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# Questions tagged [units]

Units are standards of measurement used for different types of quantities.

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### Laser beam diameters: What are the units of 1/e^2?

I own an argon ion laser head with a beam diameter of '0.70' (using $1/e^2$ width). What is the unit on the 0.70? And can it be converted to something more intuitive, such as meters?
68 views

### How do you properly derive atomic units from SI units?

I'm learning atomic units and I'm having trouble figuring out how to derive the atomic units version of energy, time, electric field, magnetic field, etc. For example the Wikipedia page has a list of ...
277 views

### What is the largest practical unit of mass? [closed]

I have seen in internet and books that the largest practical unit for mass is Chandrasekhar limit which is 1.4 times the mass of sun. It gives the minimum mass for a star to become a neutron star. But ...
314 views

### Multiplication of units

I can understand the operation of dividing two units for example: 1 m/s it means that the object covers the distance of 1m in one second but really I can not understand the operation of multiplication ...
I'm studying Calculus, not physics, but was curious how units work when we consider curl over a 2D velocity field. Given $\mathbf{F} = M(x,y)\mathbf{i} + N(x,y)\mathbf{j}$ the curl is defined to be $(\... 0answers 122 views ### How is thrust per unit power measured? Typically, thrust efficiency is measured in specific impulse, as thrust per fuel consumption rate: $$I_\text{sp} = \frac{F_\text{thrust}}{\dot{m}_\text{fuel}} \quad \text{or} \quad I_\text{sp} = \... 1answer 62 views ### Converting units, ternary phase diagram [closed] I have a task with a ternary phase diagram with units given in mol/mol, but in the description I am also given a feed-to-solvent ratio at 2.5 kg/kg. The molecular masses (and in the equation you see ... 1answer 61 views ### Understanding the statcoulomb conceptually I've just learned about the statcoulomb, which is basically a way to express charge when we didn't have units of charge (I think), with the definition$$1 \mathrm{statC} = 1 \mathrm{dyne}^{1/2} \... 6answers 639 views ### Give an interpretation of what$c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$actually means From my later school days on, the formula that impresses me the most is $$c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$$ I know how this can be derived, both from Maxwell Equations and more intuitively. I ... 3answers 3k views ### What is a “decade” as a unit of measure (ex. a decade of the EM spectrum)? Reading through papers and online sources about radio galaxies, I kept stumbling across a term--a "decade" of the electromagnetic spectrum. Radio galaxy emission encompasses "11 decades of the EM ... 5answers 136 views ### The value of$c$in$E=mc^2$In Einstein’s formula$E = m c^2$,$c$is the speed of light. However the speed of light is not a unique number. It can be expressed in kilometres per second or miles per second for example, which ... 2answers 831 views ### Complete list of fundamental properties [closed] What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per ... 3answers 85 views ### Is energy always measured in units of mass × length$^2$/time$^2$in physics? Is it always$ M\,L^2/T^2$? Is special relativity different from general relativity regarding the units of energy? 1answer 880 views ### What unit of measurement is a BeV? I am studying this paper by Gleeson and Axford from 1968. They often measure the energy of a particle in BeV: does it mean GeV, like a Billion of eV? 1answer 54 views ### How can distance and temperature be commensurable? I am sorry to intrude. The discipline I know something about is metaphysics rather than physics itself. And on Stack Exchange I spend my time on English Language and Usage. I am struggling ... 0answers 23 views ### Mixed unit conversion I have a silly question. I am not used to different unit systems and I am stuck with trivial problem. I have expression for energy of harmonic oscillator: $$\Delta E = \frac{1}{2}\Omega^2\Delta Q^2. ... 2answers 611 views ### Units on the electrostatic force equation So here we have the electrostatic equation: Fe = Ke * q1 * q2 / r^2 The units of the constant Ke are N*m^2/C^2 And this is because we want it to cancel out with the product of q1 and q2 which unit ... 1answer 151 views ### The Gauss's law for gravitational field and the unit system Here g is the gravitational field, G is the gravitational constant, and M is the total mass in the volume V. I wonder if this formula holds for any unit system. That is, does the coefficient ... 2answers 198 views ### Definition of stress-energy tensor The image from the wiki article on the stress energy tensor gives T_{00} as 1/c^2 times the energy density. I believe this is incorrect and that the 1/c^2 factor should be dropped. Am I ... 3answers 140 views ### Dimensionless consistency and quantities I am a chemical engineering student learning about dimensionless quantities. This is a practice question that I am trying. The Van der Waals equation of state can be used to predict the behaviour ... 2answers 123 views ### How does the gradient affect units in physics? I intuitively understand the gradient in a mathematical sense, especially the fact that it points in the direction of maximum increase and easily tells us the function's sensitivity to change in each ... 2answers 150 views ### Minimal substitution, four-potential and units When we make the minimal substitution \begin{equation*} p^\mu\rightarrow p^\mu+\frac{e}{c}A^\mu \end{equation*} the four-potential A^\mu must be proportional to 1/e in order to ensure the whole ... 2answers 179 views ### Is there a SI unit for space-time? Space and time are routinely combined into space-time nowadays, which implies that the SI meter and second should be combined into a single SI unit such as [meter-second]. So far, I haven't come ... 4answers 238 views ### Why do physicists multiply the time component of a space-time vector by c to define the conventional four-vector? When I think of a space-time vector, I think of it as a geometrical object in Minkowski space with the physical properties of magnitude and orientation wrt other space-time vectors. I can use four ... 0answers 274 views ### Are Heaviside-Lorentz unit and Gaussian unit natural units? I am studying electromagnetism and have encountered the Heaviside-Lorentz unit and Gaussian unit. In these unit systems, the constant c keeps occurring. For example, the Maxwell equations are of ... 3answers 190 views ### Are there any physical quantities whose units are defined using n-th roots, logs, sins, etc. of SI units? All the SI derived units I know are defined as products of SI units raised to an integer power (e.g. the coulomb is measured in \text{s}\cdot\text{A}, the pascal in \text{kg}\cdot\text{m}^{-1}\cdot\... 2answers 60 views ### Why c needs to be added in numerator and denominator while solving a de Broglie equation word problem? 15.3 Calculate the de Broglie wavleength of a 0.05 eV ("theremal") neutron. Making a nonrelativistic calculation,$$ \lambda=\frac{h}{p}=\frac{h}{\sqrt{2m_0K}}=\color{red}{\frac{hc}{\sqrt{2\left(... 1answer 77 views ### Dimensionality Dilemma: Dimensional Analysis Invalidates my Mathematical Model I am trying to derive an equation that describes the rotational motion of an "auto-unravelling system": systems comprised of a material (string, chain, cloth etc.) wound around a cylinder and left to ... 1answer 302 views ### What's the matter with Planck mass$M_P\$ in Einstein-Hilbert action?
The Einstein-Hilbert (EH) action is often written as $$S_{EH}=\frac{c^4}{16\pi G}\int d^4x \sqrt{-g}R\tag{1}$$ and often as $$S_{EH}=\int d^4x \sqrt{-g}M_P^2 R\tag{2}.$$ Comparing (1) and (2), one ...