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Charge flow and electrical power Where does a battery’s power go? 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 Mechanical and electrical power measure the same rate Charge and current4.1 Energy, power and resistance4.2 Power3.3.3 Resistance4.2.3 Resistivity4.2.4 Power in circuits4.2.5 Series and parallel circuits4.3.1 Internal resistance4.3.2 Potential dividers4.3.3
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Power equations must use the component’s operating point

OCR A AS Physics · Two ideas, one explanation

Power can be calculated from the voltage and current at a component’s operating point. A filament lamp’s resistance changes as it heats up, so one fixed resistance cannot describe its whole graph.

P = VI = I²R = V²/R

Follow the reasoning

  1. Read voltage and current for the same operating point.
  2. Calculate power from their product.
  3. If using a resistance formula, calculate R = V/I at that point.
Specification outcomes (3)
  • 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
  • 4.2.3(a) Define resistance using R = V / I and state the unit ohm.
  • 4.2.3(c)(i) Describe and sketch I–V characteristics for resistor, filament lamp, thermistor, diode and LED.
Open the official OCR AS specification ↗

H156 v2.0 · May 2026

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