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A uniform resistance wire of length 100 cm is connected across an ideal DC voltage source of 28 V . A testing branch containing a green light emitting diode (LED) of threshold voltage 2 V in series with a Zener diode is connected between one end of the wire and a sliding jockey. When the jockey is tapped at a distance of 25 cm from the end, the LED barely begins to emit light. Assuming the Zener diode is in its rever

Correct answer

5

Step-by-step solution

Voltage gradient along the uniform wire = 28 100 = 0.28 V/cm . Voltage across the 25 cm segment is V = 0.28 25 = 7 V . Since the LED just begins to glow, the branch draws negligible current. The voltage across the segment equals the sum of the voltage drops required to turn on the LED and break down the Zener diode. V = V_ LED + V_ Z 7 = 2 + V_ Z V_ Z = 5 V . Answer: 5

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