PSLE Science · Primary 5 and 6
Electrical Circuits
Electrical circuits are part of the Systems and Energy themes in PSLE Science. You need to know what makes a closed circuit, which materials conduct electricity, and how the number and arrangement of batteries and bulbs change how brightly bulbs light up. Circuit questions are very common, and many are about predicting which bulbs light up when a switch is opened or a bulb fuses. The key skill is tracing each path around the circuit, and this guide shows you how.
What the syllabus expects
- Identify the parts of an electric circuit and read simple circuit diagrams
- Explain that a bulb lights up only when it is part of a closed circuit
- Classify materials as electrical conductors or electrical insulators
- Predict how the number and arrangement of batteries and bulbs affect the brightness of bulbs
- Compare bulbs in series and in parallel, including what happens when one bulb fuses
- Describe how to use electricity safely
Closed circuits, conductors and insulators
An electric circuit is a path along which electricity flows. A simple circuit has a battery, which is the source of electrical energy, connecting wires, a bulb and a switch. Electricity flows only when there is a closed circuit, which is an unbroken path from one end of the battery, through the bulb, and back to the other end. Opening a switch makes a gap, so the circuit becomes an open circuit and the bulb goes out. A fused bulb also breaks the circuit, because the thin wire inside it, the filament, is broken. Materials that allow electricity to pass through them are electrical conductors, such as metals like copper, iron and aluminium, and pencil lead. Materials that do not allow electricity to pass through them are electrical insulators, such as plastic, rubber, wood, glass and dry cloth.
Batteries in series and in parallel
In a circuit diagram, each battery is drawn as a long line and a short line, with the long line as the positive end. Batteries are in series when they are joined one after another, positive end to negative end, in a single line. The more batteries there are in series, the greater the electric current and the brighter the bulbs. For example, a bulb connected to two batteries in series is brighter than the same bulb connected to one battery. Batteries are in parallel when each is on its own branch, connected side by side. Batteries in parallel make a bulb about as bright as one battery would, but they last longer before running down. So if a question asks how to make bulbs brighter, add batteries in series.
Bulbs in series and in parallel
Bulbs in series are on one single path, one after another. The more bulbs there are in series with the same batteries, the dimmer each bulb becomes, because the electric current through them is smaller. If one bulb in series fuses, the only path is broken, so all the bulbs go out. Bulbs in parallel are each on a separate branch, and each branch makes its own closed circuit with the batteries. Each bulb in parallel is about as bright as a single bulb connected to the same batteries. If one bulb in parallel fuses, only its own branch is broken, so the other bulbs stay lit. This is why the lights in a home are connected in parallel: one faulty light does not switch off the rest.
Switches and electrical safety
Where a switch is placed decides which bulbs it controls. A switch on the main path, which every branch must pass through, controls all the bulbs. A switch on one branch controls only the bulb or bulbs on that branch. To decide, trace each bulb's path from one end of the batteries back to the other and check whether the switch is on it. Electricity can be dangerous. Tap water conducts electricity, so never touch switches or plugs with wet hands. Do not poke metal objects into sockets, and do not plug too many appliances into one socket, because the wires can overheat and cause a fire. Replace wires whose plastic covering is damaged, because the bare metal wire could give an electric shock.
Worked example
Two batteries are connected in series. Bulbs P and Q are connected in parallel to the batteries. Switch S1 is on the branch with bulb P only. Switch S2 is next to the batteries, on the main path that all the electricity must pass through. (a) Which bulb lights up when S1 is open and S2 is closed? (b) Both switches are closed and bulb P fuses. What happens to bulb Q? Explain. (c) Both switches are closed and both bulbs work. A third battery is added in series. What happens to the brightness of P and Q?
- Trace the paths first. Every path passes through S2. Only the path through P also passes through S1. The path through Q does not pass through S1.
- For (a), S2 is closed, so the main path is complete. S1 is open, so P's branch has a gap. Q's branch forms a closed circuit, so only Q lights up.
- For (b), the fused filament breaks only P's branch. Q is on a separate branch that still forms a closed circuit with the batteries.
- So Q stays lit, and it stays about as bright as before, because each branch in parallel works on its own.
- For (c), more batteries in series give a greater electric current through each branch, so both bulbs become brighter.
Answer: (a) Only Q lights up. (b) Q stays lit, because it is on a separate branch that still forms a closed circuit. (c) Both P and Q become brighter.
Common mistakes
- Thinking a bulb lights up as long as it touches the battery. The bulb must be part of an unbroken path from one end of the battery back to the other.
- Thinking that adding bulbs in parallel makes each bulb dimmer. Each bulb in parallel is about as bright as one bulb alone. It is bulbs in series that become dimmer.
- Thinking batteries in parallel make bulbs brighter. They give about the same brightness as one battery but last longer. Batteries in series make bulbs brighter.
- Assuming a switch controls every bulb. A switch controls only the bulbs whose path passes through it.
- Calling water an electrical insulator. Tap water conducts electricity, which is why wet hands near switches are dangerous.
Exam tips
- Trace each path with your finger from one end of the batteries back to the other. A bulb lights up only if its path is complete.
- Use the words 'closed circuit' and 'open circuit' in explanations, not just 'connected' or 'not connected'.
- For brightness questions, count the batteries in series and the bulbs in series first, and treat each parallel branch on its own.
Practice questions
1. Easy: Why are electrical wires made of copper and covered with plastic?
Show answer
Copper is an electrical conductor, so it lets electricity flow through the wire. Plastic is an electrical insulator, so the covering stops electricity from passing to anyone who touches the wire.
2. Medium: Circuit A has 1 battery and 1 bulb. Circuit B has 1 battery and 2 bulbs in series. Circuit C has 2 batteries in series and 1 bulb. All bulbs and batteries are identical. Arrange the circuits from the brightest bulb to the dimmest.
Show answer
C, A, B. C has more batteries in series than A, so its bulb is brighter. B has the same battery as A but two bulbs in series, so each bulb is dimmer than A's bulb.
3. Harder: Ali wants to find out which of these objects are electrical conductors: a steel spoon, a plastic ruler, a pencil lead and a rubber band. Describe how he can test them and predict his results.
Show answer
He sets up a circuit with a battery, a bulb and wires, leaving a gap between two wire ends. He places each object across the gap in turn. If the bulb lights up, the object is a conductor. The bulb lights up with the steel spoon and the pencil lead, which are conductors, and not with the plastic ruler or the rubber band, which are insulators.
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