What Makes a Bulb Light?
A bulb lights when electricity flows through it. For electricity to flow there must be a closed circuit: an unbroken loop that starts at one end of a cell, goes along a wire to the bulb, through the thin wire inside the bulb (the filament), and back along another wire to the other end of the cell. The cell is the energy source. It pushes the electricity round the loop, and the bulb changes some of that electrical energy into light and heat.
If there is a gap anywhere in the loop, the circuit is an open circuit and no electricity flows anywhere in it, not just at the gap. That is why one loose wire switches off the whole torch. On the board above, try taking away a single wire from a working circuit: the bulb goes out at once, even though the wire you removed was nowhere near it.
Switches: Opening and Closing the Loop
A switch is simply a safe way of making and breaking a gap. When the switch is closed, its metal lever touches both contacts and completes the circuit. When the switch is open, the lever is lifted and there is an air gap, so the circuit is open and the bulb goes off. Children often say a switch is “on” when it is open, because a door that is open lets you through. In electricity it is the other way round: an open switch stops the electricity, and a closed switch lets it through.
Conductors and Insulators
Materials that let electricity flow through them easily are electrical conductors. Materials that do not are electrical insulators. In the tester, any object in the gap that lights the bulb is a conductor.
| Object | Made of | Bulb lights? | Conductor or insulator |
|---|
Almost all metals are good conductors, which is why wires have copper inside. Plastic, rubber, wood, glass and cotton are insulators, which is why wires are covered in plastic and why the handles of electrical tools are made of plastic or rubber. One surprise is pencil lead: it is not really lead but graphite, a form of carbon, and it conducts even though it is not a metal. The bulb is usually a little dimmer with pencil lead than with a paper clip.
Circuit Symbols
Drawing a picture of every cell and bulb takes a long time, so scientists use circuit symbols. Press Circuit symbols above the board to switch between pictures and symbols. These are the symbols used in Singapore primary science:
- Cell: a long thin line and a short thick line. The long line is the positive (+) end. Two or more cells joined together make a battery.
- Bulb: a circle with a cross inside it.
- Switch: a short lever between two dots; the lever is drawn lifted when the switch is open and flat when it is closed.
- Wire: a straight line. Circuit diagrams are drawn with straight lines and square corners, even though real wires bend.
Series and Parallel Circuits
When bulbs are joined one after another in a single loop, they are in series. There is only one path for the electricity, so it must pass through every bulb in turn. When bulbs are on separate branches, each joined across the cell, they are in parallel. The electricity has more than one path to take.
| Change | What happens to the bulbs | Why |
|---|---|---|
| Add a second bulb in series | Both bulbs are dimmer | Each bulb makes it harder for electricity to flow, so less electric current flows round the single loop |
| Add a second cell in series (same way round) | The bulb is brighter | Two cells push harder than one, so more current flows |
| Add a second bulb in parallel | Each bulb is as bright as one bulb alone | Each branch is joined straight across the cell, as if it were on its own |
| One bulb blows in series | All the bulbs go out | The only path is broken |
| One bulb blows in parallel | The other bulbs stay lit | Only that branch is broken; the others are still complete loops |
Parallel circuits use up the cells faster, because more current flows from the cell in total, but they are much more useful in buildings: every light at home is on its own branch, so you can switch one off without plunging the whole house into darkness.
Short Circuits
Electricity takes the easiest path. If a bare wire joins the two ends of a bulb, almost all the electricity rushes through the wire instead of the filament, and that bulb goes out. This is a short circuit around the bulb. Any other bulb still in the loop actually gets brighter, because there is now one fewer bulb in the way. If a wire joins the two ends of the cell with nothing else in between, the whole cell is short-circuited: a very large current flows, no bulbs light, the cell goes flat quickly and it can become hot. The board warns you when this happens. Try the “short circuit” example and move the extra wire to see the difference.
