SI Units (International System of Units) – Types and Advantages

In electrical and electronics engineering, we must deal with measurable quantities. Therefore, as electrical engineers, we have to communicate in a standard language that all professionals can understand, irrespective of the country in which the measurement is performed. Such a standard language of measurement is the International System of Units or SI System.

In this article, we will study the international system of units, types of SI units, and advantages and disadvantages of the SI system.

What is the International System of Units?

The international system of units, abbreviated as SI units, is a modern form of the metric system. It is the most widely used system of measurement, both in science and engineering. The international system of units was adopted by the General Conference on Weights and Measures in the year 1960.

The international system of units consists of seven base units. Every base unit represents different types of physical quantities. From the seven base units, several other units of measurement can be derived. SI units are common used in electrical and electronics engineering.

SI Units and their Definitions

The seven base units of the international system of units are described below.

Meter

Meter is the SI unit of length. It is represented by the letter m. One meter is defined as the length of the path travelled by light in vacuum in a time interval of 1/299,792,458 seconds.

Kilogram

Kilogram is the SI unit of mass. It is denoted by kg. One kg is defined as the mass of a platinum-iridium cylinder kept in the international bureau of weights and measures, Paris. It was originally defined as the mass of 1 litter of pure water.

Second

Second is the SI unit of time. It is denoted by s. One second is defined as the time taken to complete 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium-133 atom.

Kelvin

Kelvin is the Si unit of thermodynamic temperature. It is denoted by the symbol K. One kelvin is defined as the (1/273.16)^th of the thermodynamic temperature of the triple point of pure water.

Ampere

Ampere is the SI unit of electric current. It is represented by the symbol A. One ampere is defined as the electric current flowing through two infinitely long parallel wires placed 1 meter apart in vacuum and experience a force of 2 × 10^-7 N.

One ampere is also be defined as one Coulomb of electric charge (charge on 6.24 × 10¹⁸ electrons) moving past a specific point in one second.

Candela

Candela is the SI unit of luminous intensity. It is denoted by the symbol cd. One candela is defined as the luminous intensity in a given direction of a source of light that emits monochromatic radiation of frequency 540 THz (Tera Hertz) and has a radiant intensity of 1/683 watts per steradian in the same direction.

Mole

Mole is the SI unit of amount of substance. It is denoted by mol. One mole is defined as the amount of substance containing 6.02214076 × 10²³ particles.

Classification of SI Units

The SI units can be classified into the following three classes −

• Fundamental Units
• Supplementary Units
• Derived Units

Fundamental Units

SI units which are independent and not related to any other unit are known as fundamental unit. The international system of units has seven fundamental units which are given in the following table −

Fundamental Unit	Quantity	Symbol of Unit
Meter	Length	M
kilogram	Mass	Kg
Second	Time	s
Ampere	Electric Current	A
Kelvin	Thermodynamic Temperature	K
Candela	Luminous Intensity	cd
Mole	Amount of Substance	mol

Supplementary Units

There are two supplementary units in the international system of units, which are given in the following table −

Supplementary Unit	Quantity	Description
Radian (rad)	Plane Angle	One radian is defined as the plane angle subtended from the center of a circle which intercepts an arc whose length is equal to the radius of the circle.
Steradian (sr)	Solid Angle	One steradian is defined as the solid angle subtended at the center of a sphere by a surface whose area is equal to the square of the radius of the sphere.

Derived Units

SI units of measurement which are derived from the fundamental units, or can be expressed in terms of the fundamental units are known as derived units. In science and engineering, there are several derived units, and it is not possible to list all the derived units in a single article. However, the following table shows some important derived units of physical quantities −

Quantity	Unit	Symbol of Unit
Electric Charge	Coulomb	C
Velocity	Meter per second	m/s
Acceleration	Meter per sq. second	m/s2
Density	Kilogram per cubic meter	Kg/m3
Volume	Cubic meter	m3

Advantages of SI System

Following are the major advantages of using the SI system of units −

• The SI system is a universal system of units. Thus, it is used in almost every country in the world.

• The SI system is a metric system that means in this system, quantities can be expressed as the exponents of 10.

• It is an absolute system of units.

• The SI system of units assigns only one unit to a particular quantity. Thus, it is a rational system of units.

• It is a coherent system of units, i.e., it has a set of fundamental units, from which all other units can be derived.