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
Accuracy of Measurement
Accuracy refers to the closeness of a measured value to a standard or known value.
Precision refers to the closeness of two
or more measurements to each other.
It occurs in all measurements.
It occurs whenever an observer estimates the last figure of a reading on an instrument.
- human reaction time
- background noise
- mechanical vibrations
It cannot be predicted.
It can be reduced by taking large numbers of readings and averaging them.
It is not random but constant.
It may cause an observer to consistently underestimate or overestimate a reading.
- 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.
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
Amount of Substance
Definitions of Base Units
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 ground-state hyperfine transition frequency of the caesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s−1.
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.
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⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
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.
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⋅m2⋅s−2⋅K−1, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
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.
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 540×1012 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⋅s3, 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
In O-Levels, the only prefixes that you need to know are nano, micro, milli, centi, deci, kilo, mega and giga.
Examples of Orders of Magnitudes
Highest energy gamma wave ray detected
Mass of the rings of Saturn
Frequency of an X-Ray
The distance light travels in a year
Speed of light in a vacuum
Diameter of the earth
Bite force of a 5.2m Saltwater Crocodile
Average mass of a grand piano
Average mass of an adult human
Energy of an American half-dollar falling 1 metre
Fastest recorded speed of a snail
Calculated speed of an amoeba (lower bound)
Force required to break a typical covalent bond
Lowest temperature produced
Mass of a HIV-1 virus
Mass of double-stranded DNA molecule consisting of 1,578 base pairs
Energy of a van der Waals interaction between atoms
One Planck length