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 (same brightness)
Conductors — allow current to flow: metals (copper, iron, steel), graphite
Symbols used:
cell (1)
battery (2 or more cells)
bulb
switch
In the questions, the energy source is called a battery — to find out how many cells it has, count the line-pairs.
Part A — Multiple choice (Booklet A style · 2 marks each)
Identical batteries and bulbs are used to set up the two circuits below.
Which of the following is correct about the brightness of the bulbs?
(1)Dimmest: A · Brightest: D, E and F
(2)Dimmest: B and C · Brightest: D, E and F
(3)Dimmest: A, B and C · Brightest: D, E and F
(4)Dimmest: B and C · Brightest: A
In Circuit 1, bulb A is in series with the whole circuit, so the full current passes through it. That current then splits between B and C, so B and C each carry only part of the current and are dimmer than A. In Circuit 2, each of D, E and F sits on its own branch directly across two batteries, so each is as bright as a single bulb on two batteries — brighter than any bulb in Circuit 1.
Beyond PSLE · not testedWhy exactly is each parallel bulb on two batteries the brightest? Each one gets the full voltage of both batteries across it, while bulbs B and C in Circuit 1 share their branch with only part of a single battery's current. Voltage is a secondary-school idea; at PSLE you compare brightness using current and the number of batteries/paths only.
Meiling set up four different circuits using identical bulbs and batteries as shown. In which circuit would bulb P be the brightest?
(1)Circuit (1)
(2)Circuit (2)
(3)Circuit (3)
(4)Circuit (4)
P is brightest when it gets the most current. Two things help: more batteries (in series) and being on its own branch (parallel) instead of sharing one path. Circuit (3) has both — two batteries and P on its own parallel branch. In (1) P only has one battery; in (2) P shares one path with another bulb (series); in (4) P has one battery and shares a series path, so it is the dimmest.
Beyond PSLE · not testedRanking the four for P: (3) two batteries across P alone → brightest; (1) one battery, full single-bulb brightness; (2) two batteries but shared between two series bulbs → about the same as one battery on one bulb; (4) one battery shared between two series bulbs → dimmest. The "shared path" idea is voltage being divided — formalised in secondary school.
Rahim made a battery-operated warning light with two bulbs, so that he is still alerted if one of the bulbs fuses. The diagram below shows how he connected the parts.
(a) In the answer space, draw and label a circuit diagram of Rahim's warning light, using the correct symbols for the battery, bulbs and switch. (2 marks)
(b) Rahim added more batteries to the circuit and found that both bulbs fused. Explain why the bulbs would not light up after they had fused, even when the switch is closed. (1 mark)
(a)The two bulbs are drawn in parallel (each on its own branch), with the battery and switch in the main part of the circuit:
(b)When a bulb fuses, the thin wire (filament) inside it melts and breaks, leaving a gap in that bulb. Since both bulbs have fused, there is a break in both branches, so the circuit is no longer complete through either bulb. No current can flow through the bulbs, so they do not light up even when the switch is closed.
Two bulbs in parallel · a fused bulb = a broken filament = a gap in the circuit
Marks for (a): one mark for using the correct symbols for battery, switch and bulb; one mark for showing the two bulbs in parallel (the whole point of the backup design). Adding more batteries pushed too much current through the bulbs, which is why they fused.
Devi wanted to set up a circuit to light a corridor using two special switches. Each switch can be turned to position A or position B. She set up the circuit so that the bulb is lit as described in the table.
Position of switch
Bulb is lit
Switch 1
Switch 2
A
A
Yes
A
B
No
B
A
No
B
B
Yes
(a) The diagram shows the parts of the circuit. Complete the circuit with wires so that it works as described in the table. (3 marks)
(b) Devi wanted to add a second bulb to the corridor without decreasing the brightness of the first bulb when it is lit. Describe how she should connect the second bulb. (1 mark)
(a)Join the two A contacts together with one wire, and the two B contacts together with another wire. Connect the battery to the common terminal of Switch 1, and connect the bulb between the common terminal of Switch 2 and the other end of the battery:
Now both switches at A complete the A–A path (bulb lit); both at B complete the B–B path (bulb lit). If the switches are at different positions, neither path is complete, so the bulb is not lit — exactly as the table requires. Either switch on its own can turn the bulb on or off, which is why this is used for corridors and staircases.
(b)Connect the second bulb in parallel with the first bulb (each bulb on its own branch). In parallel, each bulb still receives the full current from the battery, so the first bulb's brightness does not change. (Connecting it in series would make both bulbs dimmer.)
Join A–A and B–B · same position → lit · add bulbs in parallel to keep brightness
Beyond PSLE · not testedThis is a real "two-way" (SPDT) switch — each switch sends the common terminal to one of two contacts. The bulb lights only when both switches route the current to the same rail (A–A or B–B). At PSLE you only need to wire it and read the table; the formal logic (an "exclusive-NOR") comes much later.
Using the parts already drawn (a battery and one switch), design a circuit that meets all the requirements below:
One switch controls all the bulbs.
The other switch controls only the brighter bulb.
Two of the bulbs have the same brightness and cannot be switched on or off independently.
Using the correct symbols, complete the circuit diagram in the space below with three bulbs, one more switch and wires.
Put the given switch in the main line with the battery (it controls everything). Then make two parallel branches: one branch has a single bulb with the second switch (this single bulb is the brighter one); the other branch has two bulbs in series (these share the battery, so they are equally dim and cannot be controlled on their own).
Why it works:
·The main switch is in series with the battery, so opening it cuts current to all three bulbs.
·The second switch is in series with only the single-bulb branch, so it controls only that one bulb.
·The single bulb is alone on its branch, so it gets the full current and is the brighter bulb. The other branch has two bulbs in series, which share the current — they are equally dim and cannot be switched on or off on their own.
Main switch in the battery's line · 2nd switch on the single (brighter) bulb · two series bulbs = equal & dim
Beyond PSLE · not testedThe single bulb is brighter because it has the battery's full voltage across it, while the two series bulbs split that voltage in half each. "Sharing voltage" is the secondary-school way of explaining why series bulbs are dimmer; for PSLE, "the current is shared between the two bulbs so each is dimmer" is the expected reasoning.
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