A switch on a wire shared by two branches can turn off both bulbs. A switch on just one branch can turn off that branch’s bulb while the other stays lit. To decide which bulbs light, trace a complete path from one battery terminal through each bulb to the other terminal.
The MOE Primary Science syllabus covers closed circuits and bulbs arranged in series and parallel. This PSLE practice uses two switches in one circuit to show why the location of a break matters.
- Find the point where the circuit splits and where the branches meet again.
- Locate each switch on the shared path or on a single branch.
- Trace a complete path through each bulb separately.
- Name the gap and the affected bulb in your explanation.
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Sketch one circuit with two paths
Draw a battery and a wire from its positive terminal to a junction. Put switch S on this wire before the junction. From that junction, draw two branches that rejoin at a second junction. Put bulb A and switch T in the upper branch. Put bulb B alone in the lower branch. Draw a wire from the second junction back to the battery’s negative terminal.
Assume the battery and bulbs work and all the wires are connected as described. Both paths include S. Only the path through bulb A includes T. Mark the switches on your drawing before deciding which bulbs light.
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Worked example | Open T and keep S closed
With S and T both closed, there is a complete path through bulb A and a complete path through bulb B. Both bulbs light. Now open T while keeping S closed. The path through bulb A has a gap at T, so no current flows through bulb A and it goes out.
Bulb B stays lit. Starting at the battery’s positive terminal, trace through S, through the lower branch and bulb B, then back to the negative terminal. The path remains closed even though the upper branch is open.
A written answer can explain the results separately. Bulb A goes out because opening T breaks the path through A. Bulb B stays lit because the path through B and the battery is still closed.
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CHAPTER 3 OF 5 · Back to contents
Test what happens when the shared switch opens
Close T again and open S. The gap at S lies on the part of the circuit that both branches use. The two paths are broken at the same gap, so both bulbs go out. Closing T completes the upper branch locally, but the full path through the battery still contains the open switch S.
To check your diagram, put a finger on the positive terminal and follow the path towards either bulb. You meet the gap at S before reaching the split. The position of S explains why the state of T makes no difference while S is open.
| Shared switch S | Branch switch T | Bulbs lit |
|---|---|---|
| Closed | Closed | A and B |
| Closed | Open | B only |
| Open | Closed | Neither |
| Open | Open | Neither |
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Move the switch and predict the result
Keep T on the upper branch and move S to the shared return wire between the second junction and the negative terminal. If S is open, both bulbs still go out. Both paths now reach the gap after the branches rejoin.
For a second variation, put S in the lower branch next to bulb B. With S open and T closed, only bulb A lights. In your explanation, use the new switch position and trace the complete route through A.
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CHAPTER 5 OF 5 · Back to contents
Frequently asked questions
Do all bulbs go out whenever a switch is open?
The result depends on which complete paths the switch breaks. In this circuit, opening T breaks the path through A while the path through B can remain complete.
Can I decide from which bulb is drawn closest to the switch?
Trace the connections. A bulb drawn far away can share the same wire and be affected. A nearby bulb on a separate path can stay lit.
How can a parent check the explanation?
Ask the child to trace the path for each bulb with a pencil and stop at an open switch. Then have them say which bulb has a complete path through the battery.
Sources and next reading
Read the official SEAB syllabus · MOE 2023 Primary Science syllabus