## International System of Units – Class 11 | Chapter – 2 | Physics Short Notes Series PDF for NEET & JEE

International System of Units: The International System of Units (SI) is the modern form of the metric system and is the world’s most widely used system of measurement. It is a comprehensive system of measurement that is based on seven base units, which are used to derive all other units of measurement. The seven base units are the meter, kilogram, second, ampere, kelvin, mole, and candela.

## International System of Units

The 7 base units of the International System of Units (SI) are:

• meter (m) for length
• kilogram (kg) for mass
• second (s) for time
• mole (mol) for amount of substance
• ampere (A) for electric current
• kelvin (K) for temperature
• candela (cd) for luminous intensity

The International System of Units (SI) is the modern form of the metric system and is the world’s most widely used system of measurement. It is based on seven base units, which are used to derive all other units of measurement:

• Meter (m): the base unit of length, defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.
• Kilogram (kg): the base unit of mass, defined as the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in France.
• Second (s): the base unit of time, defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
• Ampere (A): the base unit of electric current, defined as the constant current that, if maintained in two parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.
• Kelvin (K): the base unit of temperature, defined as 1/273.16 of the thermodynamic temperature of the triple point of water.
• Mole (mol): the base unit of amount of substance, defined as the amount of substance that contains as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 0.012 kilograms of carbon-12.
• Candela (cd): the base unit of luminous intensity, defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

## SI Derived Units

The derived units are unlimited as they are formed by different operations on the base units. For derived units, the dimensions are expressed in terms of the dimensions of the base units. The derived units might also be expressed with the combination of base and derived units.

There are several derived units in physics. Some of the most widely used SI derived units in physics are given below.

Unit(s) Name SI Unit SI Unit Symbol Expressed in SI Base Unit Expressed in other SI units
Force, Weight Newton N kg⋅m⋅s-2
Frequency Hertz Hz s-1
Electric charge Coulomb C s⋅A
Electric potential (Voltage) Volt V kg.m2.s-3.A-1 W/A
Inductance Henry H kg.m2.s-2.A-2 Wb/A
Resistance, Impedance, Reactance Ohm Ω kg.m2.s−3.A−2 V/A
Electrical conductance Siemens S kg−1.m−2.s3.A2 Ω−1
Magnetic flux Weber Wb kg.m2.s−2.A−1 V⋅s
Magnetic flux density Tesla T kg.s−2.A−1 Wb/m2
Energy, Work, Heat Joule J kg.m2.s−2 N⋅m = Pa⋅m3
Power, Radiant flux Watt W kg.m2.s−3 J/s
Luminous flux Lumen lm cd cd⋅sr

## Significance of the International System of Units

The International System of Units (SI) is significant for several reasons:

• Standardization: The SI provides a standardized system of measurement that is used worldwide. This allows for consistent and accurate communication of measurements across international borders, which is important for scientific research, industry, and trade.
• Simplification: The SI simplifies measurement by providing a set of base units and a simple set of rules for combining these units to derive other units of measurement. This reduces confusion and errors caused by the use of different measurement systems.
• Precision: The SI provides precise definitions for each unit of measurement, which ensures that measurements are accurate and consistent.
• Evolution: The SI is designed to evolve and adapt to changing scientific and technological needs. As new measurement techniques and technologies emerge, the SI will continue to be refined and updated to reflect these advances.
• Universal Acceptance: The SI is widely accepted and used in many countries and industries around the world. This universal acceptance helps to promote international cooperation and collaboration in scientific research and technological development.

### SI Conventions

The SI conventions are a set of rules and guidelines for using the International System of Units (SI) correctly and consistently. These conventions include:

• Spelling and symbols: The symbols for SI units are always written in lowercase letters, except for liter and those derived from the name of a person (such as newton and pascal). The unit name is spelled out in full when it appears in text, and the symbol is used only when a numerical value is given.
• Prefixes: The SI prefixes are used to indicate multiples or fractions of a unit. They are always written in lowercase letters, and the prefix symbol is attached to the unit symbol without a space.
• Multiplication and division: When combining units, multiplication and division are indicated by a space or a solidus (/), respectively. For example, the unit of velocity is meters per second (m/s), and the unit of acceleration is meters per second squared (m/s^2).
• Order of magnitude: Units are written in order of magnitude, with the largest units first and the smallest units last. For example, a length of 3.5 meters and a time of 4.2 seconds would be written as 3.5 m and 4.2 s, respectively.
• Uncertainty and significant figures: The uncertainty of a measurement and the number of significant figures should be indicated when reporting numerical values. This helps to convey the accuracy of the measurement and allows for proper comparison with other measurements.

## SI Units Prefixes

SI prefixes are used for forming decimal multiples and submultiples of SI units. The grouping of a prefix symbol attached to a unit symbol form a new inseparable unit symbol.

Multiplying Factor Name (symbol) Scientific Notation
1 000 000 000 000 000 000 000 000 yotta (Y) 1024
1 000 000 000 000 000 000 000 zetta (Z) 1021
1 000 000 000 000 000 000 exa (E) 1018
1 000 000 000 000 000 peta (P) 1015
1 000 000 000 000 tera (T) 1012
1 000 000 000 giga (G) 109
1 000 000 mega (M) 106
1 000 kilo (k) 103
100 hecto (h) 102
10 deca (da) 101
1 100
0.1 deci (d) 10-1
0.01 centi (c) 10-2
0.001 milli (m) 10-3
0.000 001 micro (µ) 10-6
0.000 000 001 nano (n) 10-9
0.000 000 000 001 pico (p) 10-12
0.000 000 000 000 001 femto (f) 10-15
0.000 000 000 000 000 001 atto (a) 10-18
0.000 000 000 000 000 000 001 zepto (z) 10-21
0.000 000 000 000 000 000 000 001 yocto (y) 10-24

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