Charge & Current — Quick Revision

Mr Toogood's Physics · Electricity basics

AQA 3.5.1.1
I = ΔQ/Δt
Current
V = W/Q
P.d.
ε = E/Q
Emf
R = V/I
Resistance

Charge & current

Current is the rate of flow of charge past a point. Charge is measured in coulombs (C); one electron carries 1.60×10⁻¹⁹ C, so 1 C ≈ 6.24×10¹⁸ electrons.

  • I = current in amps (A)
  • ΔQ = charge in coulombs (C)
  • Δt = time in seconds (s)

Rearranged: Q = IΔt — the area under a current–time graph, or the gradient of a charge–time graph gives I.

Conventional current and electron flow

Conventional current flows + → − (opposite to electron flow).

Conventional current flows from + to − terminal. Electrons (negative) actually flow − to +. Always use current to mean conventional current.

Kirchhoff's laws

Kirchhoff's 1st law at a junction

Current in at a junction = current out.

1st law (charge conservation): current entering a junction = current leaving it. In series, current is the same everywhere; in parallel, branch currents sum to the total.
Potential difference around a circuit loop

Sum of p.d.s around a loop = emf of that loop.

2nd law (energy conservation): the emf supplied around any closed loop equals the sum of the p.d.s across the components in that loop.

Exam essentials

P.d. vs emf

  • P.d.: work done per unit charge as it passes through a resistance — charge loses energy.
  • Emf: energy given per unit charge by the source — charge gains energy.
  • Charge returning to the cell has 0 V electrical potential.

Meters

  • Ammeter: in series; resistance ≈ 0 Ω so it doesn't affect current.
  • Voltmeter: in parallel; resistance ≈ ∞ so no current flows through it.

Common slips

  • Current is constant in a series loop — don't assume it drops across resistors.
  • Use ΔQ, not total Q, in I = ΔQ/Δt.
  • mAh is a charge unit — convert to coulombs via Q = I t.