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A Levels Physics (9702)•9702/13/M/J/19
Question 34 from 9702/13/M/J/19

Explanation

Reversed wiring requires both switches in matching Z2 position Steps:

  • Original setup lights lamp with switches in opposite positions via crossed travelers.
  • Reversing X and Z connects supply to Z and routes X through the second switch, swapping traveler logic to require same positions.
  • Trace paths: supply enters at Z; second switch (Y common) must connect to Z2 for matching traveler.
  • First switch must then connect X to Z2 to complete path to lamp via shared Z2 wire.

Why D is correct:

  • Both to Z2 aligns travelers, closing circuit from supply at Z through Z2 to lamp, following reversed two-way switch logic.

Why the others are wrong:

  • A: Y1 and Z1 mismatch travelers, leaving path open.
  • B: Y1 and Z2 select opposite travelers, no connection.
  • C: Z1 and Z2 use different travelers, circuit incomplete.

Final answer: D

Topic: Practical circuits

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