Are units that obvious ?

There are 2 types of physical quantities, the Fundamental\color{#D61F06}{\textbf{Fundamental}} and derived\color{#D61F06}{\textbf{derived}} ones.

Fundamental quantities are those which do not depend upon other physical quantities, there are 7 fundamental quantities\color{#D61F06}{\textbf{7 fundamental quantities}}. They have own defined units.

Derived quantities\color{#D61F06}{\textbf{Derived quantities}} are some function of the Fundamental quantities\color{#D61F06}{\textbf{Fundamental quantities}}, and they are infinite in number. Their units are depending on the units of Fundamental quantities\color{#D61F06}{\textbf{Fundamental quantities}} in their dimensional analysis.


That's how quantities are defined, we all know this.


But do you know how UNITS\color{#20A900}{\textbf{UNITS}} are defined ?

(Note, here I don't mean that 1 metre1 \text{ metre} is defined as 100cm100 cm)


Here are the 7 fundamental quantities\color{#3D99F6}{\textbf{Here are the 7 fundamental quantities}} with their units and definition of their unit.\color{#3D99F6}{\textbf{with their units and definition of their unit.}}


Their format- Quantity—Unit—Definition \color{#69047E}{\textbf{Quantity}} \color{#20A900}{\textbf{---Unit}} \textbf{---Definition}


1.Masskilogram\mathbf{1.}\color{#69047E}{\textbf{Mass}}-\color{#20A900}{\textbf{kilogram}}

The mass of a platinum-iridium cylinder kept in the National Bureau of Weights and Measurements, Paris


2.Lengthmetre\mathbf{2.}\color{#69047E}{\textbf{Length}}-\color{#20A900}{\textbf{metre}}

The distance travelled by light in vacuum in 1299792458th \dfrac{1}{299792458}^{th} part of a second or also defined as 1650763.73 times wavelength emitting from Kr86Kr^{86}


3.Timesecond\mathbf{3.}\color{#69047E}{\textbf{Time}}-\color{#20A900}{\textbf{second}}

The time interval in which Cesium-133 atom vibrates 91926317709192631770 times.


4.Temperaturekelvin\mathbf{4.}\color{#69047E}{\textbf{Temperature}}-\color{#20A900}{\textbf{kelvin}}

1273.16\dfrac{1}{273.16} fraction of thermodynamic temperature of triple point of water.


5.Electric Currentampere\mathbf{5.}\color{#69047E}{\textbf{Electric Current}} - \color{#20A900}{\textbf{ampere}}

The amount of electric current that produces a force of 2×107N2\times 10^{-7} \textbf{N} per unit length, that acts between two parallel wires of infinite length and negligible cross-section area placed at 1m distance in vacuum.


6.Luminous Intensitycandela\mathbf{6.}\color{#69047E}{\textbf{Luminous Intensity}}-\color{#20A900}{\textbf{candela}}

The amount of intensity on 160000m2\frac{1}{60000} m^2 area of blackbody in the direction perpendicular to its surface at freezing point of platinum 20422042 K at pressure of 101325101325 N/m2N/m^2


7.Quantity of substancemole\mathbf{7.}\color{#69047E}{\textbf{Quantity of substance}}-\color{#20A900}{\textbf{mole}}

The amount of substance which has same number of elementary entities as in 12 gm of carbon-12.


Source- NCERT textbook. (Felt like sharing because this was new to me and may be new to many of us!)

Note by Aditya Raut
5 years, 1 month ago

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Well, the kilogram definition sucks. Since it was originally defined as, the cylinder has decreased mass compared to its sisters. But, there is a definition that could be better. Watch Sixty Symbols recent video on the demise of the kilogram.

Sharky Kesa - 5 years, 1 month ago

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well, check out this -

www.youtube.com/watch?v=ZMByI4s-D-Y

Kartik Sharma - 5 years, 1 month ago

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Ahh yes, the Veritasium video. I still think that he Sixty Symbols definition is much better.

Sharky Kesa - 5 years, 1 month ago

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I edited the format of this note 4 times ( !!!!! ) so that it is well readable. And still doing improvements ! -_-

Aditya Raut - 5 years, 1 month ago

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No colors this time?

Satvik Golechha - 5 years, 1 month ago

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@Satvik Golechha Thanks for reminding !

Aditya Raut - 5 years, 1 month ago

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@Aditya Raut @Aditya Raut D'you know of a way by which we can invent colors on LaTeX?? For example there are 60 million colors and not all have LaTeX names, if we could use them somehow...

Satvik Golechha - 5 years, 1 month ago

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How precisely can we find the mass of that platinum-iridium cylinder? :D

Satvik Golechha - 5 years, 1 month ago

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Mmm. That Platinum-Iridium cylinder is made of the 2 most expensive elements. Devious plan

ZhiJie Goh - 5 years, 1 month ago

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but someone told me there are 11 fundamental quantities..... what about four you think...

Vicky Rohilla - 5 years, 1 month ago

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I don't know, there are 2 more supplementary qualities, the plane angle\textbf{plane angle} and the solid angle\textbf{solid angle}. but they are not fundamental, they're supplementary.

Aditya Raut - 5 years, 1 month ago

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Thanks abt these important info

Mohamed Raafat - 5 years, 1 month ago

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Mass -kilogram dude need a perfect definition......

Abu The-boss - 5 years, 1 month ago

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I read somewhere that 1kg is defined as mass of 1cubic decimeter of water at 4°C

Aman Sharma - 5 years, 1 month ago

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That's the old definition.

Sharky Kesa - 5 years, 1 month ago

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Ohhhhh okkk

Aman Sharma - 5 years, 1 month ago

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Yeah! They teach this at the +1 level.

Ameya Salankar - 5 years, 1 month ago

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Kilogram should have a better definition. ..btw where did it come from...

Rajsuryan Singh - 5 years, 1 month ago

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In the new ncert definition of luminous intensity is different

Satvik Choudhary - 5 years, 1 month ago

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@Aditya Raut I have read that luminous intensity is fundamental quantity. But I think it can be derived using other fundamental quantities. So it should not be fundamental. Even NCERT writes a note about Luminous Intensity.

Pranjal Jain - 4 years, 9 months ago

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