Imagine you had to make curtains and needed to buy fabric. The shop assistant would need to know how
much fabric you needed. Telling her you need fabric \(\text{2}\) wide and \(\text{6}\) long
would be insufficient — you have to specify the unit (i.e. \(\text{2}\)
metres wide and \(\text{6}\) metres long). Without the unit the information is
incomplete and the shop assistant would have to guess. If you were making curtains for a doll's
house the dimensions might be \(\text{2}\) centimetres wide and \(\text{6}\) centimetres long!
It is not just lengths that have units, all physical quantities have units (e.g. time, temperature,
distance, etc.).
Physical Quantity
A physical quantity is anything that you can measure. For example, length,
temperature, distance and time are physical quantities.
There are many different systems of units. The main systems of units are:
SI units
c.g.s units
Imperial units
Natural units
SI units (ESAF)
We will be using the SI units in this course. SI units are the internationally agreed upon
units.
SI Units
The name SI units comes from the French Système
International d'Unités, which means international
system of units.
There are seven base SI units. These are listed in Table 1.1. All physical quantities have units which can
be built from these seven base units. So, it is possible to create a different set of
units by defining a different set of base units.
These seven units are called base units because none of them can be expressed as combinations
of the other six. These base units are like the \(\text{26}\) letters of the alphabet
for English. Many different words can be formed by using these letters.
Base quantity
Name
Symbol
length
metre
\(\text{m}\)
mass
kilogram
\(\text{kg}\)
time
second
\(\text{s}\)
electric current
ampere
\(\text{A}\)
temperature
kelvin
\(\text{K}\)
amount of substance
mole
\(\text{mol}\)
luminous intensity
candela
\(\text{cd}\)
Table 1.1: SI base units
The other systems of units (ESAG)
The SI Units are not the only units available, but they are most widely used. In Science
there are three other sets of units that can also be used. These are mentioned here for
interest only.
c.g.s. units
In the c.g.s. system, the metre is replaced by the centimetre and the
kilogram is replaced by the gram. This is a simple change but it means
that all units derived from these two are changed. For example, the
units of force and work are different. These units are used most often
in astrophysics and atomic physics.
Imperial units
Imperial units arose when kings and queens decided the measures that were to
be used in the land. All the imperial base units, except for the measure
of time, are different to those of SI units. This is the unit system you
are most likely to encounter if SI units are not used. Examples of
imperial units are pounds, miles, gallons and yards. These units are
used by the Americans and British. As you can imagine, having different
units in use from place to place makes scientific communication very
difficult. This was the motivation for adopting a set of internationally
agreed upon units.
Natural units
This is the most sophisticated choice of units. Here the most fundamental
discovered quantities (such as the speed of light) are set equal to
\(\text{1}\). The argument for this choice is that all other quantities
should be built from these fundamental units. This system of units is
used in high energy physics and quantum mechanics.
Combinations of SI base units (ESAH)
To make working with units easier, some combinations of the base units are given special
names, but it is always correct to reduce everything to the base units. Table 1.2 lists some
examples of combinations of SI base units that are assigned special names. Do not be
concerned if the formulae look unfamiliar at this stage - we will deal with each in
detail in the chapters ahead (as well as many others)!
It is very important that you are able to recognise the units correctly. For example, the
newton (\(\text{N}\)) is another name for the kilogram metre
per second squared (\(\text{kg·m·s$^{-2}$}\)), while the
kilogram metre squared per second squared
(\(\text{kg·m$^{2}$·s$^{-2}$}\)) is called the joule
(\(\text{J}\)).
Quantity
Formula
Unit expressed in base units
Name of combination
Force
\(ma\)
\(\text{kg·m·s$^{-2}$}\)
\(\text{N}\) (Newton)
Frequency
\(\frac{1}{T}\)
\(\text{s$^{-1}$}\)
\(\text{Hz}\) (Hertz)
Work
\(Fs\)
\(\text{kg·m$^{2}$·s$^{-2}$}\)
\(\text{J}\) (Joule)
Table 1.2: Some examples of combinations of SI base units assigned special
names
Prefixes of base units (ESAI)
Now that you know how to write numbers in scientific notation, another important aspect of
units is the prefixes that are used with the units. In the case of units, the prefixes
have a special use. The kilogram (\(\text{kg}\)) is a simple example. \(\text{1}\)
\(\text{kg}\) is equal to \(\text{1 000}\) \(\text{g}\) or \(\text{1} \times
\text{10}^{\text{3}}\) \(\text{g}\). Grouping the \(\text{10}^{\text{3}}\) and the
\(\text{g}\) together we can replace the \(\text{10}^{\text{3}}\) with the prefix k
(kilo). Therefore the k takes the place of the \(\text{10}^{\text{3}}\). The kilogram is
unique in that it is the only SI base unit containing a prefix.
