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Are units that obvious ?

There are 2 types of physical quantities, the \(\color{Red}{\textbf{Fundamental}}\) and \(\color{Red}{\textbf{derived}}\) ones.

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

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


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


But do you know how \(\color{Green}{\textbf{UNITS}}\) are defined ?

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


\(\color{Blue}{\textbf{Here are the 7 fundamental quantities}}\) \(\color{Blue}{\textbf{with their units and definition of their unit.}}\)


Their format- \( \color{Purple}{\textbf{Quantity}} \color{Green}{\textbf{---Unit}} \textbf{---Definition} \)


\(\mathbf{1.}\color{Purple}{\textbf{Mass}}-\color{Green}{\textbf{kilogram}}\)

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


\(\mathbf{2.}\color{Purple}{\textbf{Length}}-\color{Green}{\textbf{metre}}\)

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


\(\mathbf{3.}\color{Purple}{\textbf{Time}}-\color{Green}{\textbf{second}}\)

The time interval in which Cesium-133 atom vibrates \(9192631770\) times.


\(\mathbf{4.}\color{Purple}{\textbf{Temperature}}-\color{Green}{\textbf{kelvin}}\)

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


\(\mathbf{5.}\color{Purple}{\textbf{Electric Current}} - \color{Green}{\textbf{ampere}}\)

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


\(\mathbf{6.}\color{Purple}{\textbf{Luminous Intensity}}-\color{Green}{\textbf{candela}}\)

The amount of intensity on \(\frac{1}{60000} m^2\) area of blackbody in the direction perpendicular to its surface at freezing point of platinum \(2042\) K at pressure of \(101325\) \(N/m^2\)


\(\mathbf{7.}\color{Purple}{\textbf{Quantity of substance}}-\color{Green}{\textbf{mole}}\)

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
2 years, 10 months 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 · 2 years, 10 months ago

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@Sharky Kesa well, check out this -

www.youtube.com/watch?v=ZMByI4s-D-Y Kartik Sharma · 2 years, 10 months ago

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@Kartik Sharma Ahh yes, the Veritasium video. I still think that he Sixty Symbols definition is much better. Sharky Kesa · 2 years, 10 months ago

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@Sharky Kesa I edited the format of this note 4 times ( !!!!! ) so that it is well readable. And still doing improvements ! -_- Aditya Raut · 2 years, 10 months ago

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@Aditya Raut No colors this time? Satvik Golechha · 2 years, 10 months ago

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@Satvik Golechha Thanks for reminding ! Aditya Raut · 2 years, 10 months 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 · 2 years, 10 months ago

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@Sharky Kesa How precisely can we find the mass of that platinum-iridium cylinder? :D Satvik Golechha · 2 years, 10 months ago

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Mmm. That Platinum-Iridium cylinder is made of the 2 most expensive elements. Devious plan ZhiJie Goh · 2 years, 10 months ago

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Thanks abt these important info Mohamed Raafat · 2 years, 10 months ago

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but someone told me there are 11 fundamental quantities..... what about four you think... Vicky Rohilla · 2 years, 10 months ago

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@Vicky Rohilla I don't know, there are 2 more supplementary qualities, the \(\textbf{plane angle}\) and the \(\textbf{solid angle}\). but they are not fundamental, they're supplementary. Aditya Raut · 2 years, 10 months 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 · 2 years, 6 months ago

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In the new ncert definition of luminous intensity is different Satvik Choudhary · 2 years, 10 months ago

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Kilogram should have a better definition. ..btw where did it come from... Rajsuryan Singh · 2 years, 10 months ago

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Yeah! They teach this at the +1 level. Ameya Salankar · 2 years, 10 months ago

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I read somewhere that 1kg is defined as mass of 1cubic decimeter of water at 4°C Aman Sharma · 2 years, 10 months ago

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@Aman Sharma That's the old definition. Sharky Kesa · 2 years, 10 months ago

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@Sharky Kesa Ohhhhh okkk Aman Sharma · 2 years, 10 months ago

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Mass -kilogram dude need a perfect definition...... Abu The-boss · 2 years, 10 months ago

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