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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 3

Topic 4.2.2 · Electric current

Current and charge

Electric current is related to the flow of charge.

In metals, conduction is the movement of free electrons.

I = Q / t where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). Current is the charge passing a point per unit time.

A charge of 12 C passes a point in a wire in 4.0 s. Calculate the current.

Worked example: current is 12 coulombs over 4.0 seconds equals 3.0 amperes
I = Q / t = 3.0 A.

Conventional current is drawn from positive to negative. Free electrons actually flow from negative to positive.

Ammeter in series with a lamp and cell: conventional current from the positive terminal, free electrons drifting the other way in the metal.
Diagram 1: Ammeter in series with a lamp and cell: conventional current from the positive terminal, free electrons drifting the other way in the metal.

Exam Traps

  • Do not mix conventional current (positive to negative) with electron flow (negative to positive). Exam diagrams use conventional current unless they ask for electrons.

Measuring current; d.c. and a.c.

An ammeter measures current. Connect it in series. Choose a range that is not overloaded but still gives a usable reading (analogue or digital).

Direct current (d.c.)
Charge flows in one direction (cells and batteries).
Alternating current (a.c.)
The direction of the current repeatedly reverses (mains).
d.c. as a current that stays in one direction; a.c. as a current that repeatedly reverses, shown on current–time sketch graphs.
Diagram 2: d.c. as a current that stays in one direction; a.c. as a current that repeatedly reverses, shown on current–time sketch graphs.

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