There are many physical quantities that we can measure like force, acceleration, time, power, charge, electric current, luminosity, etc. However, all of them are independent. For example, force can be written as mass times acceleration. So, from the plethora of physical quantities, we can choose certain base quantities from which we can define others.

By, defining these base standards precisely we can define other derived quantities very precisely. For, this reason many base or fundamental units are derived from universal constants such as the speed of light which has been measured extremely accurately.

For example,

Velocity = \( \frac {Displacement}{Time}\)

The units for velocity can be written in terms of the units of distance and time. Thus, we can express speed in\( \frac { metres}{second}\) or m/s. In this case, the unit of velocity is derived from the fundamental units: meter and seconds.

There are seven fundamental units of measurement defined by the International System or SI. These define quantities that cannot be expressed by any other quantity. Sometimes, the unit concerned is chosen for convenience. For example, electric current (Ampere) is chosen as a fundamental Units of Measurement instead of electric charge (Coulomb) since it is easier to measure electric current.

In this case, the charge becomes a derived unit as charge = current × time.

Thus 1 Coulomb = 1 Ampere × 1 second.

**The SI fundamental Units Of Measurement:**

**Length – Meter (m):**

It is defined as the length of the path traveled by light in an interval of exactly \( \frac {1}{299 792 458}\) s. It is based on the fundamental quantity, the speed of light in a vacuum which is c=299 792 458 m/s.

**Time – Second (s):**

The time taken by 9 192 631 770 periods of oscillations of the light emitted by a cesium -133 atoms corresponding to the transition between two hyper-fine levels of the ground state. This is determined by using highly precise atomic clocks.

**Mass – Kilogram (kg):**

It is the mass of a prototype platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Paris, France. Copies of this cylinder are kept by many countries which use them to standardize and compare weights.

**Electric current – Ampere (A):**

The constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section when placed 1 m apart in vacuum, would produce a force equal to \(2\times 10^{-7}\) Newton per meter of length between these conductors. While, it may appear that electric charge should have been used as base unit, measuring current is far easier and hence is chosen as the standard base unit.

**Temperature – Kelvin (K):**

It is exactly \( \frac {1}{273.16} \) of the thermodynamic temperature of the triple point of water. The triple point of water is a fixed temperature and pressure at which the solid, liquid and gaseous states can exist at the same time.

**Amount of a substance – Mole (mol):**

The mole is the amount of substance which contains as many entities as there are atoms in 0.012 kg of carbon-12. A mole contains an Avogadro number of entities. Check out our chemistry articles to know more about the Avogadro number.

**Luminous Intensity – Candela (cd):**

It is the luminous intensity of a source that emits radiation of a constant frequency of \(540\times 10^{12}\)Hz with a radiant intensity of \( \frac {1}{683 }\) Watt per steradian in any given direction.

Stay tuned with byju’s to learn more about temperature, electric current and many more.