Units of Measurement and SI Unit

In this chapter, we will cover units for measuring objects including CGS, FPS, MKS and SI Units.

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Summary

1. 1. The seven fundamental S.I units are the kilogram, meter, second, Kelvin, ampere, candela, and mole.
   2. Derived units are based on the above seven fundamental units. Some examples are derived units are Angstrom unit, Acre, Pascal, etc
   3. In CGS system, the units of length, mass and time are centimeter(cm), gram(g), and second(s).
   4. In FPS system, the units of length, mass and time are the foot, pound and second(s).
   5. In MKS system, the units of length, mass and time are the metre, kilogram and second(s).
   6. International System (SI) of Units is modified MKS system. The units of length, mass and time in SI system are the metre, kilogram and second(s).
   7. Seven Fundamental Quantities with SI Units are given below. There are two supplemental quantities plane angle(unit is radian) and solid angle (unit is steridian) added to represent angles.
      

      S.No	Quantity	S.I Unit
      1	Mass	kilogram
      2	Length	meter
      3	Time	second
      4	Temperature	kelvin
      5	Electric Current	ampere
      6	Luminous Intensity	candela
      7	Amount of Substance	mole

   8. SI Prefix for Multiples and Sub-Multiples
      

      Multiple	Prefix	Symbol
      10¹⁵	Peta	P
      10¹²	Tera	T
      10⁹	Giga	G
      10⁶	Mega	M
      10³/1000	Kilo	k
      100	Hecto	h
      10	Deca	da
      10⁻¹	deci	d
      10⁻²	centi	c
      10⁻³	milli	m
      10⁻⁶	micro	µ
      10⁻⁹	nano	c
      10⁻¹²	pico	c
      10⁻¹⁵	femto	c

   9. Other Units for conversion during exam
      1. One Angstrom = 1 Å = 10⁻¹⁰ m
      2. One Astronomical Unit = 1 AU = 1.496 x 10¹¹ m
      3. One Light Year = 9.467 x 10¹⁵ m
      4. One Parsec = 3.08 x 10¹⁶ m
      5. One Quintal = 100 Kg
      6. One Tonne = 1000 Kg
      7. One Pound = 0.4536 kg
      8. One hour = 3600 seconds
      9. One Day = 86400 seconds
      10. One Year = 3.15 x 10⁷ seconds
      11. One Century = 100 years

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Table of Contents

1. What is Unit
2. Physical Quantity -Work and Energy
3. Fundamental Units and Derived Units
4. System of Units
5. SI Units
6. Base Units
7. Reference

What is Unit

Let’s start with the introduction to the unit. What is a unit? The unit is an internationally accepted reference standard to measure a physical quantity. When we talk of units we have several physical quantities so we need a reference to measure the physical quantity for example let us suppose your mom asks you to go to the shop and buy some rice now how would you convey the shopkeeper how much rice you want so you need a unit to identify the measurement of the rice, for example, you go to the shopkeeper and you say that you want 5 kg rice so kg is an internationally accepted reference which everybody would understand so similarly we have many other units for example kilogram to measure mass of any substance liter to measure volume of any substance meter to measure distance or length of anything and many other units like that if we see we find that the number of units is comparatively lesser than the number of physical quantities why is it so because there are many physical quantities which are represented by the same unit.

Physical Quantities – Work and Energy

We have a physical quantity work, we have another physical quantity, energy. Work and Energy are two different physical quantities but both of them are expressed by the same unit Joule, similarly, there are many situations where many physical quantities are defined in terms of the combination of existing units, for example, we say velocity. Velocity is a physical quantity now the unit which is used to measure velocity is meter per second, where a meter is already a unit for length and second is already a unit of time. so the unit for velocity is derived from already existing units, therefore it is observed that because of these kinds of constraints it is observed that the number of units is comparatively less than the number of physical quantities so what did we study here what is unit it is an internationally accepted reference to measure a physical quantity.

Fundamental and Derived Units

Let us now discuss the types of unit broadly units are classified into two types fundamental or base units and derived units fundamental units are those units on the basis of which all other units are defined so there is a total of seven fundamental units which we will discuss later. Examples of fundamental units are the kilogram, meter, second, Kelvin, etc. Similarly, derived units are always derived from fundamental units. This means that their origin is the fundamental unit, an example is meter square, for example, the unit for area is meter square, now meter is a fundamental unit so meter square is a derived unit. Similarly the unit for volume that is meter cube unit, for velocity meter per second or the unit for density that is kg per meter cube, they are all derived from the basic units that are kg, meter and second.

System of Units

These fundamental units plus the derived units together are known as the system of units, so what did we learn here units are classified into two types, fundamental units and derived units. Fundamental units are the basic units from which all other units are derived. Derived units are definitely they originate from the fundamental units. Fundamental units plus derived units together are called as the system of units.

SI Unit

What does SI stand for? It stands for System International, SI is a globally accepted system for measurement what happened was that in earlier days different parts of the world used different systems of units according to their own convenience so it so happened that if people traveled from one part of the world to another they faced problem communicating with each other regarding units. For example, let us say if you traveled to somewhere in Hong Kong, they used their own Hong Kong System of Units so it would be very difficult for you to communicate units there. So, scientists together decided to invent a globally accepted system for measurement so that everybody can use the same system for measurement, so then came the SI unit that is the system international unit.

Classification of SI Unit

In SI unit there are two classifications of units the base units and the derived units as we already discussed in SI there are seven base units which are defined and when we talk of derived units any other unit other than the seven base units are all derived units so all other units except the seven base units, they are all derived units, the two units Radian and steradian, these two units were given special importance before but now they are treated as derived units because many people have confusion they say that Radian and steradian they are base units but actually they are derived units.

Radian and Steradian

What is Radian? It is the unit to measure the angle (called as the plane angle) and steradian is the unit to measure the solid angle, okay so this was just as this was just a piece of information for you that Radian and steradian are nothing but they are derived units.

Seven Base Units in SI System

We will see what are the seven base units in SI system. The seven base units are meter, which is the unit for length, kilogram the unit for mass, second the unit for time, ampere the unit for electric current,Kelvin the unit of temperature, mole the unit for amount of a substance and Candela that is the unit value of intensity now other than these units any other unit that comes to your mind that is derived from these units.

External Reference:

NIST SI Base and Dervied Units Page
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