When writing combinations of base SI units, place a dot (·) between the units to
indicate that different base units are used. For example, the symbol for metres
per second is correctly written as \(\text{m·s$^{-1}$}\), and not as
\(\text{ms$^{-1}$}\) or \(\text{m/s}\). Although the last two options will be
accepted in tests and exams, we will only use the first one in this book.
In science, all the prefixes used with units are some power of \(\text{10}\). Table 1.3 lists some of these
prefixes. You will not use most of these prefixes, but those prefixes listed in
bold should be learnt. The case of the prefix symbol is very important.
Where a letter features twice in the table, it is written in uppercase for exponents
bigger than one and in lowercase for exponents less than one. For example M means mega
(\(\text{10}^{\text{6}}\)) and m means milli (\(\text{10}^{-\text{3}}\)).
Prefix
Symbol
Exponent
Prefix
Symbol
Exponent
yotta
Y
\(\text{10}^{\text{24}}\)
yocto
y
\(\text{10}^{-\text{24}}\)
zetta
Z
\(\text{10}^{\text{21}}\)
zepto
z
\(\text{10}^{-\text{21}}\)
exa
E
\(\text{10}^{\text{18}}\)
atto
a
\(\text{10}^{-\text{18}}\)
peta
P
\(\text{10}^{\text{15}}\)
femto
f
\(\text{10}^{-\text{15}}\)
tera
T
\(\text{10}^{\text{12}}\)
pico
p
\(\text{10}^{-\text{12}}\)
giga
G
\(\text{10}^{\text{9}}\)
nano
n
\(\text{10}^{-\text{9}}\)
mega
M
\(\text{10}^{\text{6}}\)
micro
μ
\(\text{10}^{-\text{6}}\)
kilo
k
\(\text{10}^{\text{3}}\)
milli
m
\(\text{10}^{-\text{3}}\)
hecto
h
\(\text{10}^{\text{2}}\)
centi
c
\(\text{10}^{-\text{2}}\)
deca
da
\(\text{10}^{\text{1}}\)
deci
d
\(\text{10}^{-\text{1}}\)
Table 1.3: Unit prefixes
There is no space and no dot between the prefix and the symbol for the unit.
Here are some examples of the use of prefixes:
\(\text{40 000}\) \(\text{m}\) can be written as \(\text{40}\)
\(\text{km}\) (kilometre)
\(\text{0,001}\) \(\text{g}\) is the same as \(\text{10}^{-\text{3}}\)
\(\text{g}\) and can be written as \(\text{1}\) \(\text{mg}\)
(milligram)
\(\text{2,5} \times \text{10}^{\text{6}}\) \(\text{N}\) can be written as
\(\text{2,5}\) \(\text{MN}\) (meganewton)
\(\text{250 000}\) \(\text{A}\) can be written as \(\text{250}\)
\(\text{kA}\) (kiloampere) or \(\text{0,250}\) \(\text{MA}\)
(megaampere)
\(\text{0,000000075}\) \(\text{s}\) can be written as \(\text{75}\)
\(\text{ns}\) (nanoseconds)
\(\text{3} \times \text{10}^{-\text{7}}\) \(\text{mol}\) can be rewritten as
\(\text{0,3} \times \text{10}^{-\text{6}}\) \(\text{mol}\), which is the
same as \(\text{0,3} \times \text{10}^{-\text{6}}\) \(\text{μmol}\)
(micromol)
Without units much of our work as scientists would be meaningless. We need to express our
thoughts clearly and units give meaning to the numbers we measure and calculate.
Depending on which units we use, the numbers are different. For example if you have
\(\text{12}\) water, it means nothing. You could have \(\text{12}\) \(\text{mL}\) of
water, \(\text{12}\) litres of water, or even \(\text{12}\) bottles of water. Units are
an essential part of the language we use. Units must be specified when expressing
physical quantities. Imagine that you are baking a cake, but the units, like grams and
millilitres, for the flour, milk, sugar and baking powder are not specified!
Importance of units
Work in groups of \(\text{5}\) to discuss other possible situations where using the
incorrect set of units can be to your disadvantage or even dangerous. Look for
examples at home, at school, at a hospital, when travelling and in a shop.
The importance of units
Read the following extract from CNN News 30 September 1999 and answer the questions
below.
NASA: Human error caused loss of Mars orbiter November 10, 1999
Failure to convert English measures to metric values caused the loss of the
Mars Climate Orbiter, a spacecraft that smashed into the planet instead
of reaching a safe orbit, a NASA investigation concluded Wednesday.
The Mars Climate Orbiter, a key craft in the space agency's exploration of
the red planet, vanished on 23 September after a 10 month journey. It is
believed that the craft came dangerously close to the atmosphere of
Mars, where it presumably burned and broke into pieces.
