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AS Daily A Level Physics question

2026-01-22 OCR A Foundations of Physics (M2) Module 4.3.1 Electrical circuits — Series and parallel; potential divider (intro) Module 4.2.2 Energy, power and resistance — Potential difference distribution in series

A technician builds a potential divider using a 6.0 V DC supply, a fixed 1.0 kΩ resistor R1 from the positive terminal to a node, and a variable resistor R2 from the node to 0 V. The output V_out is measured across R2 (node to 0 V). As R2 is adjusted from much smaller than 1.0 kΩ to much larger than 1.0 kΩ, which statement must be true about V_out?

  1. A V_out is close to 0 V when R2 is very large, and approaches 6.0 V when R2 is very small.
  2. B V_out stays at about 3.0 V for any R2 value, because series resistors share the supply equally.
  3. C V_out tends to 6.0 V when R2 is much larger than 1.0 kΩ, and tends to 0 V when R2 is much smaller than 1.0 kΩ. (correct)
  4. D As R2 increases from very small to very large, V_out first rises then falls, reaching a maximum when R2 equals 1.0 kΩ.

Answer

The correct answer is C.

Correct: C — V_out tends to 6.0 V when R2 is much larger than 1.0 kΩ, and tends to 0 V when R2 is much smaller than 1.0 kΩ. When R2 ≫ R1 almost all the supply appears across R2; when R2 ≪ R1 almost none does. A reverses the limiting behaviour: a larger share of the voltage goes to the larger resistance, not the smaller. B assumes a fixed half-supply regardless of values, which only happens when R1 = R2. C states the correct limits for a potential divider taken across R2. D suggests a peak at R2 = R1, but V_out increases monotonically with R2 toward 6.0 V and does not exhibit a maximum at equal resistances.

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