Key Concepts — Electricity & Circuits
| Term | Meaning | Example |
|---|---|---|
| Conductor | A material that allows electricity to flow through it | Copper wire, iron nail, aluminium foil |
| Insulator | A material that does not allow electricity to flow through it | Rubber, plastic, wood, glass |
| Closed circuit | A complete, unbroken path — current flows and bulbs light up | Switch in the ON position |
| Open circuit | A broken path — current cannot flow, bulbs do not light up | Switch in the OFF position, broken wire |
| Series circuit | Components connected in ONE single loop | Fairy lights (old type) — one breaks, all go out |
| Parallel circuit | Components connected in SEPARATE branches | Home electrical wiring — one light off, others stay on |
⚡ Series Circuit
One single loop. If one bulb is removed or broken, the circuit opens and all bulbs go out. Adding more bulbs makes each bulb dimmer.
⚡ Parallel Circuit
Separate branches. If one bulb is removed, the other stays lit. Each bulb receives the full battery voltage — bulbs are brighter than in series.
| What changes? | Series Circuit | Parallel Circuit |
|---|---|---|
| Add a bulb | All bulbs get dimmer | Brightness of other bulbs stays the same |
| Remove a bulb | All bulbs go out | Other bulbs stay on |
| Add a battery | All bulbs get brighter | All bulbs get brighter |
| One bulb blows / breaks | Circuit opens — all bulbs off | Other branches still work — other bulbs on |
Multiple-Choice Questions (1 mark each)
Q1Which of the following materials is the best conductor of electricity?
Q2A circuit has one battery and one bulb. The switch is open. What will happen to the bulb?
Q3Two identical bulbs, X and Y, are connected in series with one battery. Bulb X is then removed from the circuit. What happens to Bulb Y?
Q4Two identical bulbs, P and Q, are connected in parallel with one battery. Bulb P is removed. What happens to Bulb Q?
Q5A student tests several objects to find out if they conduct electricity. Which object will complete the circuit and light up the bulb?
Q6Three identical bulbs are connected in series with one battery. Compared to a single bulb connected to the same battery, the three bulbs will be ___.
Q7The wires in an electrical plug are coated with plastic. Why is plastic used for the coating?
Q8A circuit has one battery and two bulbs in series. A second identical battery is added in series with the first. What happens to the brightness of the bulbs?
Q9Which of the following correctly describes a parallel circuit?
Q10The lights in HDB flats are wired in parallel. What is the main advantage of this arrangement?
Open-Ended Questions
Write your answers in the boxes below. Click Show Model Answer to check.
Q11A student sets up a circuit with one battery and three identical bulbs (A, B, C) in series. All three bulbs light up. [3 marks]
(a) The student unscrews Bulb B. What happens to Bulbs A and C? Explain why.
(b) The student then replaces Bulb B and adds a fourth identical bulb in series. What happens to the brightness of all the bulbs? Explain why.
✅ Model Answer
(a) Bulbs A and C go out. In a series circuit, all components are connected in a single loop. When Bulb B is removed, the circuit is broken (open circuit), so current cannot flow and neither Bulb A nor Bulb C can light up.
(b) All bulbs get dimmer. Adding a fourth bulb in series means the battery's electrical energy is now shared among four bulbs instead of three, so each bulb receives less energy and glows less brightly.
Q12Explain why the wiring in a Singapore home uses parallel circuits rather than series circuits. Give two reasons. [2 marks]
✅ Model Answer
Reason 1: In a parallel circuit, each appliance or light is on its own branch. If one is switched off or breaks down, the others continue to work. In a series circuit, if one appliance fails, the whole circuit breaks and all appliances stop working.
Reason 2: In a parallel circuit, each appliance receives the full supply voltage, so all appliances work at their proper brightness/power. In a series circuit, the voltage is shared, making appliances work less effectively.
Q13Siti wants to find out whether a mystery object is a conductor or an insulator. Describe a simple experiment she could carry out using a battery, two wires and a bulb. State the result for a conductor and the result for an insulator. [3 marks]
✅ Model Answer
Procedure: Siti connects the battery and bulb with two wires to form an incomplete circuit, leaving a gap. She places the mystery object in the gap so the two wire ends touch either side of the object.
Result — Conductor: The bulb lights up. The object allows current to flow through it, completing the circuit.
Result — Insulator: The bulb does not light up. The object does not allow current to flow, so the circuit remains open.
Q14Two identical bulbs are connected in parallel with one battery. The brightness of each bulb is recorded. Both bulbs are then reconnected in series with the same battery. Compare the brightness of the bulbs in the two arrangements. Explain your answer. [3 marks]
✅ Model Answer
The bulbs are brighter in the parallel circuit than in the series circuit.
In the parallel circuit, each bulb is on its own branch and receives the full voltage of the battery, so each bulb shines at full brightness.
In the series circuit, both bulbs are in one loop and the battery's voltage is shared equally between them. Each bulb receives only half the voltage, so each bulb is dimmer.
Q15A string of decorative lights in a shopping mall stops working completely when one bulb breaks. Are these lights connected in series or parallel? Explain how you know, and suggest how the wiring could be improved. [3 marks]
✅ Model Answer
The lights are connected in series. In a series circuit, all components share one single loop. When one bulb breaks, the circuit is broken (open circuit) and no current can flow, so all the other bulbs go out as well. This matches what happened.
Improvement: The lights should be rewired in parallel. In a parallel circuit, each bulb is on its own branch. If one bulb breaks, the remaining bulbs continue to receive current through their own branches and stay lit.
⚠️ Common PSLE Exam Mistakes
Mistake 1: Saying a bulb gets brighter when another bulb is added in parallel. The brightness of each bulb stays the same in parallel — each branch has its own full voltage.
Mistake 2: Confusing open and closed circuits. A closed circuit = complete path = bulb lights up. An open circuit = broken path = no current flows.
Mistake 3: Saying graphite (pencil lead) is an insulator. Graphite is actually a conductor — it contains free electrons. A pencil can complete a circuit through its graphite core.
Mistake 4: Forgetting that in a series circuit, removing one bulb breaks the entire circuit — the gap left behind is an open circuit, not a short circuit.
📋 Key Facts to Memorise
- Conductors allow current to flow (metals, graphite). Insulators do not (rubber, plastic, wood, glass).
- A complete (closed) circuit is needed for current to flow and bulbs to light up
- Series: one loop — remove one bulb → all go out; add a bulb → all get dimmer
- Parallel: separate branches — remove one bulb → others stay on at same brightness
- Adding batteries (in series) → more energy → bulbs get brighter in both circuit types
- Home wiring is parallel so each appliance works independently at full power
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