Charge and current
Electric current is a rate of flow of charge. Connect the macroscopic current to the motion and number of charge carriers in a conductor.
Specification outcomes (10)
- 4.1.1(a) Define electric current as the rate of flow of charge, I = ΔQ / Δt.
- 4.1.1(b) Define the coulomb as the unit of charge.
- 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(d) Understand that net charge on a particle or object is quantised and a multiple of e.
- 4.1.1(e) Understand current as the movement of electrons in metals and movement of ions in electrolytes.
- 4.1.1(f) Distinguish between conventional current and electron flow.
- 4.1.1(g) Apply Kirchhoff’s first law as the conservation of charge at a junction.
- 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.
- 4.1.2(c) Distinguish between conductors, semiconductors and insulators in terms of number density of charge carriers.
H156 v2.0 · May 2026
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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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