Electric charge and current
Electricity is the flow of electric charge. In metals, charge is carried by free electrons; in liquid electrolytes, by ions. Although electrons flow from negative to positive, the conventional current is defined as flowing from positive to negative — a historical convention you must use in exams.
Charge
Charge is measured in coulombs C. The charge on a single electron is tiny: $-1.6 \times 10^{-19}$ C. One coulomb is the charge of about $6.25 \times 10^{18}$ electrons.
Current
Current is the rate of flow of charge.
$I = \dfrac{Q}{t}$
- $I$ in amperes A
- $Q$ in coulombs C
- $t$ in seconds (s)
Equivalently: $Q = It$.
A 1 A current means 1 coulomb of charge passes a point every second.
✦Worked example— Worked examples
Example 1. A current of 0.30 A flows for 5.0 minutes. Find the total charge.
- Convert time: 5 × 60 = 300 s.
- $Q = It = 0.30 \times 300 = 90$ C.
Example 2. 60 C of charge flow through a resistor in 4.0 minutes. Find the current.
- $t = 240$ s.
- $I = Q/t = 60 / 240 = 0.25$ A.
Example 3. The current in a wire is 2.0 A. How many electrons pass a point each second?
- 1 A = 1 C/s, so 2 A = 2 C/s.
- Number of electrons per second $= 2 / (1.6 \times 10^{-19}) \approx 1.25 \times 10^{19}$ electrons.
Conservation of charge at a junction
In any junction in a circuit, the total current flowing in equals the total current flowing out. This is Kirchhoff's first law (you don't need the formal name at GCSE, but you need the idea):
Sum of currents in = sum of currents out.
For a parallel branching: if 0.6 A enters a junction and splits into two branches of 0.4 A and ? — the missing branch must be 0.2 A.
Why charge can't be lost
Charge is conserved — like energy. It cannot be created or destroyed. In a circuit it just flows from one place to another. In a series circuit the same charge passes through every component (current is the same everywhere); in a parallel circuit the charge splits at junctions.
Direction of current
- Conventional current — from positive to negative terminal of the battery (the convention used in all exams).
- Electron flow — actual movement of electrons, from negative to positive.
- They are equal in magnitude but opposite in direction. Stick to conventional current in answers unless told otherwise.
⚠Common mistakes— Pitfalls
- Forgetting to convert time to seconds before substituting.
- Confusing current (rate) with charge (total). Q = I × t.
- Saying "current flows out of a battery and is used up". It isn't — the same current returns to the battery.
- Using electron-flow direction in circuit diagrams instead of conventional.
➜Try this— Quick check
A charge of 1500 C passes through a wire in 5.0 minutes. Find the current.
- t = 300 s.
- I = Q/t = 1500/300 = 5.0 A.
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