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A student connects four identical light emitting diodes (LEDs) to a 2 V DC supply as shown - Scottish Highers Physics - Question 24 - 2019
Question 24
A student connects four identical light emitting diodes (LEDs) to a 2 V DC supply as shown. Which of the LEDs P, Q, R, and S will light? A. P only B. Q only C. Q a... show full transcript
Worked Solution & Example Answer:A student connects four identical light emitting diodes (LEDs) to a 2 V DC supply as shown - Scottish Highers Physics - Question 24 - 2019
Step 1
Step 2
Determine the voltage across each LED
Answer
Since the total supply voltage is 2 V and the connections are such that the voltage drop across each LED in series is approximately equal, the voltage divider rule applies. In this case, Q will share the 2 V with P, and the rest (R and S) will have the same 2 V from the parallel connection.
Step 3
Check which LEDs will light
Answer
Given that all LEDs are identical and require a threshold voltage to light, LED Q will light as it sees the full voltage from the supply in both paths. However, LED P also receives part of this voltage, thus will light too. On the other hand, LEDs R and S won't light since they rely on the voltage provided by Q, which is at the edge of its threshold. Therefore, only LED Q is active.
Step 4