Electrical principles - Terminology

Units of measure

There are seven base units in the International System (SI) for measuring physical quantities.

The units are:

ampere – unit of electric current (symbol for ampere is A, symbol for current is I)
newton – unit of force (symbol N)
metre – unit of length (symbol m)
kelvin – unit of thermodynamic temperature (symbol K)
kilogram – unit of mass (symbol kg)
pascal – unit of pressure (symbolPa)
second – unit of time (symbol s).

Derived units

Derived units
Unit	Unit for	Symbol	Description
coulomb	electrical charge	C	The quantity of electric charge transferred each second by a current of one ampere (nominally equal to 6.24 x 1018 electrons)
farad	capacitance	F	The capacity that exists between two plates of a capacitor if the transfer of one coulomb from one plate to the other creates a potential difference of one volt
henry	inductance	H	If the rate of change of current in a circuit is one ampere per second, and the resulting electromotive force is one volt, then the inductance of the circuit is one henry
hertz	frequency	Hz	The number of periodic oscillations per second
joule	energy and work	J	One joule of work is required to move one coulomb through an electrical potential difference of one volt
ohm	resistance	Ω	If a device dissipates one watt of power with one ampere of current flowing through, it has a resistance of one ohm (R = P/I²)
volt	potential difference or voltage	V	The potential difference existing between two points on a conductor carrying a current of one ampere when the power dissipated is one watt
watt	power	W	The power used when energy is expended at the rate of one joule per second

Submultiple and multiple units

There are times when we need to measure very large or very small amounts of an electrical quantity. This can lead to a number made up of six or more digits, eg 1,000,000 or 0.000001.

In order to simplify these numbers, we use prefixes, eg 5 million ohms can be written as 5,000,000Ω or simplified to 5MΩ. M is the symbol for mega, which is the prefix meaning 'million'.

The following table lists the prefixes you are likely to use.

Prefixes
Prefix	Symbol	Meaning	Value	Factor
nano	n	1,000,000,000^th	0.000,000,001	10^-9
micro	u	1,000,000^th	0.000,001	10^-6
milli	m	1,000^th	0.001	10^-3
centi*	c	100^th	0.01	10^-2
deci*	d	10^th	0.1	10^-1
kilo	k	1,000 x	1,000	10³
mega	M	1,000,000 x	1,000,000	10⁶
giga	G	1,000,000,000 x	1,000,000,000	10⁹

The first five prefixes are submultiples, ie they are smaller in value than the basic unit. The remaining prefixes are multiples,and they have larger values than the base unit.

*centi and deci are only used in relation to metre, the unit of measure.

Examples of submultiple and multiple:

nano – 33nV equals 0.000000033 of a volt
micro – 33uV equals 0.000033 of a volt
milli – 33mV equals 0.033 of a volt
kilo – 33KV equals 33,000 volts
mega – 33MV equals 33,000,000 volts
giga – 33GV equals 33,000,000,000 volts.

Calculations with submultiple and multiple units

When doing calculations with mixed quantities (any combination of base, multiple or submutliple), all of the quantities should be converted to the base unit.

The conversion is done by moving the decimal point either left or right, based on the factor, as shown in the table above.

To convert 33nV to volts:

33 is multiplied by 0.000,000,001to get 0.000000033V (decimal point moved nine places to the left on the number 33.0).

To convert 33mV to volts:

33 is multiplied by 0.001 to get 0.033V (decimal point moved three places to the left on the number 33.0).

To convert 33GV to volts:

33 is multiplied by 1,000,000,000 to get 33,000,000,000V (decimal point moved nine places to the right on the number 33.0).

The conversion rule is:

submultiple to a base – move the decimal point to the left
multiple to a base – move the decimal point to the right.

When reading the value displayed on an electrical meter (particularly a multimeter), the value shown on the meter could be a base, multiple or submutliple.