# Questions tagged [si-units]

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

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### Electric potential and Electronvolt [closed]

I am studying the electric potential energy and for the first time i red about the electric potential. The book Halliday et al. states that the electric potential can be defined as V = U/q where q is ...
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### Why is current measured in coulombs per second and not in electron per second?

If electricity is produced by flow of electron, that is current, then current should be measured in electron per second, why is it measured in coulombs per second?
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### Definition of 1 second [closed]

"One second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom." Can we alternatively define it as, Frequency ...
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### Is the plasma frequency independent from the relative dielectric constant of the solid crystal?

I have found many unclear explanations regarding plasma frequency in 3D condensed matter. Based on many textbooks and Wikipedia, the plasma frequency is $f_p=(\frac{e^2n}{m*\epsilon_0})^{0.5}/(2\pi)$...
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### Why is Coulomb's constant not unity? [duplicate]

Just as in Newton's 2nd law the value of constant was completely arbitrary (as said in ncert textbook class 12 physics) and was chose to be 1. Why not Coulomb's constant is chosen as unity?
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### Definition of a joule relative to gravitational PE

Why was it determined to calculate as 'pushing' an object (at the same height) over 1m instead of 'dropping' an object at 1 meter. In other words, dropping a 1kg object at 1m would have 9.8J, but ...
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### Constant Values with non SI units

I'm trying to code a program to study the energy levels of Helium using the variational method, but I'm not sure my constant values are correct. I'm using Angstroms as the unit of distance and $eV$ ...
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### Why is ampere still a base unit? [duplicate]

