Basic electrical quantities

Three quantities to be aware of

In an electrical circuit, three quantities are used to describe what is happening:

• Voltage U, expressed in volts (V), measured using a voltmeter connected in parallel across the component.
• Current I, expressed in amperes (A), measured using an ammeter connected in series with the circuit.
• Resistance R, expressed in ohms (Greek symbol omega, denoted here as Ohm), which characterises a component’s ability to oppose the flow of current.

A simple illustration

An electrical circuit is often compared to a water circuit:

Electrical quantity	Hydraulic equivalent
Voltage U (V)	water pressure
Current I (A)	water flow rate
Resistance R (Ohm)	narrowness of the pipe

The narrower the pipe (higher resistance), the more pressure (voltage) is required to achieve the same flow rate (current).

The ohmic conductor

An ohmic conductor (or resistor) is a component whose resistance R remains constant regardless of the voltage applied. This is the case with the resistors used in the classroom, which are marked with coloured bands indicating their value in ohms.

Key points

• U is measured in volts (V).
• I is measured in amperes (A).
• R is measured in ohms (Ω).
• The voltmeter is connected in shunt, whilst the ammeter is connected in series.

These three quantities are linked by a simple mathematical relationship which we will explore in the next lesson: Ohm’s law.