Unit & Measurement Notes | Study Science & Technology for UPSC CSE - UPSC

Measurement

Measurement is a technique in which the properties of an object are determined by comparing them to a standard.

• Measurement requires tools to provide scientists with a quantity. A quantity describes how much of something there is and how many there are. 
• Scientists use a system of measurement still commonly referred to as the “metric system.”  
• It was the first standardized system of measurement, developed in France in the 1790s. Today, this form is the standard form of measurement in every country except the United States.

Unit

A unit is a particular physical quantity, defined and adopted by the convention, with which other particular quantities of the same kind are compared to express their value.

The Seven Base Units Of Measurement

Length – Meter (m)

• It is defined as the length of the path traveled by light in an interval of exactly 1299792458 s. 
• It is based on the fundamental quantity, the speed of light in a vacuum which is c=299 792 458 m/s.

Time – Second (s)

• The time taken by 9 192 631 770 periods of oscillations of the light emitted by a cesium -133 atoms corresponding to the transition between two hyper-fine levels of the ground state. 
• This is determined by using highly precise atomic clocks.

Mass – Kilogram (kg)

• It is the mass of a prototype platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Paris, France. 
• Copies of this cylinder are kept by many countries which use them to standardize and compare weights.

Electric current – Ampere (A)

• The constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section when placed 1 m apart in vacuum, would produce a force equal to 2×10−7 Newton per meter of length between these conductors. 
• While, it may appear that electric charge should have been used as a base unit, measuring current is far easier and hence is chosen as the standard base unit.

Temperature – Kelvin (K)

• The SI unit of temperature is Kelvin. It is exactly 1273.16  of the thermodynamic temperature of the triple point of water. 
• The triple point of water is a fixed temperature and pressure at which the solid, liquid and gaseous states can exist at the same time.

Amount of a substance – Mole (mol)

• The mole is the amount of substance which contains as many entities as there are atoms in 0.012 kg of carbon-12. 
• A mole contains an Avogadro number of entities. Check out our chemistry articles to know more about the Avogadro number.

Luminous Intensity – Candela (cd)

• It is the luminous intensity of a source that emits radiation of a constant frequency of 540×1012Hz with a radiant intensity of 1683  Watt per steradian in any given direction.

Derived Units

The units of measurement derived from the seven base units specified by the International System of Units is known as SI derived units. 

• They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation. 
• The names of SI-derived units, when written in full, are always in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter.
• For example, the symbol for hertz is “Hz”, but the symbol for metre is “m.” In the table below, we have given a list of derived units. 

The International System of Units

In earlier times scientists of different countries were using different systems of units for measurement.

• In CGS system they were centimeter, gram, and second respectively.

• In FPS system they were foot, pound, and second respectively.

• In MKS system they were meter, kilogram, and second respectively.

• The system of units that is at present internationally accepted for measurement is the Système Internationale d’ abbreviated as SI.
• The SI was developed and recommended by General Conference on Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work.
• Seven Fundamental Quantities of International System of Units are: 

Units of Long Distance

• Light year = distance travelled by the light in one year = 10¹⁶m
• Parsec (Parallactic second) is the distance corresponding to a parallax of one second of arc,
• 1 Parsec = 3.26 light year

• Light year = distance travelled by the light in one year = 10¹⁶m
• Parsec (Parallactic second) is the distance corresponding to a parallax of one second of arc,
• 1 Parsec = 3.26 light year

Units of Short Distance

Mass & Weight 

• Mass is defined as the amount of matter an object has. One of the qualities of mass is that it has inertia Mass is a measure of how much inertia an object shows.
• The weight of an object on earth depends on the force of attraction (gravity) between the object and earth.