What are quantities and units?
Whatever is measurable in this universe is called a quantity. Example given: length of an object, its mass and time can be measured, that’s why these are quantities. But no device has been invented to date which can measure concepts like jealousy, pride, pleasure, and sadness. That’s why these are not quantities, in terms of Physics. So you can guess, there has to be a standard to measure a quantity. This standard is called a unit.
Standards to measure a certain quantity could vary from country to country. That’s why various systems of units have been in use. Like, C.G.S. system, F.P.S. system, M.K.S. system, S.I. system, etc.
Basic and derived quantities
Quantities are basically of two types: basic and derived quantities. You don’t need the idea or concept of any other quantity when expressing a basic quantity. Example given: time. To realize what time is, you don’t need to know about the concept of another quantity, other than time itself. That’s why, time is a basic quantity. Another few basic quantities apart from time are distance or length, mass, temperature, amount of substance (number of moles), etc.
On the other hand, a quantity to express or realize which, you need to incur concepts of another one or more quantities, is a derived quantity. Example given: speed and velocity. Speed is the distance traveled in unit time. In other words, to realize what speed is, you first have to know what distance and time are. That’s why, speed is a derived quantity.
Scalar and Vector quantities
There is another classification of quantities: scalar and vector quantities. This classification is based on which quantities have directions and which others don’t. Think about time, temperature and mass – to express any of these quantities, only magnitude is required, direction isn’t required. That’s why, these are scalar quantities.
On the other hand, to express a quantity like weight and force, not only magnitude, but also direction is required. That’s why, these are vector quantities. If you change the direction of such a quantity while keeping the magnitude unchanged, the result or effect of the quantity will change.
Assume that I applied a force of 10 units magnitude on an stationery object towards east and another force of the same magnitude but with a different direction on another stationery object. At the end of a certain time period (say, five seconds) both the objects will have the same speed and kinetic energy (neglecting friction and energy loss), but their directions of velocities will be different.
Since their effects are different, they are unequal forces, even if their magnitudes are the same. This is because the two forces have different directions. So you realize that force is a vector quantity. Another vector such as this is velocity.
Various systems of units
Previously different countries had different systems of units in them. These units of systems were the C.G.S. system, the F.P.S. system and the M.K.S. system. As a result, when a person traveled from one country to another, they would be in trouble regarding systems of units. To eradicate this problem, scientists in 1960 came to a consensus in an international conference that there would be a common system of units across the whole world. Among these scientists were many famous physicists and chemists. SI stands for International System of Units.
Seven quantities have been considered as basic quantities in this system of units. These are: length or distance, mass, time, temperature, amount of substance, electric current, and luminous intensity. SI units of these quantities are meter, kilogram, second, Kelvin, mole, Ampere, and Candella.
The following chart presents the units of the important basic units.
|Name of System of Units||C.G.S.||F.P.S.||M.K.S.||S.I.|
|Full format of System's name||Centimeter Gram Second System||Foot Pound Second System||Meter Kilogram Second system||International System of Units|
|Unit of Length||Centimeter||Foot||Meter||Meter|
|Unit of Mass||Gram||Pound||Kilogram||Kilogram|
|Unit of Time||Second||Second||Second||Second|
It is evident from the chart that SI has basically absorbed the MKS system. Just notice that both the systems have common units of length and mass.
Unfortunately, SI hasn’t been fully implemented in many countries. Example given, countries like USA have been widely using units like foot and pound. People from these countries aren’t accustomed to use internationally recognized units like meter and kilogram. The purpose around which SI was built hasn’t fully been served yet.
Charge instead of Electric current
I think charge should have been considered as a basic quantity instead of electric current. Because you know electric current is the amount of charge passed through the cross-section of a conductor in unit time, by definition. So, to define electric current, you need the concepts of charge and time. On the other hand, charge is a unique thing that doesn’t need the help of any other physical quantity to be defined. That’s why, in my opinion, charge should be declared a basic or fundamental quantity instead of electric current.