Topic 3: Electric Circuits — Edexcel A-Level Physics
Test yourself on Topic 3: Electric Circuits with PEARSON EDEXCEL A-Level practice questions.
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Topic 3: Electric Circuits explained
Electric current measures how quickly charge passes a point in a circuit.
Read the full explanation
If a quantity of charge ΔQ flows through a cross-section in a time interval Δt, the average current is I = ΔQ/Δt, so current is measured in coulombs per second, which is the ampere (1 A = 1 C s⁻¹). Rearranging gives ΔQ = IΔt and Δt = ΔQ/I. In a metal the charged particles are electrons, each carrying charge of magnitude 1.60 × 10⁻¹⁹ C, so a steady current of 2.0 A for 30 s transfers ΔQ = 2.0 × 30 = 60 C, corresponding to 60 ÷ (1.60 × 10⁻¹⁹) = 3.75 × 10²⁰ electrons. Current direction is conventionally the direction of positive charge flow, opposite to electron drift in metals. In an MCQ, identify the two quantities involved, check the time is in seconds, and choose the option consistent with the rearranged equation.
Your focus
- Explain that electric current is the rate of flow of charged particles.
- Apply I = ΔQ/Δt, including rearranged forms, to calculate current, charge or time.
- Convert time and charge into consistent SI units and interpret the result in amperes.
Topic 3: Electric Circuits exam tips
Marking Points
- States that current is the rate of flow of charged particles, i.e. charge transferred per unit time.
- Uses I = ΔQ/Δt correctly, with charge in coulombs and time in seconds, giving current in amperes.
- Rearranges the equation correctly, for example ΔQ = IΔt or Δt = ΔQ/I, when the question supplies current and time or current and charge.
- Converts time intervals to seconds before substitution and keeps consistent units throughout.
- Recognises that 1 A = 1 C s⁻¹ and that conventional current direction is opposite to electron flow in a metal.
Examiner Tips
- 💡Write down the rearranged equation before substituting so the operation is clear and units can be checked.
- 💡Check the unit of every quantity: charge in C, time in s, current in A, and convert prefixes such as mA or μs first.
- 💡Sanity-check the answer: a current of a few amperes over tens of seconds should give tens of coulombs, not a tiny fraction.
Common Mistakes
- Treating current as a quantity of charge rather than a rate: the correction is that current equals charge divided by time, so the same charge in half the time is double the current.
- Substituting time in minutes or milliseconds without converting: the correction is to convert every time to seconds before dividing.
- Confusing the charge on one electron with the total charge transferred: the correction is to find total charge from IΔt first, then divide by the elementary charge 1.60 × 10⁻¹⁹ C if the number of particles is required.