# THE SI system and units of measurement

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```							Units of Measurement
THE SI SYSTEM AND UNITS OF MEASUREMENT
MEASUREMENT

 Measurement is used to measure quantities.
 Quantity is something that has magnitude, size,
or amount (volume).
 In the late 18th century, scientists used the
metric system. The metric system is a precursor
to the SI System.
 Scientists all over the world use a single
measurement system called Le Systeme
International d’Units, abbreviated SI.
SI SYSTEM OF MEASUREMENT

 This system was presented in 1960 by a
General Conference of Weights and Measures
and has both base units and derived units.
 It is based on the metre-kilogram-second
system and replaces both the foot-pound-
second system and the centimetre-gram-
second system.
 The International System of units has been
developed and agreed internationally in the
interests of world health.
SI SYSTEM OF MEASUREMENT
   It overcomes languages barriers, enabling an exchange of
health information within a country and between nations to be
made without the misunderstandings which arise when each
country, or even a separate hospital within a country, uses its
own units of measurements for reporting tests. It is therefore
important for health authorities and laboratories to adopt the
new system of units. They are not difficult and are already
being used in most medical publications and journals, by the
manufacturers of chemicals and reagents, and by most
scientific and medical organizations.
   There are seven SI base units, i.e. metre, kilogram, second,
mole, ampere, Kelvin and candela.
SI BASE UNITS

   The most common base units that we will study
include:
Quantity                  Unit
Quantity                Unit name
Symbol                 abbreviation

Length         l        meter           m

Mass          m       kilogram        kg

Time          t        second          s

Temperature      T        Kelvin          K
SI BASE UNITS

•   To enable the measurement of quantities larger
or smaller than the base units or derived units,
the SI Units system also includes a set of
prefixes. The use of a prefix makes a unit larger
or smaller. The ranges of SI unit prefixes are
listed in the tables 1 and 2
TABLE 1
Prefix   Symbol   Function       Divided by
deci       d        10-1            10
centi      c        10-2            100
milli      m        10-3           1000
micro      µ        10-6         1000000
nano       n        10-9       1000000000
pico       p       10-12      1000000000000
femto      f       10-15     1000000000000000
TABLE 2
Prefix   Symbol   Function   Multiply by

deca     da       101        10

hector   h        102        100

kilo     k        103        1000

mega     M        106        1000000

Giga     G        109        1000000000

tera     T        1012       1000000000000

peta     P        1015       1000000000000000
LENGTH
 The SI standard unit for length is the meter.
 A distance of 1m is about the width of an average
doorway.
 To express longer distances, the kilometer (km) is
used. One kilometer is equal to 1000 m.
 To express shorter distances, the centimeter (cm)
is used. One centimeter is equal to 1/100 of a
meter.
 Length can be measured using a meter stick or
rulers.
MASS
 Mass is a measure of the quantity of matter.
The standard unit for mass is the kilogram (kg).
 The gram (g), which is 1/1000 of a kg is used
for measuring masses of small objects. Mass is
typically measured using a balance.
 Mass is different from weight. Weight is a
measure of the gravitational pull on matter
(Newton). The weight of an object increases as
gravity acts on it.
TIME
 The standard unit of measurement for time is the
second (s).
 Time can be measured using stop watches, clocks,
count down timers, and other time pieces.
 Larger amounts of time are measured in minutes
and hours.
 There are 60 seconds in one minute. There are 60
minutes in one hour. Given that there are 24 hours
in one day, how many seconds are there in one
day?
TEMPERATURE
 The standard unit of measurement for
temperature is degrees Kelvin (K).
 Temperature can also be measured in degrees
Celsius (°C) and degrees Fahrenheit (°F).
 To convert degrees Celsius (°C) to degrees
Fahrenheit (°F) multiply by 1.8 and then add 32.
To convert degrees Fahrenheit to degrees Celsius,
subtract 32 and then divide by 1.8
 ° K= °C + 273
 Temperature is measured using a thermometer.
(measures the degree of heat or coolness)
DERIVED UNITS

   Derived units are combinations of base units.
They are produced by multiplying or dividing
standard units. The derived units we will study
include:
Quantity                   Unit
Quantity                  Unit
symbol                     abbreviation

Area        A       square meter        m2

Volume       V       cubic meter         m3

kilograms per
Density      D                         kg/m3
cubic meter
AREA
 Area is length times the width. It is expressed
as square meters.
 Area can also be expressed as cubic
centimeters.
 What is the area of a rectangle that has an a
length of 6 cm and a width of 16 cm?
16 cm

   1           6 cm
   11111cmam
VOLUME
 Volume is the amount of space occupied by an
object. The derived SI unit for volume is cubic
meters. The cubic meter is rather large, so a more
common unit of cubic centimeters is more
commonly used.
 Non-SI units are also used to measure volume
such as the milliliter (mL) and the liter (L), which is
1000 cm3. There are 1000 mL in 1 L.
used to determine the volume of liquids.
DENSITY

   Density is the ratio of mass to volume, or mass
divided by volume. It can be written:
density=mass/volume or D=m/V
Density is a characteristic physical property of a
substance that does not depend on the size of
the sample. As the mass of an object
increases, its volume increases.

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