# Tag Info

## Hot answers tagged si-units

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• 51.5k
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### Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

The underlying reason that 1 coulomb seems like a large amount of charge is that most charged particles in ordinary settings are nonrelativistic -- moving at speeds $v$ much less than the speed of ...
• 3,870

### Is there any truth to interpreting definition of a second as corresponding to oscillations?

The definition for the cesium clock is: 9192631770 cycles per second is frequency of the radio waves which cause maximum resonance, a physically measurable condition, in the cesium atoms. This ...
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### The definition of 1 kelvin

To answer this question it may help to take an example from a more familiar area of physics, and then discuss temperature. For a long time the kilogram (the SI unit of mass) was defined as the mass of ...
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### How long is a second?

A second is a second long by definition, but if you measure any time in seconds, the number of seconds you infer will be subject to an error of at least $\mathcal O(10^{-15})$ because of the ...
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### Why do we use the electron volt?

Addressing only why it is used/useful in science today, not why or how it came about The other answers seem to come from a particle physicist's point of view; for a chemist the electronvolt is ...
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### Why are "degrees" and "bytes" not considered base units?

Another answer (and a linked question) addresses the fact that that the SI derived unit for angles is the radian, which is a ratio of lengths. See e.g. The bit/byte question is interesting. In ...
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### Why is the length of the Kelvin unit of temperature equal to that of the Celsius unit?

Kelvins aren't really all that natural either; or rather, they are just as arbitrary as Celsius. You need another arbitrary quantity--the Boltzmann constant--to get the temperature unit to work with ...
• 24.2k
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### Why is the Planck constant an exact number with defined value?

Planck's constant relates two different types of quantities, namely energy and frequency. That means it is a conversion factor which converts the units of quantities from one form to another. If the ...
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### What is a joule? I find the definition confusing

Maybe imagine the ball is being pulled by a weight, or a spring, back here on Earth. The weight falls down and pulls the ball along: It is easy to see that the weight goes down 1 meter when the ball ...
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### Is there any truth to interpreting definition of a second as corresponding to oscillations?

You're correct and the video is mistaken. In fact, if cesium atoms were constantly oscillating between the two hyperfine states, cesium beam clocks wouldn't work at all! In its simplest form, a ...
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### Do units have to make sense in Physics?

Units in physics are units of the real world. They must be consistent, because the real world is. After choosing a definition of necessary fundamental units (as defined by the SI system e.g. - I am ...
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### What's exactly the new definition of kilogram, second and meter?

The SI system is now defined entirely by physical constants. There are no more “prototype” artifacts. How it works is thus: https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-concise-EN....
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### Why is the Planck constant an exact number with defined value?

Before May 2019, Planck's constant was not defined by an exact value and instead was measured experimentally to be $6.626069934(89)\times10^{−34}\ \mathrm{J\cdot s}$. However, it is worth noting what ...
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### What technology is needed for an individual to reproduce the current SI meter and kilogram from scratch?

I don't see any particular reason why you couldn't reproduce the Foucault experiment with bronze age technology. Foucault measured the speed of light to better than 1% - not quite the 0.1% requested, ...
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### Why is the mole/"amount of substance" a dimensional quantity?

Tim, your reservations about the quantity called "amount of substance" is completely justified and many authors argue the same as you do. Let me expand some: "matter" or "substance" can be quantified ...
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