Common Misconceptions
- “Electricity is used up by the first bulb.” In a series circuit the bulbs are equally bright, whichever is nearest the cell. The same current flows all the way round the loop.
- “One wire from the cell is enough.” The loop must go back to the other end of the cell. A bulb with only one wire never lights.
- “All metal things are magnetic, so conductors are magnetic.” Conducting electricity and being attracted to a magnet are different properties. Aluminium foil and copper conduct well but are not magnetic.
- “More bulbs always means more light.” In series, more bulbs with the same cells means each bulb is dimmer.
- “Turning one cell round does not matter.” Cells must face the same way to work together. If two cells face opposite ways their pushes cancel and the bulb stays off. Tap a cell on the board to see it.
Safety First
Cells (batteries) are safe for experiments because they give only a small push. The mains electricity in wall sockets is many times stronger and can kill. Never poke anything into a socket, never use wires from the mains for experiments, keep water away from electrical appliances, and never open a mains plug. Do not short-circuit a cell on purpose: it gets hot. Button cells can be very dangerous if swallowed, so keep them away from young children.
How to Use This Page
- Build on the board. Tap a part, then tap a place between two dots, or drag the part straight there. Start with the “one bulb” example and change one thing at a time.
- Read the panel beside the board. It says whether the circuit is open or closed and why each bulb is lit, dim or off.
- Test objects in the conductor tester and see the results table fill up.
- Choose your year and a challenge, then press Start. P5 challenges ask you to make the bulb light, predict open and closed circuits, and sort conductors. P6 challenges cover series and parallel circuits, brightness and faults. Each round earns up to three stars.
- Print a worksheet with circuit symbols, circuits to predict, a conductor table and an answer page.
Links to the Primary Science Syllabus
Electrical circuits are taught in upper primary science in Singapore. Children learn that an electric circuit needs an energy source such as a cell and a closed loop, that materials can be grouped into electrical conductors and insulators, that circuits are drawn with standard symbols, and how the number and arrangement of cells and bulbs in series and parallel affect the brightness of the bulbs. Examination questions often show a circuit diagram with one fault, such as an open switch, a blown bulb or an insulator in the loop, and ask which bulbs light and why. The challenges on this page practise exactly that reasoning.
Tips for Parents and Teachers
- Predict, then test. Before changing the circuit, ask which bulbs will light. Then make the change and talk about any surprises.
- Trace the loop with a finger. Start at the cell and follow the wires. If your finger can get all the way back to the other end of the cell through the bulb, the bulb lights.
- Use full answers. “The bulb does not light because the switch is open, so the circuit is not closed and no electric current flows.”
- Try it for real. A cell holder, two small bulbs in holders and crocodile-clip wires cost very little and make the board on screen come alive.
Nothing to Install, Nothing Uploaded
The circuit lab runs entirely in your browser on any phone, tablet or computer, with no account and no download. Your year, challenge and stars are kept only in your own browser; circuits and answers are never sent to us.
Frequently Asked Questions
Electricity can only flow when there is a complete, unbroken loop from one end of the cell, through the bulb, and back to the other end of the cell. A gap anywhere, such as an open switch, a missing wire or an insulator in the way, stops the flow everywhere in that loop, so the bulb stays off.
In a series circuit there is only one path, and every bulb in it makes it harder for the electricity to flow. With two bulbs in series and one cell, less electric current flows through each bulb than with one bulb, so both bulbs are dimmer. Adding another cell in series, facing the same way, makes them bright again.
Each bulb in a parallel circuit is on its own branch connected to the cell. If one bulb blows, only its own branch is broken. The other branches are still complete loops, so the other bulbs stay lit, and just as brightly. This is why the lights in a house are connected in parallel.
Yes. Pencil lead is not lead at all: it is mostly graphite, a form of carbon. Graphite is one of the few non-metals that conducts electricity, so a bulb lights when pencil lead is put in the gap of a circuit, although it may be a little dimmer than with a metal paper clip.