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Charge flow and electrical power Mean drift velocity is a motion quantity Current connects microscopic drift with charge transfer Kirchhoff’s first law conserves charge Charge and current4.1 Energy, power and resistance4.2 Kinematics3.1.1 Charge4.1.1 Mean drift velocity4.1.2 Series and parallel circuits4.3.1
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Current connects microscopic drift with charge transfer

OCR A AS Physics · Two ideas, one explanation

Current can be found by counting charge per second or by considering how carriers drift through a conductor. These are two descriptions of the same charge flow.

I = ΔQ/Δt = nAev

Follow the reasoning

  1. Identify charge per carrier and carrier number density.
  2. Use the cross-sectional area through which the current flows.
  3. Relate the carrier drift to the macroscopic current.
Specification outcomes (5)
  • 4.1.1(a) Define electric current as the rate of flow of charge, I = ΔQ / Δt.
  • 4.1.1(c) Know that the elementary charge e equals 1.6 × 10⁻¹⁹ C and that electrons and protons carry charges −e and +e respectively.
  • 4.1.1(e) Understand current as the movement of electrons in metals and movement of ions in electrolytes.
  • 4.1.2(a) Define and understand mean drift velocity of charge carriers.
  • 4.1.2(b) Use and apply I = A n e v, where n is the number density of charge carriers.
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H156 v2.0 · May 2026

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