# Are units that obvious ?

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

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

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

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

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

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

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

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

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

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

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

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 $Kr^{86}$

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

The time interval in which Cesium-133 atom vibrates $9192631770$ times.

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

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

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

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.

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

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 $101325$ $N/m^2$

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

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!)

6 years, 3 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.

- 5 years, 10 months ago

Thanks abt these important info

- 6 years, 2 months ago

In the new ncert definition of luminous intensity is different

- 6 years, 2 months ago

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

- 6 years, 2 months ago

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

- 6 years, 2 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.

- 6 years, 2 months ago

Yeah! They teach this at the +1 level.

- 6 years, 2 months ago

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

- 6 years, 2 months ago

That's the old definition.

- 6 years, 2 months ago

Ohhhhh okkk

- 6 years, 2 months ago

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

- 6 years, 3 months ago

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

- 6 years, 3 months ago

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.

- 6 years, 3 months ago

well, check out this -

- 6 years, 3 months ago

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

- 6 years, 3 months ago

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

- 6 years, 3 months ago

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

- 6 years, 3 months ago

No colors this time?

- 6 years, 3 months ago

Thanks for reminding !

- 6 years, 3 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...

- 6 years, 2 months ago