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Charge flow and electrical power Charge and current4.1 Energy, power and resistance4.2 Charge4.1.1 Mean drift velocity4.1.2
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Charge flow and electrical power

OCR A AS Physics · Two ideas, one explanation

Current measures charge transferred per second, while potential difference measures energy per unit charge. Multiplying them gives energy transferred per second.

I = ΔQ/Δt V = W/Q P = VI

Follow the reasoning

  1. Identify the current through the component.
  2. Find the potential difference across that same component.
  3. Multiply to obtain power and use energy = power × time if power is constant.
Specification outcomes (4)
  • 4.1.1(a) Define electric current as the rate of flow of charge, I = ΔQ / Δt.
  • 4.1.1(e) Understand current as the movement of electrons in metals and movement of ions in electrolytes.
  • 4.2.2(d) Use energy transfer equations W = VQ and W = EQ.
  • 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
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H156 v2.0 · May 2026

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PhysicsUK Solve11 marks

A wearable warning indicator is connected to a supply that maintains 3.0 V across the indicator whenever it is on. During each 0.20 s pulse, a charge counter measures 0.012 C passing through it. Current is constant during a pulse and zero between pulses. The indicator produces one identical pulse every 1.0 s; it produces 600 pulses in 10 minutes. Ignore any current used by other electronics. (a) Calculate the current while the indicator is on. [2] (b) Calculate its electrical power while it is on. [2] (c) Calculate the electrical energy transferred during one pulse using charge and potential difference. [2] (d) Calculate the total electrical energy transferred in 10 minutes and the average power over that time. [3] (e) A pupil says the current leaving the indicator must be smaller because the indicator has used up some electrons. Explain why this statement is wrong, distinguishing charge transfer from energy transfer. [2]

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