A physical quantity is a quantity that can be measured. It consists of a numerical magnitude and a unit. They are found on signboards and labels almost everywhere in our daily lives.
Altogether, there are seven basic physical quantities, or base quantities. Length and Time are two of them. Listed here are the seven basic physical quantities
- Base Quantity : Length Name of SI unit : Metre Symbol for SI unit : m
- Base Quantity : Mass Name of SI unit : Kilogram Symbol for SI unit : kg
- Base Quantity : Time Name of SI unit : Second Symbol for SI unit : s
- Base Quantity : Electric Current Name of SI unit : Ampere Symbol for SI unit : A
- Base Quantity : Thermodynamic Temp Name of SI unit : Kelvin Symbol for SI unit : K
- Base Quantity : Luminous Intensity Name of SI unit : Candela Symbol for SI unit : cd
- Base Quantity : Amount of substance Name of SI unit : mole Symbol for SI unit : mol
The units of these seven base quantities are known as SI units, from the French Le Systeme International d'Unites. The five that we have to learn are length, mass, time, electric current and temperature.
Other common physical quantities such as area, volume and speed are derived from these seven quantities. That is why these are called derived quantities. e.g. Speed is derived from length and time.
Area is derived from length and width. Volume is derived from length, width and height.
The reason as to why we need SI units was because in the past, people used parts of their bodies and things around them as units of measurement. Thats how measuring terms like foot, yard and horsepower came about. However, these units of measurement caused much confusion as they varied from person to person. In 1968, scientists decided to adopt one universal set of units - the SI units
Using decimal notation, the distance between air molecules would be represented as 0.00000001m. It would be both tiring and confusing if we needed to mention this a few times. The more convenient thing to do would be to use prefixes to represent the above quantity. In this case it can be represented as 0.01µm (micrometre) where µ represents the submultiple 10-6. The prefixes listed below are very useful in expressing physical quantities that are either very big or very small. There is another way to express the same quantity (0.01µm) and that is to use the standard form. In this case, it will be expressed as 1 x 10-8 m.
It takes some time to memorize this table completely. However, there are still a few more common quantities that are expressed in standard form
- One Kilometre (km) 1 x 103 m
- One Milliampere (mA) 10-3 A
- Three Megajoules (MJ) 3 x 106 J
- Six Microcoulombs (µC) 6 x 10-6 C
- Eight Nanoseconds (ns) 8 x 10-9 s
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