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# Questions tagged [si-units]

A set of internationally accepted units to aid in communication of measurements.

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### Why was the Planck constant $h$ fixed to be exactly 6.62607015×10−34 and not some other value?

So apparently in May, all of the SI base units were redefined to be relative to the Planck Constant $h$, instead of relying on physical objects like the Kilogram Prototype in a Paris Basement. Planck'...
18 views

### Units in Representation Of Energy Density Spectrum

In most of the texts, I referred to The Axis being labelled with Units as shown above. But I also found few images with different Notation (1/Units) Being Followed as shown Below. The units of the ...
34 views

### Help understanding unit: Micromoles per square meter per second

The context is light, illumnination, photons. The units seem to suggest something different from the definitions I have found: $$\frac{µmol}{m^2 s}$$ This, to me, says I have one millionth of a ...
34 views

### Which luminous efficacy function does BIPM reference?

The definition of candela given by BIPM is as follows: The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the ...
10k views

### Why is the mole/“amount of substance” a dimensional quantity?

According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why? I get why the mole is useful as a unit. In ...
20k views

### Why is the ampere a base unit and not the coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to ...
22 views

### Quantity, kind of quantity, and dimension

The International Vocabulary of Metrology defines a kind of quantity as the following: However, to me NOTE 1 EXAMPLE 1 and NOTE 1 EXAMPLE 2 are contradictory with NOTE 2 EXAMPLE. NOTE 2 EXAMPLE ...
40 views

### What physical quantity does Mass Moment of Inertia times Torque squared times Seconds may represent?

In some calculus I performed involving a mass moment of inertia, a torque squared and time, the resulting SI units were (kg.m^2) (kg.m.s^(-2))^2 (s) = (kg^3) (m^6) (s^(-3)) What may this quantity ...
53 views

Can someone please tell me how much 100 mw/ m-1 is converted to v /m? In terms of electromagnetic radiation? I have a gaussmeter that reads about 10-20 volts per meter in one area for electric field ...
138 views

### Gauss' law for two point charges and a factor of $4\pi$

When we model electric field as something that represents the "flow" of something that is conserved, we can prove that the flux due to a single point charge, through the following surface, Flux ...
73 views

### Is there a name for the unit “Ampere meter”?

Motivation: I'm doing a homework problem involving a rod sliding freely down a pair of parallel conducting rails. I've got a quantity of unit $\mathrm{A \cdot m}$ and want to know what I should name ...
320 views

### Why is there no constant factor in the equation $E=mc^2$? [duplicate]

As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
57 views