Introduction
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} $ |
| 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} $ |
Prefixes
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 |