The ampere is still a base unit, according to the SI brochure. However, in my perception the recent redefinition of units effectively defines the Coulomb as e/(1.602 176 634 × 10^−19), and the ampere ...
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If $g_{\mu \nu}$ is dimensionless, it follows that $$\Gamma ^\lambda _{\alpha \beta} = \frac{1}{2} \sum_\sigma {g^{\lambda \sigma} \left(\frac{\partial g_{\sigma \beta}}{\partial x^\alpha} + \frac{\... 2answers 41 views ### Source for updated values of physical constants Looking for the value of the Molar Gas constant. I am unsure because NIST has 8.314 462 618 J/(mol*K) while Wikipedia says ... 1answer 51 views ### Black Body radiation units in CGS As all of us know the equation of black body is$$B_\nu = \frac{2 h v^3}{c^2} \times (\exp(h\nu/K_BT) - 1 ) ^{-1}$$I know the following units [h] = erg \cdot sec, [\nu] = Hz and [c] = cm/s. ... 2answers 66 views ### Power of a river I'm trying to calculate the power of the Mississippi, using the section of the river as it passes through New Orleans. I have the flow rate of a river, turned into a mass flow rate of 16,992,000 \:\... 1answer 55 views ### What is the advantage of natural units over SI units? Why do physics professionals often use various different systems of units instead of SI units. Especially I ask about when constants like c or \hbar are put to 1....what is the advantage of this? 3answers 333 views ### Why was the Planck constant h fixed to be exactly 6.62607015\times10^{−34}\text{Js} and not some other value? So apparently in May, all of the SI base units were redefined to be relative to the Planck Constant h, instead of relying on physical objects like the Kilogram Prototype in a Paris Basement. Planck'... 0answers 22 views ### Units in Representation Of Energy Density Spectrum In most of the texts, I referred to The Axis being labelled with Units as shown above. But I also found few images with different Notation (1/Units) Being Followed as shown Below. The units of the ... 1answer 53 views ### Help understanding unit: Micromoles per square meter per second The context is light, illumnination, photons. The units seem to suggest something different from the definitions I have found:$$\frac{µmol}{m^2 s}$$This, to me, says I have one millionth of a ... 1answer 32 views ### Quantity, kind of quantity, and dimension The International Vocabulary of Metrology defines a kind of quantity as the following: However, to me NOTE 1 EXAMPLE 1 and NOTE 1 EXAMPLE 2 are contradictory with NOTE 2 EXAMPLE. NOTE 2 EXAMPLE ... 1answer 54 views ### Please help with calculation /conversion Can someone please tell me how much 100 mw/ m-1 is converted to v /m? In terms of electromagnetic radiation? I have a gaussmeter that reads about 10-20 volts per meter in one area for electric field ... 3answers 76 views ### Is there a name for the unit “Ampere meter”? Motivation: I'm doing a homework problem involving a rod sliding freely down a pair of parallel conducting rails. I've got a quantity of unit \mathrm{A \cdot m} and want to know what I should name ... 1answer 59 views ### What unit system is used in Feynman's Lectures on Physics, Pt. 2 In part II, lecture 18, of Feynman's Lectures on Physics, on Table 18-1 Feynman writes Ampère's law as$$ c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}. $... 5answers 328 views ### Why is there no constant factor in the equation$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 ... 2answers 109 views ### Why is the ångström not a metric unit? And why is the ångström spelt with the Scandinavian letters “å” and “ö”? The website here http://unitsofmeasure.org/ucum.html tells us whether every unit is metric or not. Metric units can be multipled by a power of 10 and can be combined with a prefix. 1 ångström is ... 2answers 175 views ### Addition of Temperature Units [duplicate] I would like to know how does the addition of temperatures works. In school, we have been taught that we should convert degrees Celsius to Kelvin unit first and then the sum will be converted back to ... 0answers 49 views ### Conversion formula between attenuation coefficient (cm^-1) and (dBcm^-1MHz^-1) So, I was presented in class the fact that (at least in Ultrasound), attenuation coefficients$\mu$can be expressed in units$\text{dBcm}^{-1}\text{MHz}^{-1}$as opposed to the form I was used to; ... 1answer 30 views ### Electrostatics, measurement of charge How did physicists come up with a standard for measuring charge? How an electrical measurement of charge was made? 0answers 13 views ### Planck's Constant to the Kibble Balance A tad confused on how the Kibble balance actually relates to the Planck Constant. There's most likely a very simple relationship that I'm missing (I feel this is a stupid question ..) If the Kibble ... 1answer 67 views ### How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units? The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \begin{equation} \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ... 1answer 157 views ### Why$1 \ \mathrm{fm}is often used in modern physics? In the International System, we all know that a one femto (or one Fermi) is equal to $$1 \ \mathrm{fm}=10^{-15} \ \mathrm{m}$$ What is the historical reason why this unit of measure was adopted as ... 0answers 77 views ### Is there a notation alternative to Hertz? Hertz, as we all know, is a unit of frequency where 1 Hz equals one cycle per second. Conversely ... 1answer 42 views ### Basic question about units of velocity and speed of a curve on a smooth manifold Frederic Schuller says that velocity has units in Hertz in The WE-Heraeus International Winter School on Gravity and Light. He says: \begin{align} [v^a]&=\frac 1 T \\ [g_{ab}]&=L^2 \\ \Big[\... 2answers 38 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 ... 6answers 75 views ### Standard Definition of speed of light and metre The speed of light is the speed at which lightwaves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per ... 3answers 71 views ### Unit of Pressure -\frac{N}{m^2}$or$\frac{kg}{cm^2}$I am came across Pressure being given with unit$\frac{kg}{m^2}$at a lot of technical papers I am reading. However, as far as I understand, Pressure is defined as$\frac{N}{m^2}$which is not same ... 1answer 90 views ### Why is a Coulomb the charge of$6.24 \times 10^{18}$electrons? [duplicate] Where does the$6.24 \times 10^{18}$number come from? How was it historically derived? I know that$1$C$=1$A s but that just pushes the question down another step, and another and another, at ... 1answer 128 views ### Confusion on the cgs, Heavside-Lorentz and SI system I am very confused about three measuring systems: cgs, Heavside-Lorentz (never used) and SI ... 2answers 128 views ### Unit confusion - How Pressure and Volume combines to Joule Imagine some helium in a cylinder with an initial volume of$1$liter ant initial pressure of$1$atm. Somehow the helium is made to expand to a final volume of$3$liters, in such a way that its ... 1answer 77 views ### Differences charge quantity and electric charge First of all,my english is not well,so sorry for reading. As a senior middle school from China mainland,I am teaching physics about electri field.I with my workmates,get a problem now.We can not get a ... 1answer 64 views ### Is there a library of physical units? There are some standard units we all understand intuitively such as: Mass (kg) Length (metres) Time (seconds) From these there are some familiar composite units which are also well understood: ... 0answers 48 views ### Please check my problem about SI prefixes [closed] 2.2 A single logic gate in a prototype integrated circuit is found to be capable of switching from the “on” state to the “off” state in 12 ps. This corresponds to: (a) 1.2 ns; (b) 120 ns; (c) 1200 ns; ... 5answers 117 views ### Units of variable acceleration If we have a function, which describes, how a displacement in space along a line varies as a function of time: e.g.:$s(t)=vt$, its units are meters because$[v]=\frac{\text{meters}}{\text{seconds}}$... 1answer 64 views ### What's the preferred way of writing units per unit? [closed] Why do we write units such as meters per second as$\mathrm{m\,s} ^{-1}$instead of$\mathrm{m/s} $. Is the latter still a valid way of expressing units? Why are there two different ways of expressing ... 3answers 220 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 ...
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### Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...