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Physical quantities are measured in units, which are standardized values. The length of a race, for example, is a physical number that can be expressed in metres (for sprinters) or kilometres (for marathon runners) (for long-distance runners). It would be extremely difficult for scientists to express and compare measured results in a meaningful way without standardized units. The International System of Units (SI) expresses all physical quantities in terms of combinations of seven fundamental physical units: length, mass, time, electric current, temperature, substance amount, and luminous intensity.

**Overview**

Since every physical quantity normally requires a unit or standard for specification, it appears that there must be an equal number of units as physical quantities. This isn’t the case. It has been discovered that if the units of any three physical quantities are arbitrarily chosen in mechanics, the units of all other physical quantities in mechanics may be expressed in terms of these. For this aim, arbitrary physical quantities such as mass, length, and time are chosen. In mechanics, a fundamental, absolute, or basic unit is any unit of mass, length, or time. Derivative units are other units that can be expressed using basic units.

A fundamental unit is a measurement unit used to measure a base quantity. A base quantity is one of a set of physical quantities that have been traditionally chosen and for which no subset quantity can be stated in terms of the others.

- Length (metre)
- Mass (kilogram)
- Time (second)
- Electric current (ampere)
- Thermodynamic temperature (kelvin)
- Amount of substance (mole)
- Luminous intensity (candela)

**The Metre**

The metre is the SI unit for length (m). The metre’s definition has evolved over time to become more exact and accurate. In 1791, the metre was defined as 1/10,000,000 of the distance between the equator and the North Pole. In 1889, the metre was redefined as the distance between two etched lines on a platinum-iridium bar, which enhanced the measurement. (The bar is now located near Paris at the International Bureau of Weights and

Measures.) Some distances might be measured more precisely in 1960 by comparing them to light wavelengths.1,650,763.73 wavelengths of orange light emitted by krypton atoms were redefined as the metre. The metre’s current definition was given in 1983: the distance light travels in a vacuum in 1/ 299,792,458 of a second.

**The Kilogram**

The kilogram is the SI unit of mass (kg). The mass of a platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris is used to define it. Many places around the world, including the National Institute of Standards and Technology in Gaithersburg, Maryland, have exact reproductions of the standard kilogram cylinder. By comparing all other masses to one of these standard kilograms, all other masses can be determined.

**The Second**

The second (s), the SI unit for time, has a long history as well. For a long time, it was defined as 1/86,400 of a typical solar day. However, due to the slowing of the Earth’s rotation, the average solar day is steadily getting longer. Because all other measures are derived from them, accuracy in the fundamental units is critical. As a result, a new criterion for defining the second in terms of a non-varying, or constant, physical event was established. The very continuous vibrating of Cesium atoms, which can be viewed and quantified, is one consistent phenomenon. The cesium atomic clock is based on this vibration. The second was redefined in 1967 as the time it takes for 9,192,631,770 Cesium atoms to vibrate.

**The Ampere**

The ampere (A), named for Andre Ampere, is a unit of the electric current measurement. When it comes to electrical currents or devices, you’ve probably heard of amperes or amps. Understanding an ampere necessitates a fundamental understanding of electricity and magnetism, which will be covered in greater depth in the book’s later chapters. In simple terms, two parallel wires with an electric current running through them will attract each other. The amount of electric current that produces an attracting force of 2.7x 10–7 newton per metre of separation between the two wires is defined as one ampere (the newton is the derived unit of force).

**Kelvins**

The kelvin is the SI unit of temperature (or kelvins, but not degrees kelvin). The absolute temperature scale was first proposed by physicist William Thomson, Lord Kelvin, who was the first to propose it. Absolute zero is the starting point for the Kelvin scale. This is the point at which all thermal energy from all atoms or molecules in a system has been gone. This temperature, 0 K, is equivalent to 273.15 degrees Celsius and 459.67 degrees Fahrenheit. The Kelvin scale, conveniently, changes in the same way that the Celsius scale does. On the Celsius system, the freezing point (0 °C) and boiling point (100 °C) of water, for example, are 100 degrees apart. These two temperatures are also separated by 100 kelvins.(Boiling point: 373.15 K; freezing point: 273.15 K).

**Amount of substance (mole)**

There are exactly 6.02214076 × 1023 elementary entities in one mole (mol). This is the Avogadro number, which is the fixed numerical value of the Avogadro constant, NA, as represented in the unit mol-1. A system’s amount of substance, denoted by the symbol n, is a count of the number of defined elementary things. An atom, a molecule, an ion, an electron, or any other particle or combination of particles can all be considered elementary entities. When employing the mole, the elementary entities must be provided, and they can be atoms, molecules, ions, electrons, other particles, or groupings of such particles.

**Luminous intensity (candela)**

The luminous intensity is a metric for describing the brightness of a light source. The luminous flux per unit solid angle is what it’s called. The luminous intensity is a photometric quantity that takes into account the human eye’s spectrum response – usually for photopic vision, which assumes a sufficiently high light intensity (luminance) for colour vision. The word is most commonly used in the approximation of a point source, that is, at distances that are large in comparison to the source’s extent. The candela = lumen per steradian (cd = lm/sr) is the SI unit for luminous intensity. The light intensity of an ordinary candle is roughly equivalent to one candela.

**Units derived from the SI**

Since they are generated by various operations on the base units, the derived units are limitless. The dimensions of derived units are represented in terms of the dimensions of base units. A combination of base and derived units can also be used to express derived units. e.g. speed, acceleration, force, etc.

**Importance of chapter for JEE Main, NEET, and Board Exams**

It is recommended that students go over these important physics SI units lists thoroughly. It is critical to write the units and dimensions corresponding to the quantity in **NEET** and **JEE** since a response is incomplete without its unit.

**The Benefits of Learning SI and CGS Units**

- SI Units and CGS Units are often asked in JEE and NEET. This chapter is having maximum weightage in board exams as well.
- These units are presented in an interactive format and are simple to comprehend, allowing students to learn more effectively.
- The units and their explanations will assist students in gaining a thorough understanding of the various themes.
- The units are graded in points to help pupils remember the material for a longer time.

**FAQs**

**Q. When were SI units established?**

**Ans:** In 1960, the SI unit was introduced.

**Q. What is the significance of the SI system?**

**Ans: **The following are some of the reasons why the SI system is crucial:

- The International System of Units (SI) is based on precise and clear standards.
- The SI system uses a base often, which makes conversions easy.
- In the SI system, prefixes such as Latin and Greek are used to refer to numbers.
- The SI units can be deduced from one another without the need for conversion factors.

**Q. What are the most often used measurement systems?**

**Ans:** The following are the most regularly used measurement systems:

- CGS system
- MKS system
- SI system

**Q. Define unit.**

**Ans:** The reference standard for measurements is defined as a unit.