SI System of Units

Introduction of the SI System

The SI system is the most modern form of the metric system, and it can be considered as rationalized MKS system. This system has been adopted by the ISO as the abbreviated name for the system of units in all languages. The SI system is nothing but an expanded form of the RMKSA system. RMKSA stands for Rationalized Meter, Kilogram, Second, Ampere. In addition to these, degree Kelvin (unit of absolute temperature), Candela (unit of luminous intensity), and Mole (amount of substance) are included in the SI system of units. This latest system of units was mainly introduced for wide application in all branches of science and engineering, but it is also new system applicable in other fields. So, now we will go to understand what it means in detail.

Base Units of the SI System

There are some quantities which are represented by a single unit of measurement. This type of unit that does not require any combination of other units for representing itself is called a base unit. Think about length! When we say 10 meter long, this-indicates the length is 10 times of 1 meter. Meter is a standard length and the act of-measuring the length of anything, is to comparing it with a 1 meter length. Length does not require any combination of other unit for representing itself, so length can be considered as a base unit. Likewise, the unit of mass does not require other units for representation, so mass is another base unit. The SI system of units has seven such base units. All other units of measurement can be derived by combining two or more of these base units of the SI system. Again, if we think about the unit of velocity, we will find that the unit of velocity is meter ⁄ second, i.e., a combination of length and time. Hence, the unit of velocity is not a base unit. It is referred to as a derived unit. The units we use in the field of engineering and for other measurement purposes are mainly SI units.

Fundamental and Derived Units in the SI System

All the mechanical quantities in this universe can be represented and derived from three base quantities, and these are length, mass and time. These base units are referred as primary fundamental units. But there are other base units in the SI system of units which are used for representing other non-mechanical physical quantities. Quantities such as electrical current, absolute temperature, luminous intensity and amount of substance are basic quantities which can be represented by the base units of the SI system in addition to the primary fundamental units. These base or fundamental units are used only when the particular physical quantities are involved, and these units are called auxiliary fundamental units.

All the units of measurement which are expressed and derived from two or more fundamental or base units of the SI system, are referred as derived units. We have already shown that velocity is a derived unit. Now we have a clear idea about fundamental and derived units in the SI system.

Base or Fundamental Units of the SI System

  Quantity Unit
Primary Fundamental Units Length meter (m)
Mass kilogram (kg)
Time second (s)
Auxiliary Fundamental Units Electric Current ampere (A)
Absolute Thermodynamic Temperature kelvin (K)
Luminous Intensity candela (cd)
Amount of Substance mole (mol)

Beside these above mentioned base units, the SI system includes two supplementary units which are known as plane angle and solid angle.

Supplementary Units of the SI System

Quantity Unit
Plane Angle radian (rad)
Solid Angle steradian (sr)

Some examples of derived units of the SI system are given below for ready reference.

Derived Units of SI System

Quantity Unit Unit Symbol
acceleration metre per second square m ⁄ s2
angular acceleration radian per second square rad ⁄ s2
angular velocity radian per second rad ⁄ s
area square metre m2
capacitance farad F → A - s ⁄ V
density kilogram per cubic metre kg ⁄ m3
dynamic viscosity newton second per square metre Ns ⁄ m2
electric charge coulomb C → A-s
electric field strength volt per metre V ⁄ m
electric current ampere A
electric potential volt V
electrical resistance ohm Ω → V ⁄ I
emf volt V
energy joule J
force newton N
frequency hertz Hz
heat joule J
illumination lux lx → lm ⁄ m2
inductance henry H → V - s ⁄ A
kinematic viscosity square metre per second m2 ⁄ s
luminous flux lumen lm → cd - sr
luninance candela per square metre cd ⁄ m2
magnetic field strength ampere per metre A ⁄ m
magnetic flux weber Wb → V - s
magnetic flux density tesla T → Wb ⁄ m2
magneto motive force ampere A ⁄ m
potential difference volt V
power watt W → J ⁄ s
pressure newton per square metre N ⁄ m2
torque newton-metre N - m
velocity metre per second m ⁄ s
volume cubic metre m3
work joule J