An investigation board concluded that NASA engineers failed to convert
English measures of rocket thrusts to newton, a metric system measuring
rocket force. One English pound of force equals \(\text{4,45}\) newtons.
A small difference between the two values caused the spacecraft to
approach Mars at too low an altitude and the craft is thought to have
smashed into the planet's atmosphere and was destroyed.
The spacecraft was to be a key part of the exploration of the planet. From
its station about the red planet, the Mars Climate Orbiter was to relay
signals from the Mars Polar Lander, which is scheduled to touch down on
Mars next month.
“The root cause of the loss of the spacecraft was a failed translation
of English units into metric units and a segment of ground-based,
navigation-related mission software,” said Arthur Stephenson,
chairman of the investigation board.
Questions
Why did the Mars Climate Orbiter crash? Answer in your own
words.
How could this have been avoided?
Why was the Mars Orbiter sent to Mars?
Do you think space exploration is important? Explain your
answer.
How to change units (ESAK)
It is very important that you are aware that different systems of units exist. Furthermore,
you must be able to convert between units. Being able to change between units (for
example, converting from millimetres to metres) is a useful skill in Science.
The following conversion diagrams will help you change from one unit to another.
Figure 1.1: The distance conversion table
If you want to change millimetre to metre, you divide by \(\text{1 000}\) (follow the
arrow from \(\text{mm}\) to \(\text{m}\)); or if you want to change kilometre to
millimetre, you multiply by \(\text{1 000}\) × \(\text{1 000}\).
The same method can be used to change millilitre to litre or kilolitre. Use Figure 1.2 to change
volumes:
Figure 1.2: The volume conversion table
Worked example 3: Conversion 1
Express \(\text{3 800}\) \(\text{mm}\) in metres.
Use the conversion table
Use Figure 1.1.
Millimetre is on the left and metre in the middle.
Decide which direction you are moving
You need to go from \(\text{mm}\) to \(\text{m}\), so you are moving from
left to right.
Very often in science you need to convert speed and temperature. The following two
rules will help you do this:
Converting speed
When converting \(\text{km·h$^{-1}$}\) to
\(\text{m·s$^{-1}$}\) you multiply by \(\text{1 000}\)
and divide by \(\text{3 600}\)
\(\left(\frac{\text{1 000}\text{
m}}{\text{3 600}\text{ s}}\right)\). For example
\(\text{72}\text{ km·h$^{-1}$} \div \text{3,6} =
\text{20}\text{ m·s$^{-1}$}\).
When converting \(\text{m·s$^{-1}$}\) to
\(\text{km·h$^{-1}$}\), you multiply by
\(\text{3 600}\) and divide by \(\text{1 000}\)
\(\left(\frac{\text{3 600}\text{
s}}{\text{1 000}\text{ m}}\right)\). For example
\(\text{30}\text{ m·s$^{-1}$} \times \text{3,6} =
\text{108}\text{ km·h$^{-1}$}\)
Converting temperature
Converting between the Kelvin and Celsius temperature scales is
simple. To convert from Celsius to Kelvin add \(\text{273}\). To
convert from Kelvin to Celsius subtract \(\text{273}\).
Representing the Kelvin temperature by \({T}_{K}\) and the
Celcius temperature by \({T}_{℃}\):
\[{T}_{K} = {T}_{℃} + 273\]
Changing the subject of a formula (ESAL)
Very often in science you will have to change the subject of a formula. We will look at two
examples. (Do not worry if you do not yet know what the terms and symbols mean, these
formulae will be covered later in the book.)
Moles
The equation to calculate moles from molar mass is: \(n=\dfrac{m}{M}\), where
\(n\) is the number of moles, \(m\) is the mass and \(M\) is the molar
mass. As it is written we can easily find the number of moles of a
substance. But what if we have the number of moles and want to find the
molar mass? We note that we can simply multiply both sides of the
equation by the molar mass and then divide both sides by the number of
moles.
\begin{align*}
n& = \frac{m}{M} \\
nM & = m \\
M & = \frac{m}{n}
\end{align*}
And if we wanted the mass we would use: \(m=nM\).
Energy of a photon
The equation for the energy of a photon is \(E=h\dfrac{c}{\lambda }\), where
\(E\) is the energy, \(h\) is Planck's constant, \(c\) is the speed of
light and \(\lambda\) is the wavelength. To get \(c\) we can do the
following:
\begin{align*}
E & = h\frac{c}{\lambda } \\
E\lambda & = hc \\
c & = \frac{E\lambda }{h}
\end{align*}
Similarly we can find the wavelength we use: \(\lambda =\dfrac{hc}{E}\) and
to find Planck's constant we use: \(h=\dfrac{E\lambda }{c}\).
Rate, proportion and ratios (ESAM)
In science we often want to know how a quantity relates to another quantity or how something
changes over a period of time. To do this we need to know about rate, proportion and
ratios.
