Charge and current
📖Definition— Definitions
- Charge (Q) — measured in coulombs C. One coulomb is the amount of charge passing a point when a current of 1 A flows for 1 s.
- Current (I) — the rate of flow of charge, measured in amperes A.
Equation: Q = I × t (charge in C, current in A, time in s).
A current can only flow in a closed loop. In a metal, the moving charge carriers are free electrons; conventional current is drawn from + to − but electrons move the opposite way.
Series circuits
All components on a single loop.
- Same current at every point: I₁ = I₂ = I₃.
- The potential difference of the supply is shared between components: V_total = V₁ + V₂ + V₃.
- Total resistance R = R₁ + R₂ + R₃.
Parallel circuits
Components on separate branches.
- Each branch has the same potential difference as the supply.
- Total current = sum of branch currents (I_total = I₁ + I₂).
- Total resistance is less than any individual resistor.
Junction rule
At any junction, the total current entering equals the total current leaving (conservation of charge). This is Kirchhoff's first law in everyday language.
✦Worked example
A 12 V battery drives 0.4 A through a single 30 Ω resistor for 2 minutes.
- Q = I × t = 0.4 × 120 = 48 C of charge.
- Energy transferred E = Q × V = 48 × 12 = 576 J.
Edexcel exam tip
For "compare series and parallel" questions, use a parallel structure: "in series, current is the same … in parallel, current splits …". Marks come in matching pairs.
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