What is SI Unit?

A unit is defined as the standard of reference chosen to measure any physical quantity. The SI unit is the abbreviation used for System International d’ units. It was introduced by the General Conference of the Weights and Measure, France, in \(1960\).

Other Systems

Apart from the SI system, other systems like cgs, fps, and mks systems are also present.

1. CGS System: This system of units was set up in France. According to this system, the unit of three fundamental quantities, i.e., length, mass, and time, are centimetre, gram, and second.
2. FPS System: This is a British system of measurement, and according to this, the units of the three fundamental quantities are foot, pound, and second. However, this system is not commonly used now.
3. MKS System: The system was also developed in France and is also known as the metric system. It uses a meter, kilogram, and second as the unit for the three fundamental quantities, i.e., length, mass, and time.

What is the SI Unit of Mass?

The SI unit of mass is the kilogram. It is defined as the mass of platinum-iridium (Pt-Ir) cylinder stored in an air-tight jar at the International Bureau of Weights and Measures in France. 

What is the SI Unit of Length?

The SI unit of length is a meter. The meter was originally defined as the length between two marks on a Pt-Ir bar kept at a temperature of \(0^\circ C\left( {273\,K} \right)\). But now, it is redefined as the length of the path travelled by light in vacuum during a time interval of \(\frac{1}{{299,792,458}}\) of a second.

Second is the SI Unit of Which Physical Quantity?

Second is the SI unit of time. The second is defined as the duration of \(9192631770\) periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-\(133\) atom.

What is Kelvin?

The kelvin is the SI unit of temperature. It is defined as the unit of thermodynamic temperature and is equal to \(\frac{1}{{273.16}}\) of the thermodynamic temperature of the triple point of water.

Though the SI unit of temperature is Kelvin, still Celsius scale of temperature is commonly used in our daily life. The kelvin and degree Celsius \(\left({^\circ C} \right)\) are related to each other as follows:

\({\text{Temperature}}\,{\text{in}}\,{\text{degree}}\,{\text{Kelvin}}(K) = ^\circ C + 273.15\)

Temperature is also measured in degrees Fahrenheit \(\left({^\circ F} \right)\). It is related to degree Celsius \(\left({^\circ C} \right)\) as follows:

\(^\circ F = \frac{9}{5}\left({^\circ C} \right) + 32\)

What is Ampere?

What is the Mole?

The mole is the SI unit of the amount of substance. The mole is defined as the amount of substance that contains as many elementary entities as there are atoms in \(0.012\) kilogram of carbon-\(12\). The elementary entities may be atoms, molecules, ions, electrons, or any other particles.

What is the SI Unit of Luminous Intensity?

The candela is the SI unit of luminous intensity. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency \(540 \times {10^{12}}\) hertz and that has a radiant intensity in that direction of \(\frac{1}{{683}}\) watts per steradian.

What are Derived Units?

The units derived from the seven basic units are called derived units. Some important commonly used derived units are as follows.

1. Area: The area of an object is obtained by multiplying its length and breadth, i.e., the area is the product of its two lengths.
\({\text{Area}} = {\text{Length}} \times {\text{Length}} = m \times m = {{\text{m}}^2}\).

2. Volume: The derived unit of volume is m³. It is obtained as follows:
\({\text{Volume}} = {\text{Length}} \times {\text{Breadth}} \times {\text{Height}}\)
\({\text{Volume}} = {\text{Lengt}}{{\text{h}}^3} = {{\text{m}}^3}\)

3. Velocity:  Velocity is defined as the distance travelled in unit time. Its derived unit is \({\text{m}}{{\text{s}}^{ – 1}}\).
\({\text{Velocity}} = \frac{{{\text{ Distance travelled }}}}{{{\text{ Time }}}} = \frac{m}{s} = {\text{m}}{{\text{s}}^{ – 1}}\)

4. Acceleration: Acceleration is defined as the rate of change of velocity with respect to time. The derived unit of acceleration is \({\text{m}}{{\text{s}}^{ – 2}}\).
\({\text{Acceleration}} = \frac{{{\text{ Velocity }}}}{{{\text{ Time }}}} = \frac{{{\text{m}}{{\text{s}}^{ – 1}}}}{s} = {\text{m}}{{\text{s}}^{ – 2}}\)

5. Density: Density is defined as the mass per unit volume. The derived unit of density is \({\text{kg}}{{\text{m}}^{ – 3}}\).
\({\text{Density}} = \frac{{{\text{ Mass }}}}{{{\text{ Volume }}}} = \frac{{{\text{kg}}}}{{{{\text{m}}^3}}} = {\text{kg}}{{\text{m}}^{ – 3}}\)

