Electric current

Electric current describes how quickly electric charge passes a point. A larger current means that more charge passes the point each second.

Current is measured in amperes ($$\text{A}$$), and charge is measured in coulombs ($$\text{C}$$). A current of one ampere means that one coulomb of charge passes a point each second.

Current, charge and time

For a constant current, the current is the charge transferred divided by the time taken:

Here, $$I$$ is current in amperes, $$Q$$ is charge transferred in coulombs, and $$t$$ is time in seconds.

For example, if $$6,\text{C}$$ of charge passes a point in $$3,\text{s}$$ at a constant rate, the current is $$6 \div 3 = 2,\text{A}$$.

Direction of conventional current

Conventional current is defined as flowing in the direction that positive charge would flow: from positive to negative. In a metal wire, electrons carry charge in the opposite direction, but the conventional current direction remains positive to negative.

Using ammeters

An ammeter measures electric current in amperes, with the symbol $$\mathrm{A}$$. To measure the current in a part of a circuit, connect the ammeter in series with that part. The current being measured then passes through the ammeter.

Reading an ammeter

An analogue ammeter uses a pointer to show the reading on a scale. A digital ammeter displays the reading as a number.

Some ammeters have selectable ranges. The selected range must be large enough to accommodate the expected current. If the current is unknown, start with a higher range to reduce the risk of exceeding the meter’s range. Then select a lower range that still accommodates the current; this can give a more precise reading.

An analogue ammeter may have more than one scale. Read the scale that corresponds to the selected range. For example, if the selected range is $$0$$–$$3,\mathrm{A}$$, use the $$0$$–$$3,\mathrm{A}$$ scale.

Electrical conduction in metals

A metal contains free electrons that can move through it. The positive metal ions remain in fixed positions, so they do not move through the metal to carry charge.

When a potential difference is applied across the metal, the free electrons have a net movement through it. Because electrons carry negative charge, this movement transfers charge through the metal and produces an electric current.

The electrons move in the opposite direction to conventional current. Conventional current is defined as the direction in which positive charge would move, from the positive terminal towards the negative terminal. The net movement of electrons is from the negative terminal towards the positive terminal.

Direct and alternating current

An electric current is a flow of electric charge. Currents can be distinguished by whether the direction of flow stays the same or reverses.

Direct current (d.c.) flows in one direction. Its magnitude—the amount of charge flowing per second—can change. For example, a current that becomes larger and then smaller is still direct current as long as it does not reverse direction.

Alternating current (a.c.) repeatedly reverses direction. Charge flows one way, then the opposite way, and continues to alternate between the two.

The distinction is therefore about direction, not whether the current has a constant magnitude.