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Charge flow and electrical power Build a resistor network from Ohm’s law A potential divider uses shared current Changing resistance makes a voltage sensor Geometry separates resistance from resistivity Power equations must use the component’s operating point Charge and current4.1 Energy, power and resistance4.2 Resistance4.2.3 Resistivity4.2.4 Power in circuits4.2.5 Series and parallel circuits4.3.1 Potential dividers4.3.3
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Changing resistance makes a voltage sensor

OCR A AS Physics · Two ideas, one explanation

An LDR or thermistor changes resistance with the environment. A potential divider converts that resistance change into an output-voltage change.

Sensor resistance changes → voltage division changes

Follow the reasoning

  1. Identify how the sensor resistance responds to the environment.
  2. Check whether output is taken across the sensor or the fixed resistor.
  3. Use the divider equation to predict the output change.
Specification outcomes (4)
  • 4.2.3(c)(i) Describe and sketch I–V characteristics for resistor, filament lamp, thermistor, diode and LED.
  • 4.2.3(d) Describe how the resistance of a light-dependent resistor (LDR) varies with light intensity.
  • 4.3.3(b) Describe potential divider circuits with variable components such as LDRs or thermistors.
  • 4.3.3(c)(i) Use potential divider equations Vout = (R₂ / (R₁ + R₂)) × Vin and V₁ / V₂ = R₁ / R₂.
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H156 v2.0 · May 2026

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