·
Measuring is to compare a physical quantity of a
thing with similar quantity as a unit.A quantity which can be measured is
called physical quantity such as length
·
A quantity that cannot be measured is not a physical
quantity such as sadness
·
In quoting any measurement of a physical
quantity, 2 items are needed to be stated
o
Numerical
value of the quantity (called magnitude)
o
And its
unit
·
The physical quantity is divided into two parts
o
Basic or fundamental quantity : the physical
quantities which the units are predetermined
o
Derived quantity : the physical quantities which
the units of which are derived from basic quantity units
·
There are seven basic quantities, they are :
|
No.
|
The
Name of Quantity
|
Unit
|
Symbol
of unit
|
|
1
|
Length
|
Meter
|
M
|
|
2
|
Mass
|
Kilogram
|
Kg
|
|
3
|
Time
|
Second
|
S
|
|
4
|
Electric
Current
|
Ampere
|
A
|
|
5
|
Temperature
|
Kelvin
|
K
|
|
6
|
Luminous
Intensity
|
Candela
|
cd
|
|
7
|
Amount
of Substance
|
mole
|
mol
|
·
There are many derived quantities, some of them
are :
|
No.
|
The
Name of Quantity
|
Unit
|
Symbol
of unit
|
|
1
|
Area
|
Square
meter
|
m2
|
|
2
|
Volume
|
Cubic
meter
|
m3
|
|
3
|
Density
|
Kilogram
/ cubic meter
|
Kg
/ m3
|
|
4
|
Velocity
|
Meter
/ second
|
m
/ s
|
|
5
|
Energy
|
Joule
|
J
|
|
6
|
Power
|
Watt
|
W
|
·
In physic there are many system of units, for
example MKS unit system (metric system), CGS unit system and FPS units (British
system of unit)
·
Because units are the references or standard of
physical quantities. The units holds the rule as follow
o
Have constant value
o
Are general in characteristic
o
Can be converted in other unit system
·
Based on the rules, then physic uses the
International system of units which is adopted from the Eleventh General
Conference in Weights and Measure, held in Paris-France in 1960. The system is
commonly called SI and this system of units is developed from MKS system /
metric system
·
One of the advantages is much easier to convert
to other unit system of the same type
·
Standard of Basic Unit
a)
The Unit of Length
In science length represents the
distance between two points. The standard unit of length is meter. Before 1967,
meter was defined as the distance between two marks in a bar of
platinum-iridium alloy that was stored in the International Bureau of Weights
and Measures in Sevres, France. From 1967 – 1983, meter was defined as
1,650,763.73 times the wavelength of a certain orange spectral line of atomic
krypton-86. after 1983, then the standard unit of length used is the distance
that light travels during an exact time interval of 1 / 299,792,458 second.
b)
The Unit of Time
In science time is defined as the
duration between two events. The standard unit of time is second and it is
defined as the time required for exactly 9,192,631,77 oscillations of an
isotope cesium-133 atom particle.
c)
The Unit of Mass
In science mass is the amount of
materials in the substance. The standard unit of mass is kilogram that is the
mass of a platinum-iridium cylinder which is also kept in the International
Bureau of Weights and Measures in Sevres, France.
d)
The Unit of Electric Current
The standard unit of electric current
is ampere that is the electric current flowing in two long parallel wires and
have the distance of one meter in vacuum space and it gives force exactly __ /
__ ( where __ / __ = 2 x 107 )
e)
The Unit of Temperature
Temperature is the quantity to show
the degree of hotness and coldness of an object. The standard unit of
temperature is Kelvin while the temperature scales is defined as equal to 273 lim
P / Po K at the constant volume of an ideal gas. The value of 273.16 K is the
temperature of triple point of water in Kelvin scale.
f)
The Unit of Luminous Intensity
The standard unit of luminous
intensity is candela that is 1 / 16 of luminous intensity resulted from 1 cm2
of the blackbody radiation glowing at temperature of frozen platinum that is
2046 K. In this case, the blackbody is the body which is able to absorb all
energies which strike it.
g)
The Unit of Amount of Substance
The standard unit of amount of
substance is mole that is the amount of substance which contains 6.02 x 1023
particles and 6.02 x 1023 particles is the Avogrado number
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