Current and Charge

Electric current is the rate of flow of electric charge.

Conventional current and electron flow

• Conventional current flows from positive to negative (the historical convention).
• Electron flow is from negative to positive (electrons are negatively charged).

In IGCSE Physics, circuit diagrams use conventional current direction unless stated otherwise.

The equation

$Q = It$

Symbol	Quantity	Unit
$Q$	Charge	C (coulombs)
$I$	Current	A (amperes)
$t$	Time	s

Current of 1 A means 1 coulomb of charge flows past a point per second.

Worked example

A current of 0.50 A flows for 3.0 minutes. Calculate the charge that flows.

Convert time: $3.0 \times 60 = 180$ s

$Q = 0.50 \times 180 = 90\text{ C}$

Measuring current

Current is measured using an ammeter, connected in series with the component.

Common errors and how to correct them

Confusing conventional current with electron flow. Conventional current goes from + to -, but electrons actually move from - to +. Know which the question asks for.

Forgetting to convert minutes to seconds. The equation uses seconds. Convert before substituting.

How to apply this in an exam

State the equation, convert time to seconds, substitute, and calculate. If asked what current means, state “the rate of flow of charge” and give the unit (ampere).