the wire and does not reach your fingers.

Material	Conductor or insulator?
Copper	Conductor
Aluminium foil	Conductor
Steel paper clip	Conductor
Plastic	Insulator
Rubber	Insulator
Dry wood	Insulator

Key idea

Circuit symbols

A circuit diagram is a simple drawing of a circuit that uses standard symbols instead of pictures.
The symbols are the same all over the world, so anyone can read your diagram and build the same circuit.
A cell is drawn as a long thin line beside a short thick line; the long line is the positive (+) end. A battery is two or more cells joined together.
Wires are drawn as straight lines with square corners, even if the real wires are curly.

The standard symbols: a cell as a long thin line and a short thick line, a battery as two cells, a lamp as a circle with a cross, a buzzer, an open switch, a closed switch and a wire

Worked example

Drawing a torch as a circuit diagram

A torch has two cells, a switch and a lamp. Draw its circuit diagram.

A circuit diagram of a torch: a battery of two cells along the top, a switch on the right side and a lamp along the bottom, joined in one rectangle of wire

Draw the battery: two cells side by side, long line to short line each timetwo cells
Draw the switch symbol in the loopit opens and closes the gap
Draw the lamp symbol: a circle with a crossthe load
Join them with straight wires and square corners into one loop

Worked example

Testing what closes a circuit

Room 5 built a circuit with a gap in it and put different objects across the gap. Which objects closed the circuit?

Object across the gap	Lamp	Circuit
Steel paper clip	Lit	Closed
Plastic ruler	Off	Open
Aluminium foil strip	Lit	Closed
Rubber eraser	Off	Open
Pencil lead (graphite)	Lit, dimly	Closed
Wooden pop stick	Off	Open

The clip and the foil are metal, and both lit the lampmetals conduct
Plastic, rubber and wood left the lamp offinsulators
Pencil lead lit it dimly: graphite conducts, but not as well as metala surprise
The paper clip, the foil and the pencil lead closed the circuit

Key idea

Try it: conductor or insulator?

Common mistakes

One wire from the battery to the lamp is enough to light it.
The lamp needs two wires so charge can go out from the battery and come back; it must be a complete loop.
When a switch is off, the circuit is closed.
Off means the switch has opened a gap, so the circuit is open and no charge flows.
The charge gets used up in the lamp, so less comes back to the battery.
The charge flows all the way round and back; it is energy that the lamp uses, not the charge.
In a diagram, a lamp is drawn as a little picture of a light bulb.
A lamp is drawn as a circle with a cross inside it, the standard symbol.

Try it at home

Find the circuit inside a torch

You need: a torch that takes AA or AAA batteries, paper, a pencil

With an adult, open the torch, take the batteries out and lay all the parts on a table.
Trace the path with your finger: from one end of the batteries, through the spring and metal strips, to the switch, the lamp and back to the other end.
Put the batteries back the right way and switch on. Then slide a small piece of paper between the batteries and switch on again.
Take the paper out and draw the torch's circuit diagram with the standard symbols.
Look for this: the torch only lights when the loop is complete; the paper is an insulator that opens the circuit.

Safety: Use only a torch with AA or AAA batteries, never one with coin or button cells, which can badly burn a small child who swallows one; ask an adult to help.

Your turn

A lamp will not light because one wire has come loose. Is the circuit open or closed?

Which of these would close the gap in a circuit: a plastic straw, a steel spoon, or a wooden pop stick?

What are the three parts every simple circuit needs?

In a circuit diagram, what does a circle with a cross inside it show?

Why do wires have a plastic coating?

Show answer

1. Open: there is a gap, so no charge flows. 2. The steel spoon, because steel is a metal and a conductor. 3. A power source (a cell or battery), wires, and a load such as a lamp. 4. A lamp. 5. Plastic is an insulator, so it keeps the charge in the wire and away from your fingers.

Try it

Circuit Construction Kit: DC

Full screen · PhET Interactive Simulations, University of Colorado Boulder, CC BY 4.0

Summary

Recap

A simple circuit is a complete loop of a power source, wires and a load.
A closed circuit has no gaps and charge flows; an open circuit has a gap and charge stops.
A switch opens or closes a gap to turn the circuit off or on.
Charge flows through conductors such as metals, not through insulators such as plastic.
Circuit diagrams use standard symbols and straight wires with square corners.

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