Alice heard from Carmen that alcohol is cooler than water. She wants to verify that statement, therefore, she decided to carry out an experiment.
She prepared a glass of water and a glass of alcohol on the table and touched them by her hands to determine how much the alcohol is cooler than water. After this simple experiment, she told Carmen that alcohol indeed feels colder than the water, but that's only because alcohol evaporates faster than water. These two liquids should have the same degree of 'coldness' according to Alice, but Carmen insisted that alcohol is indeed colder than water because it 'feels' that way.Now, suppose that you were Alice, how would you resolve this problem?You probably have figured out that to reach a conclusion, we need a physical quantity called "temperature" and an apparatus to measure the temperature, which is the thermometer. Otherwise, this question can be discussed in endless subjective ways without reaching a consensus from everyone. So what actually is a physical quantity? A physical quantity is a quantity that can be measured. To investigate the magnitude (magnitude means "size") of a physical quantity, we need to do measurements using instruments. The scientist has subdivided physical quantities into two categories, eg. fundamental/base quantities and derived quantities. Base Quantity and Derived Quantity 1.)There are seven base quantities, as tabulated as follows:
Base Quantity SI Units symbol
Mass kilogram kg
Length meter m
Time second s
Temperature Kelvin K
Electric current Ampere A
Light intensity candela cd
Amount of substance mole mol

2.)A derived quantity is a quantity that is a combination of two or more base quantities by mean of multiplication, division or both. 3.) Some example of derived quantities:
Derived Quantity Relation with Base Quantity
Area Length x width
Volume Length x width x Height
Density Mass / Volume
Speed Distance / Time
Acceleration Distance / (Time x Time)
Force Mass x Acceleration
Moment of force Force x Perpendicular Distance
Pressure Force / Area

International System of Units

The International System of Units (abbreviated as "SI ) is the most widely used system of measurement in the world. SI consists of a set of base units, a set of derived units with special names, and a set of decimal-based multipliers that are used as prefixes.  

Units 1.)A unit is a particular physical quantity, defined and adopted by the convention, with which other particular quantities of the same kind are compared to express their value. 2.)The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent.The SI derived units for these derived quantities are obtained from these equations and the seven SI base units as shown in the below table.

Derived quantity SI derived unit Symbol
area square meter $ m^2 $
volume cubic meter $ m^3 $
speed, velocity meter per second $ m/s=ms^{-1} $
acceleration meter per second squared $ m/s^2=ms^{-2} $
wave number reciprocal meter $ m^{-1} $
mass density kilogram per cubic meter $ kg/m^{3}=kgm^{-3} $
specific volume cubic meter per kilogram $ m^3/kg=m^3kg^{-1} $
current density ampere per square meter $ A/m^2=Am^{-2}$
magnetic field strength ampere per meter $ A/m=Am^{-1} $
amount-of-substance concentration mole per cubic meter $ mol/m^3=molm^{-3} $
3.)For ease of understanding and convenience, 20+ SI derived units have been given special names and symbols.

Derived quantity Name Symbol Expression in terms of other SI units Expression in terms of SI base units
plane angle radian rad - $ $
frequency hertz Hz - $ \textrm{s}^{-1}$
force newton N - $ \textrm{m kg } \textrm{s}^{-2} $
pressure, stress pascal Pa $ \textrm{N} / \textrm{m}^2 $ $ \textrm{m}^{-1} \textrm{kg} \textrm{s}^{-2} $
energy, work, quantity of heat joule J $ \textrm{N m }$ $ \textrm{m}^2 \textrm{kg} \textrm{s}^{-2} $
power, radiant flux watt W $ \textrm{J/s} $ $ \textrm{m}^2 \textrm{ kg} \textrm{ s}^{-3} $
electric charge, quantity of electricity coulomb C - $ \textrm{s A} $
electric potential difference, electromotive force volt V $ \textrm{W/A} $ $ \textrm{m }^2 \textrm{kg } \textrm{s}^{-3} \textrm{ A}^{-1} $
capacitance farad F $ \textrm{C/V} $ $ \textrm{m}^{-2} \textrm{kg}^{-1} \textrm{s}^4 \textrm{A}^2 $
electric resistance ohm $\Omega$ $ \textrm{V/A} $ $ \textrm{m}^2 \textrm{kg} \textrm{s}^{-3}\textrm{A}^{-2} $
magnetic flux weber Wb $ \textrm{V s} $ $ \textrm{m}^2 \textrm{kg} \textrm{s}^{-2}\textrm{A}^{-1} $
magnetic flux density tesla T $ \textrm{Wb}/\textrm{m}^2 $ $ \textrm{kg} \textrm{s}^{-2}\textrm{A}^{-1} $
inductance henry H $ \textrm{Wb/A }$ $\textrm{m}^2 \textrm{kg} \textrm{s}^{-2}\textrm{A}^{-2} $
activity (of a radionuclide) becquerel Bq - $ \textrm{s}^{-1} $


1.)Prefixes are the preceding factor used to represent very small or very large physical quantities in SI units. 2.)For instance, "20km" means $ 20*1000 $ metres and "1 nm" means $ 1*10^{-9} $ metres. Here, the "k" in km is the symbol of prefix "kilo" which means "one thousand", whereas the "n" in km is the symbol of prefix "nano" which means "one billionth". 3.)Below show a list of prefixes:

Prefix Decimal Multiplier Symbol
Femto $ 10^{-15} $ f
pico $ 10^{-12} $ p
nano $ 10^{-9} $ n
micro $ 10^{-6} $ $\mu$
milli $ 10^{-3} $ m
centi $ 10^{-2} $ c
deci $ 10^{-1} $ d
kilo $ 10^3 $ k
Mega $ 10^6 $ M
Giga $ 10^9 $ G
Tera $ 10^{12} $ T

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