6. Force: Force is defined as the product of mass and acceleration. The derived unit of force is \({\text{kgm}}{{\text{s}}^{ – 2}}\).
\({\text{Force}} = {\text{Mass}} \times {\text{Acceleration}} = {\text{kg}} \times {\text{m}}{{\text{s}}^{ – 2}} = {\text{kgm}}{{\text{s}}^{ – 2}}\)
In SI system, \({\text{kgm}}{{\text{s}}^{ – 2}}\) is referred as Newton. It is represented as \(N\). Thus, 
\(1\,{\text{N}} = 1\,{\text{kgm}}{{\text{s}}^{ – 2}}\)

7. Pressure: Pressure is defined as the force per unit area. The derived unit of pressure is \({\text{kg}}{{\text{m}}^{ – 1}}{{\text{s}}^{ – 2}}\).
\({\text{Pressure}} = \frac{{{\text{ Eorce }}}}{{{\text{ Area }}}} = \frac{{{\text{kgm}}{{\text{s}}^{ – 2}}}}{{~{{\text{m}}^2}}} = {\text{kg}}{{\text{m}}^{ – 1}}{{\text{s}}^{ – 2}}\)

In SI system, \({\text{kg}}{{\text{m}}^{ – 1}}{{\text{s}}^{ – 2}}\) is referred as Pascal. It is represented as \(P\)

One pascal is defined as the force of \(1\,{\text{N}}\) applied to an area of \(1\,{{\text{m}}^2}\).

The Prefixes Used in the SI Unit System

The SI units of some of the physical quantities are either too small or too large. Therefore, to change the order of magnitude, these are expressed by using prefixes before the name of the units. The various prefixes used are given below.

Multiple	Prefix	Symbol
\({10^{ – 1}}\)	deci	\({\text{d}}\)
\({10^{ – 2}}\)	centi	\({\text{c}}\)
\({10^{ – 3}}\)	milli	\({\text{m}}\)
\({10^{ – 6}}\)	micro	\(\mu \)
\({10^{ – 9}}\)	nano	\({\text{n}}\)
\({10^{ – 12}}\)	pico	\({\text{p}}\)
\({10^{ – 15}}\)	femto	\({\text{f}}\)
\({10^{ – 18}}\)	atto	\({\text{a}}\)
\({10^{ – 21}}\)	zepto	\({\text{z}}\)
\({10^{ – 24}}\)	yocto	\({\text{y}}\)
\({10^{1}}\)	deka	\({\text{da}}\)
\({10^2}\)	hector	\({\text{h}}\)
\({10^3}\)	kilo	\({\text{k}}\)
\({10^6}\)	mega	\({\text{M}}\)
\({10^9}\)	giga	\({\text{G}}\)
\({10^{12}}\)	tera	\({\text{T}}\)
\({10^{15}}\)	peta	\({\text{P}}\)
\({10^{18}}\)	exa	\({\text{E}}\)
\({10^{21}}\)	zetta	\({\text{Z}}\)
\({10^{24}}\)	yotta	\({\text{Y}}\)

Advantages of SI Unit System

The advantages of the SI unit system are as follows:

1. SI system is based on a certain set of fundamental units. Therefore, it is a coherent system.
2. SI system assigns only one unit to a particular physical quantity; therefore, it is a rational system.
3. SI unit can be expressed as a multiple of the power of \(10\). Hence, it is a SI metric system.
4. It is the internationally accepted system of unit.

Summary

In the SI Units of Measurements article, you have gained knowledge on seven SI units of measurement, derived units, advantages of SI unit systems, etc. It will be applicable for measuring and converting all the physical quantities used in the study of science.

FAQs on SI Units of Measurements

Q.1. How do you convert SI units?
Ans: The SI units are converted by multiplying or dividing with a certain numerical value. 
Example: The length \(2\,{\text{km}}\) is expressed in SI unit as \({\text{2}} \times 1{{\text{0}}^3}{\text{m}}\) or \(2000\,{\text{m}}\).

Q.2. What is the necessity of units in measurement? What are various SI units?
Ans: The value of a physical quantity is expressed in the products of numerical values. The numerical value alone does not give any accurate information. Therefore, a unit is important in measurement. 
The various SI units are meter, kelvin, kilogram, second, mole, candela and ampere.

Q.3. Why are SI units used in measurement?
Ans: The SI units of measurement are accepted internationally, and these are derived from the fundamental unit. It can be expressed in a metric system also.

Q.4. What is the metric SI unit for measuring length?
Ans: The metric units used for measuring length are millimetre, centimetre, metre, kilometres. The metric SI unit for measuring length is metre.

Q.5. What is the SI unit of measurement?
Ans: SI unit is the abbreviation used for System International d’ units. It is the metric system used for the international standard of measurements.

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