Physical Quantities
Physical attributes that are measurable are known as Physical Quantities. A physical quantity always consists of a numerical magnitude and a unit. 
Examples of Physical Quantities
200 km 
12.3 dB 
23 Hz 
47.3 °C 
300 kN 
Accuracy of Measurement
Accuracy refers to the closeness of a measured value to a standard or known value. 
Precision
Precision refers to the closeness of two
or more measurements to each other. 
Random Errors
It occurs in all measurements.
It occurs whenever an observer estimates the last figure of a reading on an instrument.
Causes:
 human reaction time
 background noise
 mechanical vibrations
It cannot be predicted.
It can be reduced by taking large numbers of readings and averaging them. 
Systematic Errors
It is not random but constant.
It may cause an observer to consistently underestimate or overestimate a reading.
Causes:
 zero error of an instrument: any indication that a measuring system gives a false reading when the true value of a measured quantity is zero
It can be eliminated if we know the sources of the errors. 
Taking Measurements
Different measuring instruments are used for measuring different quantities. The choice of instrument will affect the precision of the measurement we obtain.
The precision of an instrument is usually equal to the smallest division of the instrument with a few exceptions such as the thermometer, ammeter and voltmeter. 


SI Units and Base Quantities
The International System of Units is the modern form of the metric system, and is the most widely used system of measurement.
It is comprised of a system of units built on seven base units. 
The Seven Base Units
Length 
metre 
m 
Mass 
kilogram 
kg 
Time 
second 
s 
Electric Current 
ampere 
A 
Temperature 
kelvin 
K 
Amount of Substance 
mole 
mol 
Luminous Intensity 
candela 
cd 
Definitions of Base Units
second 
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed groundstate hyperfine transition frequency of the caesium133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s^{−1}. 
metre 
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m⋅s^{−1}, where the second is defined in terms of the caesium frequency ΔνCs. 
kilogram 
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10^{−34} when expressed in the unit J⋅s, which is equal to kg⋅m^{2}⋅s^{−1}, where the metre and the second are defined in terms of c and ΔνCs. 
ampere 
The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^{−19} when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of ΔνCs. 
kelvin 
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10^{−23} when expressed in the unit J⋅K^{−1}, which is equal to kg⋅m^{2}⋅s^{−2}⋅K^{−1}, where the kilogram, metre and second are defined in terms of h, c and ΔνCs. 
mole 
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076×10 ^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N A
, when expressed in the unit mol ^{−1} and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. 
candela 
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^{12} Hz, Kcd, to be 683 when expressed in the unit lm⋅W^{−1}, which is equal to cd⋅sr⋅W^{−1}, or cd⋅sr⋅kg^{−1}⋅m^{−2}⋅s^{3}, where the kilogram, metre and second are defined in terms of h, c and ΔνCs. 
Not necessary information


Prefixes and Orders of Magnitude
The SI system also establishes a set of twenty prefixes to unit names and unit symbols
that may be used when specifying multiples and fractions of the units. This is useful for
expressing physical quantities that are either very big or very small. 
Table of Prefixes
yotta 
Y 
10^{24} 
zetta 
Z 
10^{21} 
exa 
E 
10^{18} 
peta 
P 
10^{15} 
tera 
T 
10^{12} 
giga 
G 
10^{9} 
mega 
M 
10^{6} 
kilo 
k 
10^{3} 
hecto 
h 
10^{2} 
deka 
da 
10^{1} 
deci 
d 
10^{1} 
centi 
c 
10^{2} 
milli 
m 
10^{3} 
micro 
μ 
10^{6} 
nano 
n 
10^{9} 
pico 
p 
10^{12} 
femto 
f 
10^{15} 
atto 
a 
10^{18} 
zepto 
z 
10^{21} 
yocto 
y 
10^{24} 
In OLevels, the only prefixes that you need to know are nano, micro, milli, centi, deci, kilo, mega and giga.
Examples of Orders of Magnitudes
3,900 YHz 
Highest energy gamma wave ray detected 
30.86 Zm 
One gigaparsec 
30 Eg 
Mass of the rings of Saturn 
30 PHz 
Frequency of an XRay 
9.461 Tm 
The distance light travels in a year 
0.3 Gm/s 
Speed of light in a vacuum 
12.742 Mm 
Diameter of the earth 
16.5 kN 
Bite force of a 5.2m Saltwater Crocodile 
2.4 hg 
Average mass of a grand piano 
7 dag 
Average mass of an adult human 
1.1 dJ 
Energy of an American halfdollar falling 1 metre 
1.6667 cHz 
1 rpm 
2.75 mm/s 
Fastest recorded speed of a snail 
0.3 μm/s 
Calculated speed of an amoeba (lower bound) 
1.6 nN 
Force required to break a typical covalent bond 
50 pK 
Lowest temperature produced 
1 fg 
Mass of a HIV1 virus 
1.65 ag 
Mass of doublestranded DNA molecule consisting of 1,578 base pairs 
3 zJ 
Energy of a van der Waals interaction between atoms 
0.000000000016 ym 
One Planck length 

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