Describe the parts of a simple electric circuit, use circuit symbols, and compare series and parallel circuits

What this dot point is asking

This dot point wants you to describe the parts of a simple electric circuit, recognise the common circuit symbols, and compare series and parallel circuits. The big idea is that an electric current is a flow of electricity that can only travel around a complete loop. A circuit needs a power source to push the current and a path with no breaks. How you connect the parts, in series or in parallel, changes how the circuit behaves.

The answer

What a circuit needs

An electric circuit is a complete loop of wire that lets an electric current flow. For a current to flow, the loop must be complete (closed) with no breaks, and it must have a power source.

The main parts of a simple circuit are:

• A cell (or battery, which is two or more cells): the source of electrical energy that pushes the current around.
• Wires: the path the current flows along.
• A switch: opens or closes the circuit to turn it on or off.
• A component that uses the energy, such as a bulb (gives light), a buzzer (gives sound), or a motor (gives movement).

If any part of the loop is broken, for example by opening the switch or a break in a wire, the current stops and the component switches off.

Circuit symbols

Instead of drawing the real parts, we use simple circuit symbols to draw a circuit diagram. You should know the common ones: a cell (a long thin line and a short thick line), a battery (two or more cells together), a switch (a line that lifts to break the circuit), a bulb (a circle with a cross inside), a buzzer, and wires (straight lines joining the parts). A neat circuit diagram uses these symbols joined by straight lines.

Series circuits

In a series circuit, all the parts are connected one after another in a single loop, so the current has only one path to follow.

The problem with a series circuit is that if one part breaks or is removed, the whole loop is broken and everything stops. If you remove one bulb from a series of bulbs, they all go out. Also, adding more bulbs in series makes each bulb dimmer, because they share the push from the cell.

Parallel circuits

In a parallel circuit, the parts are connected on separate branches, so the current has more than one path to follow.

The advantage is that each branch works on its own. If one bulb is removed or fails, the others stay lit, because the current can still flow around the other branches. Each bulb also gets the full push from the cell, so they stay bright. This is why the lights and sockets in a house are wired in parallel.

Comparing the two

The simple comparison: a series circuit is one single loop, so one break stops everything and extra bulbs share the energy. A parallel circuit has separate branches, so each part works on its own and one break does not stop the rest.

Examples in context

Example 1. A string of fairy lights. Older fairy lights were wired in series, so when one tiny bulb blew, the whole string went dark and it was hard to find the broken bulb. Newer lights are designed so the rest stay on, which is the advantage of independent branches like in a parallel circuit.

Example 2. The lights in your home. The lights and sockets in a house are wired in parallel. That is why you can switch the kitchen light on without the bedroom light coming on, and why one blown bulb does not turn off every light in the house. Each is on its own branch and gets the full voltage.

Try this

• Cue. State what happens to a bulb when the switch in its circuit is opened, and why. The bulb goes out, because opening the switch breaks the complete loop so no current can flow.

• Cue. In a series circuit of three bulbs, one bulb is removed. State what happens to the other two. They both go out, because removing one bulb breaks the single loop and stops the current to all of them.

• Cue. Explain why house lighting is wired in parallel rather than in series. In parallel each light is on its own branch, so each can be switched on its own, each gets the full voltage, and one failed bulb does not turn off the others.