6.1 Definition of current

Electric current is the rate of flow of charge. If charge Q passes a point in time t, current I = Q/t. One ampere means one coulomb of charge passes each second. Current is not “used up” by a component; charge entering a component per second equals charge leaving it in steady operation.

In metallic conductors, electrons are the mobile carriers. In electrolytes, positive and negative ions carry charge, and in semiconductors both electrons and effective positive holes may contribute. The O Level circuit model generally uses conventional current.

6.2 Conventional current and electron flow

Conventional current is defined as the direction in which positive charge would move, from the positive terminal of a source through the external circuit to the negative terminal. In a metal, electrons drift in the opposite direction.

This convention was established before electrons were discovered and remains universal in circuit diagrams and equations. Do not reverse component symbols or current arrows simply because the physical carriers are negative.

6.3 Calculating charge

The equation Q = It is useful for finding the charge delivered by a current. Use current in amperes and time in seconds to obtain charge in coulombs. A current of 0.40 A for 30 s transfers 12 C.

For a varying current, the simple product applies only if an average current is intended. On a current–time graph, charge is represented by the area under the graph, though detailed integration is beyond this syllabus.

6.4 Measuring current

An ammeter is connected in series so the same charge flow passes through it and the component. An ideal ammeter has negligible resistance. Connecting an ammeter directly across a cell may produce a dangerously large current because the meter offers a low-resistance path.

Choose the highest range first when the expected current is unknown, then reduce the range for better resolution. Check zero, polarity for analogue d.c. meters, and units. A reading of 250 mA is 0.250 A.

6.5 Direct current

Direct current, d.c., flows in one direction. Cells, batteries and many electronic power supplies provide d.c. A d.c. value may be constant or may vary while remaining in one direction; the defining feature is that polarity does not repeatedly reverse.

On a current–time graph, steady d.c. is a horizontal line above or below zero. A rectified supply may pulse but still remain one sign.

6.6 Alternating current

Alternating current, a.c., repeatedly reverses direction. Mains electricity is a.c. because generators produce a changing e.m.f. A sinusoidal a.c. waveform has equal positive and negative half-cycles when centred on zero.

Frequency states how many complete cycles occur each second. In a 50 Hz supply, the cycle repeats 50 times per second and current direction reverses 100 times per second. The period is 1/50 s = 0.020 s.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. How much charge passes when 0.25 A flows for 80 s?
2. Why must an ammeter not be connected directly across a cell?
3. A supply is 60 Hz. Find its period.