Electric circuits

What makes a complete circuit, current, voltage and resistance, circuit symbols and diagrams, series and parallel circuits, measuring with ammeters and voltmeters, conductors and insulators, and electrical safety.

Flick a light switch and the globe comes on instantly, yet the electrons in the wire are crawling along at less than a millimetre per second, slower than a snail. The wire is already packed with electrons. Closing the switch pushes them all at once, like water in a full hose, so the energy arrives at the globe almost immediately.

What is a circuit?

An electric circuit is a complete loop that charged particles (electrons) can flow around. Every working circuit needs:

an energy source, such as a cell, battery or power point, which pushes the electrons around;
conducting wires, usually copper, to carry them;
a load: something that transforms the electrical energy into a useful form, such as a lamp, motor or speaker;
usually a switch to open (break) or close (complete) the loop.

If there is a gap anywhere in the loop, the circuit is open and no current flows. A short circuit happens when current finds an easy path straight back to the source without going through the load. Lots of current flows, wires get very hot and batteries can be damaged or even catch fire.

Current, voltage and resistance

Quantity	What it means	Unit	Measured with
Current	How much charge flows past a point each second	ampere (A), often "amp"	An ammeter, connected in series (in the loop)
Voltage	How much energy each bit of charge carries; the "push" from the source	volt (V)	A voltmeter, connected in parallel (across the part)
Resistance	How much a part opposes the flow of current	ohm (Ω)	Calculated from voltage and current

A water model helps. The battery is like a pump that lifts water up (voltage), the flow of water through the pipes is the current, and a narrow section of pipe is like a resistance. The water isn't used up as it goes round; it delivers energy and returns to the pump. Electrons are the same: the current going into a lamp equals the current coming out. What the lamp uses is the energy they carry.

A typical AA cell supplies 1.5 V, a car battery 12 V and an Australian power point 230 V. A battery is really two or more cells joined together.

Circuit diagrams

Drawing real-looking batteries and globes would be slow and messy, so scientists use standard circuit symbols. Circuit diagrams are drawn with straight lines and square corners.

Standard circuit symbols. In a cell, the longer line is the positive terminal.

Series and parallel circuits

In a series circuit there is only one path for the current. In a parallel circuit each lamp has its own branch.

In a series circuit, all the parts are connected one after another in a single loop.

The same current flows through every part.
The battery's voltage is shared between the lamps, so adding more lamps makes each one dimmer.
If one lamp breaks, the loop is broken and everything goes out. Old Christmas tree lights were wired this way, so one dead globe meant hunting through every globe on the string.

In a parallel circuit, the parts are on separate branches.

Each branch gets the full battery voltage, so each lamp is as bright as it would be on its own.
The current splits between the branches, so the battery supplies more current in total and goes flat sooner.
If one lamp breaks, the others stay on. Houses are wired in parallel so that each appliance can be switched on and off by itself.

InteractiveSeries and parallel circuit lab

Try this

Start with one bulb, then add a second in series. What happens to the brightness and the ammeter reading?
Switch to parallel with two bulbs. Compare the brightness and the voltmeter reading with series.
Click a bulb to break it. Which circuit keeps working?
In which circuit does the battery have to supply the most current? What does that mean for battery life?

Conductors and insulators

Electrical conductors let current flow easily. Metals are good conductors because they have free electrons; copper is used for most wiring and gold for tiny connections in phones because it doesn't corrode. Graphite (pencil "lead") and salty water also conduct. Insulators, such as plastic, rubber, glass, dry wood and air, hardly conduct at all. Wires are coated in plastic so that current can't leak into you or into other wires.

Pure water is a poor conductor, but tap water, sweat and seawater contain dissolved salts that let them conduct. That's why you should never touch switches with wet hands or use mains appliances near a bath or pool.

Test yourself

Which instrument measures current?
Two identical lamps are in series with a battery. One lamp blows. What happens to the other?
Why are houses wired in parallel?
Which of these is an electrical insulator?
Voltage is measured in
A 6 V battery is connected to three identical lamps in series. The voltage across each lamp is

Name each circuit symbol.

Which type of circuit is described?

A circuit must be a complete . Current is measured in with an ammeter connected in . Copper is used for wires because it is a good . A circuit lets a large current bypass the load.

4 marks

Mia builds a circuit with a battery, a switch and two lamps in series. She wants to be able to remove one lamp without the other going out, and wants both lamps to be brighter. Describe what change she should make, and explain why it works. You may describe or sketch a circuit diagram.

Your answer