Electricity units

Learning objectives

2.1Electricity unitsUse the following units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V) and watt (W)

An electrical measurement is complete only when its unit identifies the physical quantity. Unit symbols are case-sensitive and are written without a plural ending or full stop.

Quantity	Unit	Unit symbol
current	ampere	A
charge	coulomb	C
energy transferred	joule	J
resistance	ohm	$\Omega$
time	second	s
voltage	volt	V
power	watt	W

A quantity symbol and its unit symbol can differ: current is represented by $I$ but measured in amperes, $A$ ; resistance is represented by $R$ but measured in ohms, $\Omega$ . The value $0.25\,A$ therefore means a current of 0.25 amperes.

$I$ is not the unit of current, and $R$ is not the unit of resistance; resistance is measured in $\Omega$ . Context also matters: $V$ is the symbol for the volt and is often used for the quantity voltage; $W$ is the watt in an electrical unit, even though $W$ can represent weight in mechanics.

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—(b) Mains electricityRead