About this experiment
A torch, a doorbell and a toy car all need the same thing: a complete loop from one end of the battery, through the device and back to the other end. In this lab children join a battery, a bulb and a switch with wires and find out for themselves which arrangements work. Four puzzles show the classic mistakes: both wires on the same part of the bulb, a gap in the loop, a switch left off, and two bulbs sharing one path.
Learning goals
- Build a simple circuit that makes a bulb light
- Explain that a circuit must be a complete loop with no gaps
- Find the two contacts on a bulb and explain why both must be used
- Use a switch to open and close a circuit
- Describe how adding cells or bulbs changes how bright the bulbs are
How to use it
- 1Before touching anything, ask: how many wires do you think you need to light one bulb? Write down a guess.
- 2Build a circuit that lights the bulb. Then try to light it with only one wire. Is it possible?
- 3Try Puzzle 1. Where do the wires touch the bulb? Move one wire so the bulb lights.
- 4Try Puzzle 2 and Puzzle 3. Explain each fault in one sentence.
- 5Add the switch into your working circuit. Does it matter where in the loop you put it?
- 6Change to 1 cell, then to 3 cells. What happens to the bulb? Why do bulbs have a voltage written on them?
- 7Try Puzzle 4. Find a way to wire two bulbs so that both are bright.
The model
Assumptions
- Each cell gives 1.5 V. The bulb is a 2.5 V, 0.3 A torch bulb treated as a fixed resistance, so brightness follows the current.
- Wires have no resistance. A wire joining + to − directly is a short circuit: the battery heats up and nothing else works.
- A bulb has two contacts: the tip underneath and the metal screw case. The filament joins them inside the glass.
- A bulb burns out when the current is more than about 1.6 times its rating. Real filaments vary.
Try this in class
- Q1Why does a bulb need two wires and not just one?
- Q2A torch does not work. Give three different things that could be wrong.
- Q3Why is it dangerous to join the two ends of a battery with just a wire?
- Q4Two bulbs in a chain are dimmer than one bulb on its own. Why?
- Q5Name three materials you could use instead of a wire to complete the circuit, and three that would not work.
Frequently asked questions
What does the lab show by the moving dashes along the wires?
They mark the wires that are part of the working loop. Inside a real wire you cannot see anything move: tiny charged particles called electrons drift through the metal all around the loop at the same time.
Is it safe to try a short circuit at school?
With one small cell it only makes the cell and the wire warm and drains the battery, but it should not be done on purpose. Never do it with mains electricity or rechargeable batteries, which can get very hot or catch fire.
Which way should Puzzle 4 be wired?
Give each bulb its own path back to the battery: one wire from + to the tip of each bulb, and one wire from the side of each bulb to −. This is called a parallel circuit. Each bulb gets the full battery voltage, so both are bright.