Physical QuantitiesPhysical attributes that are measurable are known as Physical Quantities. A physical quantity always consists of a numerical magnitude and a unit. 
Examples of Physical Quantities200 km  12.3 dB  23 Hz  47.3 °C  300 kN 
Accuracy of MeasurementAccuracy refers to the closeness of a measured value to a standard or known value. 
PrecisionPrecision refers to the closeness of two
or more measurements to each other. 
Random ErrorsIt 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 ErrorsIt 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 MeasurementsDifferent 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 QuantitiesThe 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 UnitsLength  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 Unitssecond  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 6.02214076×1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, 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 MagnitudeThe 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 Prefixesyotta  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 Magnitudes3,900 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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