Closed vs open circuits. Components: battery, wire, bulb, switch. Conductors and insulators. Effect of number of batteries and bulbs in series and parallel. Circuit diagrams.
series circuitparallel circuitconductors
Electrical systems — key facts
Series circuit
One pathCurrent flows through all components
More bulbsEach bulb dimmer
More batteriesAll bulbs brighter
Switch offALL bulbs go out
Bulb removedALL bulbs go out (circuit broken)
Parallel circuit
Multiple pathsCurrent splits at each branch
More bulbsSame brightness each
More batteriesAll bulbs brighter
Switch offOnly that branch goes out
Bulb removedOther bulbs stay on
Conductors — allow current to flow: metals (copper, iron, steel), graphite
The diagram shows a circuit with a battery, a switch and a bulb. The switch is open. Which statement correctly describes the circuit?
ACurrent flows and the bulb lights up.
BThe circuit is incomplete, so no current flows and the bulb does not light up.
CThe circuit is complete and the bulb lights up dimly.
DCurrent flows through the switch but not through the bulb.
An open switch creates a gap in the circuit — the path is broken and no current can flow. A closed (complete) circuit with no gaps allows current to flow. The switch is the most basic way to open and close a circuit.
Three identical bulbs X, Y and Z are connected as shown. Bulb X is connected in series with the battery. Bulbs Y and Z are connected in parallel with each other, and this parallel combination is in series with X. What happens when bulb Y is removed from the circuit?
AX goes out. Z stays on at the same brightness.
BX, Y and Z all go out.
CX and Z stay on. Z becomes brighter. Y goes out.
DX goes out. Z becomes dimmer.
Removing Y breaks only Y's branch — Z's branch remains complete, so Z stays on. With Y removed, all current now flows through Z alone, so Z becomes brighter. X is in series with the whole parallel combination, so X also stays on. However, because the overall resistance of the circuit increases when one parallel branch is lost, less total current flows — so X actually becomes slightly dimmer.
A student sets up a circuit with a battery, a bulb and two connecting wires with bare ends. She tests different objects by placing each one between the two wire ends and observing whether the bulb lights up.
The table shows her results.
(a) State one conclusion that can be drawn from this experiment. (1 mark)
(b) The student wants to test whether a graphite pencil lead is a conductor or insulator. She places the pencil between the two wire ends and the bulb glows dimly. What can she conclude? (1 mark)
(c) A classmate suggests the student should use the same length of each object tested. Why is this important for the experiment to be a fair test? (1 mark)
(a)Metals (copper, steel) are electrical conductors, while non-metals (rubber, wood, plastic) are electrical insulators. Accept any valid conclusion linking material type to conductivity.
(b)Graphite (pencil lead) is a conductor — it allows current to flow (the bulb lit up). However, it is a weaker conductor than metals as the bulb only glowed dimly.
(c)Using the same length ensures that the length is kept as a constant (unchanged) variable. A longer object may resist current flow more than a shorter one, so if lengths differ, the test is not fair — the results could be affected by length rather than the material alone.
Metals → conductors · Non-metals → insulators (graphite is an exception)
Graphite is the exception students forget. Most non-metals are insulators, but graphite (pencil lead) is a conductor. It always appears as a trick item in PSLE electrical questions.
The diagram shows two circuits, P and Q. Both circuits use identical batteries and identical bulbs.
(a) In which circuit, P or Q, will the bulbs be brighter? Explain your answer. (2 marks)
(b) Bulb P1 is removed from Circuit P. What happens to bulb P2? Explain why. (1 mark)
(c) Bulb Q1 is removed from Circuit Q. What happens to bulb Q2? Explain why. (1 mark)
(a)Bulbs in Circuit Q (parallel) will be brighter. In Circuit P (series), the current must flow through both bulbs, so it is shared — each bulb receives less current and glows dimmer. In Circuit Q, each bulb has its own separate path, receiving the full current from the battery.
(b)P2 goes out. Removing P1 creates a gap (break) in the only path for current to flow. Since series circuits have only one path, no current can flow through any part of the circuit.
(c)Q2 stays on (and becomes brighter). Removing Q1 only breaks Q1's branch. Q2 has its own separate path to the battery, so current continues to flow through Q2. With Q1 gone, all the current now flows through Q2 alone — making it brighter.
Series: one path, shared current, dimmer · Parallel: separate paths, full current, brighter
The most tested contrast in PSLE circuits: removing a bulb from a series circuit kills all bulbs; removing a bulb from a parallel circuit only affects that branch and makes the remaining bulbs brighter.
A student wants to light up three rooms in a doll's house. Each room has one bulb. She wants each room's bulb to be switched on and off independently, without affecting the other rooms.
(a) Should she connect the bulbs in series or parallel? Explain your answer. (2 marks)
(b) She currently uses 1 battery and finds the bulbs are too dim. She adds a second identical battery in series with the first. What effect does this have on the brightness of each bulb? (1 mark)
(c) She now has 3 bulbs in parallel and 2 batteries in series. One bulb burns out (breaks). Predict what happens to the remaining two bulbs, and explain your answer. (2 marks)
(a)Parallel. In a parallel circuit, each bulb has its own separate branch. Switching one bulb on or off (or removing it) does not affect the other branches — the other bulbs stay on. In a series circuit, switching off or removing one bulb would break the entire circuit and all bulbs would go out.
(b)Adding a second battery in series increases the current in the circuit. Each bulb receives more current and becomes brighter.
(c)The remaining two bulbs stay on and become brighter. Since the bulbs are in parallel, the burnt-out bulb only breaks its own branch. The other two branches are unaffected. With one bulb gone, the total current is now shared between two bulbs instead of three — so each remaining bulb receives more current and glows brighter.
Parallel for independence · Series batteries for more brightness · Parallel bulb out → rest brighter
Part (a) connects circuit knowledge to a real-world design purpose — a very common PSLE Booklet B approach. The key phrase to use is "each bulb has its own separate branch" to earn the mark.
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