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AS Daily A Level Physics question

2026-04-21 OCR A DC Circuits (AS) — Meter loading (qualitative) OCR Physics A (H156) Module 4.3.2: Practical circuits – potential dividers and loading by meters (qualitative)

In a lab, two 100 kΩ resistors are connected in series across a fresh 9.0 V battery to create a 4.5 V midpoint. A student measures the p.d. across the lower 100 kΩ resistor using a digital voltmeter with 1.0 MΩ input resistance. The battery’s internal resistance can be neglected. Which statement best describes the effect of the meter on the reading?

  1. A The reading remains exactly 4.5 V because a voltmeter draws no current.
  2. B The reading becomes slightly higher than 4.5 V because the meter adds extra resistance in series with the lower resistor.
  3. C The reading becomes slightly less than 4.5 V because the meter’s finite input resistance is in parallel with the lower resistor, reducing its effective resistance and the divider output. (correct)
  4. D The reading drops to much less than 4.5 V because the meter effectively short-circuits the lower resistor.

Answer

The correct answer is C.

Correct: C — The reading becomes slightly less than 4.5 V because the meter’s finite input resistance is in parallel with the lower resistor, reducing its effective resistance and the divider output. A assumes an ideal voltmeter; with 1.0 MΩ comparable to 100 kΩ, some current is drawn and loading occurs. B treats the meter as a series element, but a voltmeter is connected in parallel with the component being measured. C is the only option consistent with a parallel load that slightly lowers the effective lower resistance (since 1.0 MΩ is large but not infinite). D exaggerates the effect; 1.0 MΩ is far from a short, so the reading is not drastically reduced.

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