8202.202.1 SI UNITS

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Standard units of measurement

Unit 202: Electrical science

Level 2 Technical Certificate in Electrical Installation

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SI units

In all fields of science, it is essential to have a set of standard units.

Published in 1960, the SI units (short form for Système International
d’Unités) are the modern form of the metric system.

There are seven base units, from which we derive many other units.

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SI units

This table shows the six units that are relevant to electrotechnical
industries. The 7th unit (the mole) is more relevant to chemistry.

Quantity

Dimension

symbol

Unit

Unit

symbol

Electric current

I

ampere

A

Length

l

metre

m

Luminous intensity

Iv candela

cd

Mass

m

kilogram

kg

Temperature

T

kelvin

K

Time

t

second

s

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Common SI units used in electrical science

Quantity

Unit/symbol

Resistance, R

Ohm, Ω

Electromotive force, E

Volt, V

Voltage, V

Volt, V

Current, I

Ampere, A

Charge, Q

coulomb, C

Power, P

Watt, W

Potential difference, V

Volt, V

Energy, W

Joule, J

Resistivity, ρ

Ohm metre, Ωm

Impedance, Z

Ohm, Ω

Inductance, L

Henrys, H (Inductive reactance in Ohms)

Capacitance, C

Farads, F (Capacitive reactance in Ohms)

Frequency, f

Hertz, Hz

Reactance, X

Ohm, Ω

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Common SI units used in electrical science

Quantity

Unit/symbol

Magnetic flux, Φ

Weber, Wb

Magnetic flux density, β

Tesla, T

Force, F

Newton, N

Area, A

Square metre, m2

Power factor, p.f.

Ratio with no units

Actual power, P

Watt, W or kW

Reactive power, Q

Volt amps reactive, VAr or kVAr

Apparent power, S

Volt amps, VA or kVA

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Common SI units used in electrical science

How many of these can you remember?

Complete Activity 1 now.

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SI units – multiples and sub-multiples

Often the units are either too small or too large to be used in their
defined quantity. For example, we commonly encounter values such as
thousandths of an ampere, or millions of ohms.

For this reason, we multiply the units up, or divide them down to derive
practical values.

For example, the metre may be broken
down into millimetres (thousandths of a
metre).

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Standard form

The multiplication or division of units is expressed in powers of 10 (10
100, 1000), where each multiple or sub-multiple is given a name and
symbol, ie, 1000 is kilo, symbol k. So 1000 metres is expressed as 1
kilometre (1 km).

To denote large or small numbers clearly, notation known as standard
form is used. For example:

1000 = 10 X 10 X 10 and in standard form this is written as 1 X 103

Note that in standard form the number multiplied is always less than 10.

Question? How would you write 200 in standard form?

Answer: 2 x 102(that is, 2 X 10 X 10)

Math Response

How would you write 126,000,000,000 in standard form?

Type answer here

Correct Answer: 1.26\times10^{11}

Deg^°

Rad

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Standard form

Any number can be put into standard form.

The decimal point is moved to give a number less than 10, and the
number of places it moves gives the power or index of 10.

For example:

2000 = 2000.0 (we just don’t normally show the decimal point)

In standard form we move the decimal point 3 places to the left, so this
becomes 2.0  103

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Standard form

For numbers less than 1, the decimal point is moved to the right.

Example:

0.0632 in standard form becomes…

6.32 X 10–2

(Note that since the decimal point has been moved to the right, to get
6.32, the index is negative.)

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Standard form

For more examples and practice with standard form, complete
Worksheet 1.

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SI units – Multiples and sub-multiples

Name

Symbol

Multiplier

tera

T

1012(1 000 000 000 000)

giga

G

109(1 000 000 000)

mega

M

106(1 000 000)

kilo

k

103(1 000)

milli

m

10-3(0.001)

micro

µ

10-6(0.000 001)

nano

n

10-9(0.000 000 001)

pico

p

10-12(0.000 000 000 001)

This table shows the common multiples used in electrotechnical industries.
Those in bold font are those which we most commonly encounter in
electrical installation science. (Note the use of standard form)

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Any questions?