An electric circuit is a loop that electricity can flow around. For a torch or any simple circuit to work, you need a power source such as a cell (battery), wire to carry the current, and a device such as a globe (also called a bulb) for the current to do work in. Every part must be joined together in one unbroken loop, with no gaps anywhere.
Scientists draw circuits using simple symbols instead of pictures. A straight line shows a wire. Two parallel lines of different length show a cell. A circle with a small loop or cross inside shows a globe. Diagrams often label these parts battery and bulb instead of cell and globe: they mean the same things. A break in the line with a small lever shows a switch, which can open or close the gap on purpose.
A circuit with no gaps anywhere is a closed circuit: current flows all the way around and the globe lights up. A circuit with a gap, even a tiny one such as an open switch or a disconnected wire, is an open circuit: current cannot flow, and the globe stays off.
Look at the circuit below. It is a picture of the real parts, not a symbol diagram: it shows the actual cell, wire, switch and globe as you would see them on a desk. It uses the everyday labels, so the cell is labelled Battery and the globe is labelled Bulb. When you DRAW a circuit yourself you use the symbols above instead of drawing the real parts.
Step 1: Is there a power source? Yes, a cell.
Step 2: Are all the parts joined by wire in one loop with no gaps? Yes.
Step 3: Is the switch open or closed? Closed.
Conclusion: this is a closed circuit, so current flows and the globe lights up.
Read each circuit described below. Decide whether the globe will light up. Write yes or no, then explain why in one sentence.
1. A cell is joined by wire to a closed switch. The switch is joined by wire to a globe. The globe is joined by wire back to the cell.
2. A cell is joined by wire to a switch. The switch is joined by wire to a globe. The globe is joined by wire back to the cell. The switch is open.
3. A cell is joined by wire to a globe. The other end of the globe is joined to a length of wire that just stops in mid-air: it is not joined back to the cell.
4. A cell is joined by wire to a closed switch, then by wire to a globe, then by wire to a second globe, then by wire back to the cell, all in a single loop.
5. A cell is joined by wire to a closed switch, then by wire to a globe, then by wire back to the cell. Partway along one wire there is a small cut, and the two cut ends do not touch.
Some materials let electric current pass through them easily. These are called conductors. Most metals are good conductors, including copper, aluminium, steel and iron. This is why the wire inside an electrical cord is made of copper: it lets current flow through easily to carry electricity where it is needed.
Other materials stop current from flowing through them. These are called insulators. Plastic, rubber, glass, wood and dry paper are all insulators. This is why the wire inside an electrical cord is covered in a layer of plastic: the plastic insulator stops the current from escaping and keeps people safe from an electric shock.
To work out whether a material is a conductor or an insulator, think about what it is made of. Metals are almost always conductors. Materials such as plastic, rubber, wood and glass are almost always insulators.
Is a steel spoon a conductor or an insulator?
Step 1: What is the spoon made of? Steel, which is a metal.
Step 2: Metals let electric current flow through them easily.
Conclusion: a steel spoon is a conductor.
Decide whether each material is a conductor or an insulator. Write the matching letter next to each material.
Circuit diagrams use symbols instead of pictures. Fill in the missing symbol names, then draw the circuits described below using the symbols.
1. Two parallel lines of different length: .
2. A straight line: .
3. A circle with a small loop or cross inside: .
4. A break in the line with a small lever: .
5. Draw a circuit diagram using the symbols above to show one cell, one closed switch, one globe and wire, all joined in a single loop.
6. Now draw the same circuit again, but this time show the switch open.
These questions are trickier. Read each one carefully and write your answer in full sentences.
1. Fill in the missing words: when a switch is open, there is a in the circuit, so current flow, and the globe .
2. A torch has a cell, wire, a switch and a globe, all connected in a closed circuit, and the switch is closed. Explain in your own words what happens to the current and the globe the moment the switch is opened.
3. A circuit has two switches, one after the other in the same loop, along with a cell and a globe. Explain why both switches must be closed for the globe to light.
4. A circuit is built correctly with a cell, wire, a switch and a globe, but somewhere in the loop a short plastic ruler has been used instead of a piece of wire, and the switch is closed. Will the globe light? Explain your answer using the word insulator.
5. A circuit is built correctly with a cell, wire, a switch and a globe, but somewhere in the loop a metal paperclip has been used instead of a piece of wire, and the switch is closed. Will the globe light? Explain your answer using the word conductor.
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