Rate:
The rate is always a change in a quantity per unit time. The average rate is
the amount by which some quantity changes divided by the time interval during which the
change takes place. So, for example, we can calculate the rate of change of
velocity per unit time \(\left(\frac{\Delta \vec{v}}{\Delta{t}}\right)\) or the rate of
change
in concentration per unit time \(\left(\frac{\Delta c}{\Delta{t}}\right)\). Note that
\(\Delta\)
represents "the change in" a given quantity, so \(\Delta \vec{v}\) is the change in
velocity, \(\Delta{c}\) is the change in concentration, and \(\Delta{t}\) is the size of
the time interval during which the change takes place.
Ratios and fractions:
A fraction is a number which represents a part of a whole and is written as \(\dfrac{a}{b}\),
where \(a\) is the numerator and \(b\) is the denominator. A ratio tells us the relative
size of one quantity (e.g. the number of moles of reactants) compared to another
quantity (e.g. the number of moles of product). \(\text{2}:\text{1}\) ;
\(\text{4}:\text{3}\), etc. Ratios can also be written as fractions as percentages
(fractions with a denominator of \(\text{100}\)).
Proportion:
Proportion is a way of describing relationships between values or between constants. We can
say that \(x\) is directly proportional to \(y\) (\(x\propto y\)) or that \(a\) is
inversely proportional to \(b\) (\(a\propto \frac{1}{b}\)). It is important to
understand the difference between directly and inversely proportional.
Directly proportional
Two values or constants are directly proportional when a change in one leads
to the same change in the other. This is a more-more relationship. We
can represent this as \(y\propto x\) or \(y=kx\) where \(k\) is the
proportionality constant. We have to include \(k\) since we do not know
by how much \(x\) changes when \(y\) changes. \(x\) could change by
\(\text{2}\) for every change of \(\text{1}\) in \(y\). If we plot two
directly proportional variables on a graph, then we get a straight line
graph that goes through the origin \(\left(0;0\right)\):
Inversely proportional
Two values or constants are inversely proportional when a change in one leads
to the opposite change in the other. We can represent this as
\(y=\frac{k}{x}\). This is a more-less relationship. If we plot two
inversely proportional variables we get a curve that never cuts the
axis:
Constants in equations (ESAN)
A constant in an equation always has the same value. For example the speed
of light (\(c=\text{2,99} \times \text{10}^{\text{8}}\text{ m·s$^{-1}$}\), Planck's
constant (\(h\)) and Avogadro's number (\({N}_{A}\)) are all examples of constants that
are used in science. The following table lists all the contants you will encounter in
this book.
Trigonometry is the relationship between the angles and sides of right angled triangles.
Trigonometrical relationships are ratios and therefore have no units. You should know
the following trigonometric ratios:
Sine
This is defined as \(\sin A = \frac{\text{opposite}}{\text{hypotenuse}}\)
Cosine
This is defined as \(\cos A = \frac{\text{adjacent}}{\text{hypotenuse}}\)
Tangent
This is defined as \(\tan A = \frac{\text{opposite}}{\text{adjacent}}\)
Using significant figures
Textbook Exercise 1.2
Write the following quantities in scientific notation:
\(\text{10 130}\) \(\text{Pa}\) to
\(\text{2}\) decimal places
\(\text{978,15}\) \(\text{m·s$^{-2}$}\)
to one decimal place
\(\text{0,000001256}\) \(\text{A}\) to
\(\text{3}\) decimal places
Solution not available at present.
For each of the following symbols, write out the unit in full
and write what power of \(\text{10}\) it represents:
\(\text{μg}\)
\(\text{mg}\)
\(\text{kg}\)
\(\text{Mg}\)
Solution not available at present.
Write each of the following in scientific notation, correct
to \(\text{2}\) decimal places:
\(\text{0,00000123}\) \(\text{N}\)
\(\text{417 000 000}\)
\(\text{kg}\)
\(\text{246 800}\) \(\text{A}\)
\(\text{0,00088}\) \(\text{mm}\)
Solution not available at present.
For each of the following, write the measurement using the
correct symbol for the prefix and the base unit:
\(\text{1,01}\) microseconds
\(\text{1 000}\) milligrams
\(\text{7,2}\) megametres
\(\text{11}\) nanolitre
Solution not available at present.
The Concorde is a type of aeroplane that flies very fast. The
top speed of the Concorde is \(\text{844}\)
\(\text{km·hr$^{-1}$}\). Convert the Concorde's top
speed to \(\text{m·s$^{-1}$}\).
Solution not available at present.
The boiling point of water is \(\text{100}\)
\(\text{℃}\). What is the boiling point of water
in Kelvin?
To convert from Celcius to Kelvin we add \(\text{273}\):
