2.1  Basic electrical quantities

The international standard units, Système international d'unités (SI units), that are commonly used for electrical quantities include coulombs, amperes, volts, ohms, watts and joules.

The fundamental unit of electricity is the negative charge of the electron or the positive charge of an ionised atom that has lost a single electron (resulting from the positive charge on the protons in the nucleus outnumbering that of the remaining electrons).

Electrical charge is measured in coulombs (symbol C). The charge on a proton is approximately 1.6 × 10⁻¹⁹ C, while the charge on an electron is the same magnitude but has the opposite sign (approximately −1.6 × 10⁻¹⁹ C). An electrical current is a flow of electric charge, measured in amperes (symbol A), where a current of one ampere is a total charge of one coulomb flowing in one second. Ampere is often abbreviated to amp.

In a circuit, positive and negative electric charges typically flow in the opposite direction to each other, as shown in Figure 8. This course will use conventional current flow, the predominant convention in the electronics industry, which is indicated in the direction taken by positive charges. In metal wires, the current is carried by electrons, which move in the opposite direction. This was not known at the time the convention was established.

Figure 8  Current flow in an example circuit showing a light bulb (lamp) powered by a battery: (a) schematic using pictures to represent the components; (b) the same circuit using standard circuit schematic symbols. By convention, current flows in the opposite direction to the flow of electrons.

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Part (a) shows a schematic circuit with a battery, lamp and switch.

Part (b) shows the same circuit but with symbols for the components.

The battery, lamp and switch are connected together by straight lines representing wires.

The symbol for a battery is two parallel lines, one longer than the other, oriented at right angles to the wire connections. The longer line represents the positive terminal and the shorter line the negative terminal.

The symbol for a lamp is a large circle containing a cross that looks like an X.

The symbol for a switch is two small circles with a gap between them. A straight line starts at the left circle and ends at a point above the right circle, showing the switch is open. If ‘pushed down’, this line would close the switch and complete the circuit.

Both (a) and (b) show electrons flowing from the negative to the positive battery terminal, and current flowing the other way.

Figure 8  Current flow in an example circuit showing a light bulb (lamp) powered by a battery: (a) schematic using pictures to represent ...

A quantity closely related to current is voltage. Voltage is a measure of the potential difference between two points. A potential difference of one volt (symbol V) will drive a coulomb of charge through a resistance of one ohm every second. Note that we usually refer to current flowing through a component such as a resistor, but to the voltage being across the component (because two points are required to define the voltage). Voltage can be expressed as the energy per coulomb of charge (J C⁻¹).