Charge: (a) electric current as rate of flow of charge; I = ΔQ/Δt

The idea

Electric current is defined as the rate of flow of charge. The equation I = ΔQ/Δt relates current I to the charge ΔQ that passes a point in time Δt. This means current is the amount of charge passing a given cross-section per unit time. For example, if 6.0 C of charge passes a point in 3.0 s, the average current is I = 6.0 C / 3.0 s = 2.0 A. The direction of conventional current is from positive to negative, opposite to electron flow in metals. In a circuit, current is measured with an ammeter connected in series. The equation applies to steady currents or average currents over a time interval. It is a fundamental relationship linking charge and time, and is used to calculate charge transfer in electrical circuits.

What earns the marks

• Current is the rate of flow of charge, I = ΔQ/Δt.
• ΔQ is the charge passing a point, Δt is the time taken.
• The unit of current is the ampere (A), where 1 A = 1 C s⁻¹.
• Conventional current direction is opposite to electron flow in metals.
• The equation can be rearranged to ΔQ = I Δt or Δt = ΔQ / I.

Common mistakes and how to put them right

• Confusing current with charge.

  Put it right: Current is measured in amperes, charge in coulombs.

• Using the equation with non-uniform current.

  Put it right: For instantaneous current, use calculus or ensure Δt is small.

• Forgetting that conventional current is opposite to electron flow.

  Put it right: State conventional current direction unless asked about electron flow.

Exam tips

• Remember the units: current in A, charge in C, time in s. Check unit consistency before calculating.
• When using I = ΔQ/Δt, ensure ΔQ is the charge passing a single point, not the total charge in the circuit.
• In multiple-choice questions, look for distractors that confuse current with charge or use incorrect rearrangements.