Electric Current

Electric current is how much electric charge flows past a point each second. Think of charge like water in a pipe: current is the flow rate. A bigger current means more charge passing each second.

Definition and Units

Current is defined as charge per unit time:

$$I = \frac{Q}{t}$$

where $I$ is current in amperes (A), $Q$ is charge in coulombs (C), and $t$ is time in seconds (s). One ampere means one coulomb of charge passes each second: $1\,\text{A} = 1\,\text{C s}^{-1}$.

Direction of Current

• Conventional current goes from positive to negative.
• In metals, electrons actually move from negative to positive (opposite to conventional current).

How Current Moves in Circuits

• Series circuits: the current is the same at every point.
• Parallel circuits (junctions): current splits. The total current entering a junction equals the total leaving it.

What Carries Current in Metals?

Metals have free electrons that can move easily. A battery or power supply pushes these electrons, creating a steady flow (current) through the wire.

Measuring Current (Ammeters)

• Use an ammeter (analogue or digital) to measure current.
• Connect it in series with the component so the same current passes through it.
• Choose a suitable range before measuring to avoid overloading.

Direct and Alternating Current

• d.c. (direct current): flows in one direction (e.g., batteries).
• a.c. (alternating current): repeatedly reverses direction (e.g., mains electricity).

Common Misconceptions

• Current is not “used up.” It is the same before and after a lamp in a series circuit.
• Ammeter goes in series, not across (parallel with) a component.