Units and Measurement
| Quantity | Name | Symbol | Definition |
| Length | Metre | m | The metre is the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. (1983) |
| Mass | Kilogram | kg | The kilogram is defined by the fixed numerical value of the Planck constant, which is 6.62607015 × 10−34 kg⋅m2⋅s−1. |
| Time | Second | s | The second is the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. (1967) |
| Electric Current | Ampere | A | The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, would produce a force of 2×10⁻⁷ newton per metre of length. (1948) |
| Thermodynamic Temperature | Kelvin | K | The kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water. (1967) |
| Amount of Substance | Mole | mol | The mole is the amount of substance in a system, which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. (1971) |
| Luminous Intensity | Candela | cd | The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10¹² hertz with a radiant intensity of 1/683 watt per steradian. (1979) |
Definition of Kilogram
In 2019, the definition of Kilogram was changed.
Since 1889, the kilogram was defined as being equal to “the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at the International Bureau of Weights and Measures, at Sevres, France.” (1889)
Problem: This is a very primitive definition based on a physical object, which might get destroyed someday. We cannot base all our physics on a physical object! We certainly need a definition that relies solely on the fundamental constants of nature.
You will find that all the units above are defined based on the fundamental properties of nature or natural substances. This is exactly what was required for a kilogram.
New definition of Kilogram
The kilogram is defined by the fixed numerical value of the Planck constant, which is 6.62607015 × 10−34 kg⋅m2⋅s−1. This definition was agreed upon by the International Bureau of Weights and Measures in 2019.
This definition is based on the Planck constant’s relationship to photon energy and frequency.
E = hn
Where h is the plank’s constant and n is the frequency of the light for which energy is being measured.
Units and Measurement form the foundation of all scientific studies. It helps in quantifying physical quantities, ensures standardization, and enables accurate communication of scientific data.
The SI system consists of seven fundamental units — meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).
Fundamental units are independent and cannot be derived from other units, whereas derived units are formed by the combination of fundamental units, like m² (area), m/s² (acceleration), or newton (force).
Accuracy refers to how close a measurement is to the true value, while precision indicates the consistency of repeated measurements. Both are essential to minimize errors and ensure reliable scientific results.
Measurement errors can be classified into systematic errors (consistent and predictable) and random errors (unpredictable variations). Minimizing these errors is crucial for accurate scientific analysis.
