# 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!)

3 years, 9 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.

- 3 years, 9 months ago

well, check out this -

- 3 years, 9 months ago

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

- 3 years, 9 months ago

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

- 3 years, 9 months ago

No colors this time?

- 3 years, 9 months ago

Thanks for reminding !

- 3 years, 9 months ago

@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...

- 3 years, 9 months ago

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

- 3 years, 9 months ago

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

- 3 years, 9 months ago

Thanks abt these important info

- 3 years, 9 months ago

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

- 3 years, 9 months ago

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.

- 3 years, 9 months ago

@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.

- 3 years, 5 months ago

In the new ncert definition of luminous intensity is different

- 3 years, 9 months ago

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

- 3 years, 9 months ago

Yeah! They teach this at the +1 level.

- 3 years, 9 months ago

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

- 3 years, 9 months ago

That's the old definition.

- 3 years, 9 months ago

Ohhhhh okkk

- 3 years, 9 months ago

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

- 3 years, 9 months ago