PHYS 2211 Module 1.2

Units and Standards

Learning Objectives

By the end of this section, you will be able to:

• Describe how SI base units are defined.
• Describe how derived units are created from base units.
• Express quantities given in SI units using metric prefixes.

We define a physical quantity either by specifying how it is measured or by stating how it is calculated from other measurements.

Measurements of physical quantities are expressed in terms of units, which are standardized values.

SI Units: Base and Derived Units

In any system of units, the units for some physical quantities must be defined through a measurement process. These are called the base quantities for that system and their units are the system’s base units. All other physical quantities can then be expressed as algebraic combinations of the base quantities. Each of these physical quantities is then known as a derived quantity and each unit is called a derived unit.

ISQ Base Quantity	SI Base Unit
Length	meter (m)
Mass	kilogram (kg)
Time	second (s)
Electrical current	ampere (A)
Thermodynamic temperature	kelvin (K)
Amount of substance	mole (mol)
Luminous intensity	candela (cd)

Check out a comprehensive source of information on SI units at the National Institute of Standards and Technology (NIST) Reference on Constants, Units, and Uncertainty.

Metric Prefixes

SI units are part of the metric system, which is convenient for scientific and engineering calculations because the units are categorized by factors of 10.

Prefix	Symbol	Meaning	Prefix	Symbol	Meaning
yotta-	Y	1024	yocto-	y	10–24
zetta-	Z	1021	zepto-	z	10–21
exa-	E	1018	atto-	a	10–18
peta-	P	1015	femto-	f	10–15
tera-	T	1012	pico-	p	10–12
giga-	G	109	nano-	n	10–9
mega-	M	106	micro-	μ	10–6
kilo-	k	103	milli-	m	10–3
hecto-	h	102	centi-	c	10–2
deka-	da	101	deci-	d	10–1