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

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What defines a unit system and how many constants can we set to 1?

Is it possible to have a system of units in which both $\frac{1}{4\pi\epsilon_0}$ and $c$ equal 1?
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What happens to the units when squaring a variable?

What happens to the units of a squared variable? For example, if I squared velocity, would the units, metres per second (${\rm m}/{\rm s}$), change as well?
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Units of angular power spectrum

I am not sure whether this question might be better suited for crossvalidated or stackoverflow, but I will give it a try here: I have a map of the full sky in in the healpix format, units are $\rm ...
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36 views

Definition of Sievert (Sv) unit - is it whole body mass?

I'm wondering about the definition of the Sievert (Sv) unit. It is J/Kg but is that Kg the mass of the whole body or just of the exposed body part? For instance, when a table says that an x-ray of a ...
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Natural units conversion

Now, in this question, in the accepted answer, to convert from Kelvin to cm^-1, he directly multiplies by the SI equivalents of $k/\hbar c$ (although in the link, he converts cm^-1 to K). However, ...
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conversion of units

I have a quantity that I need to convert to different units. First, I need to note that the value is in the natural units system. The quantity is in cm^-1 (or, to be accurate, $cm^{-1}\hbar c /k$, ...
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What unit system puts a bunch of powers of ten in Maxwell's equations?

In what unit system can we get the Maxwell equation in the following form (section 1.2 from Lewin's book Advanced Theory of Waveguide) $$\nabla\times \vec E = -10^8\mu \frac{\partial \vec H} ...
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When can two quantities be added together?

Whenever two things are to be added together, one typically needs to check whether this actually makes sense, and an addition is said to make sense, in principle, when the units match up. Yet, ...
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Terminology: Gauge stress?

When a material is loaded with a force, the stress at some location in the material is defined as the applied force per unit of cross-sectional area. If I have a material submerged in pressurized ...
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73 views

What is the difference between unit and dimensions?

If I say Height of a block = 2m, then would "Height" be called as a dimension
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Question on units for integrating F vs x graph

I had a homework problem with this graph of F(Newtons) vs x(meters). The graph has a straight line (constant slope) from (0,0) to (8,10). And the slope ((10-0)/(8-0)) came out to be 1.25, making the ...
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36 views

When to use Kelvin over Rankine and vice versa

Kelvin and Rankine are both used by scientists over Celsius and Fahrenheit to record heat, but are they both used interchangeably, dependant on the country or preference of an individual, or are there ...
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37 views

What is the unit of information as defined by information theory?

It appears to be defined by probability; however, does it have some unit that indicates its 'information level' in terms of probability?
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51 views

Converting Langley to Watts

I'm reading a really old paper which looks at estimating the surface energy fluxes for The Great Lakes. They show all of their values as Langleys per day. Can this be converted to Watts per meter ...
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56 views

Can you explain this “Theory of Everything”? [duplicate]

I recently come across with an amazing equation of Theory of Everything; I wonder if TOE has been formulated (???) I found this equation on a website, check it out ...
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3answers
150 views

Messed up units!

In the article, Environment-assisted quantum transport, $\gamma$ is a constant equal to $2\pi kT/\hbar*E_{R}/(\hbar\omega_{c})$ where $T$ is the temperature, $k$ is the Boltzmann constant. ...
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45 views

Why is intermolecular potential energy given in 'per mole'?

I was just wondering that if intermolecular potential is the interaction potential energy of two atoms/molecules, then why do we give its value in J/mole? I don't understand why 'per mole' ...
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Units of vector differential operator del ($\nabla$)

My book says that $\left[\nabla \cdot (\vec E \times \vec H)\right] = \mathrm{W/m^3}$. I see that $\vec E$ is in $\mathrm{V/m}$ and $\vec H$ is $\mathrm{A/m}$, so these multiplied is $\mathrm{W/m^2}$, ...
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Which units should I use in molecular dynamics simulation?

I have written a simple molecular dynamics simulation program. The simulation runs fine but the physical properties (in particular, I have calculated temperature) are off by many scales. I understand ...
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36 views

Applying operators to the wave function, getting the physical units

Reading the wikipedia entry about operators, in particular the table at the end listing all operators, I have several questions regarding an $N$-particle system or statements that I wonder whether ...
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2answers
86 views

What is $c$ in the Lorentz force expression?

The usual Lorentz force expression I am familiar with is this: $$\vec F=q(\vec E+\vec v \times \vec B)$$ I have seen some other versions lately that include an extra factor $1/c$: $$\vec ...
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1answer
73 views

What is the significance of Planck charge?

It seems for me that Planck units are somehow connected to limits where our current knowledge breaks down because of (quantum) gravitational effects. Please correct me if I'm wrong. For example ...
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1answer
129 views

What is the reason we originally and still use the non-SI unit, the Jansky?

The Jansky is the unit for spectral flux density. It is defined as $$1 {\rm \ Jy} = 10^{-26} {\rm W \ m^{-2} \ Hz^{-1}}$$ in terms of Watts per square meter per Hertz. I've never quite understood ...
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How much is if my internet provider says “we have increased your WiFi connection by 2 decibels”?

I understand the definition of decibel as a logarithmic unit for intensity of anything. I just cannot imagine its "actual size" when comes to the wireless connection. Can you provide something ...
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Do bras and kets have dimensions?

I'm trying to understand more intuitively what bras and kets are, but some aspects of them remain a mystery to me. We usually think of $\psi (x)$ as having dimension of $[1/\sqrt{L}]$ so that ...
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1answer
24 views

Can I describe resistance without using an Ohm?

Can resistance be described with units $\rm cm^2/sec$? If so, how would this (resistance) relate to permeability?
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Experimentally finding units of physical quantity?

Say you had a new physical quantity you wanted to determine the units for. How do you go about this? For the strength of an electromagnet for example, you could carry out a simple experiment like the ...
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59 views

Natural unit conversion

I'm a bit confused about different notions of "natural units" that I encounter occasionally. I'm familiar with Planck units, and in particular I can understand the conversion between, say, metres and ...
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50 views

What are the units of the creation and annihilation operators?

The creation and annihilation operators - also known as ladder operators are; $ \hat{a}^\dagger$ and $\hat{a}$ respectively. Using the equation $\hat{H} = \hbarω\left(\hat{a}^\dagger \hat{a} + ...
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28 views

Stroboscope's unit

Can someone please tell me why does stroboscope's unit is rps or rpm. Revolution per minute/second. Although the stroboscope used was an electronic stroboscope. Is it has something to do with the ...
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226 views

How to derive $E=mc^{2}$?

Is there some way to derive $E=mc^{2}$? I can understand that energy in something is proportional to its mass, but the $c^{2}$ part. I have no idea. It seems like the way the units are going it would ...
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Units of eigenvectors

Consider for example a mass matrix $M$, $\lambda$ one eigenvalue and $X$ a corresponding eigenvector. Then $[M]=\text{mass}$ (the brackets indicate the "unit operator"), and $MX=\lambda X$ so ...
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730 views

What is the fundamental units for coulomb?

Everything I saw suggests coulomb has no fundamental units. So then how is Newton/Coulomb equivalent to Volt/meter?
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Recalculating lux from EV

I have to calculate amount of light from IR emmiter. The only equipement, that I can use is Sekonic Light Meter like this one: http://www.ebay.com/bhp/sekonic-l-358 The problem is that i need to have ...
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2answers
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What unit system does Fahrenheit belong to?

Wikipedia's page for Imperial Units does not list Fahrenheit. The corresponding page for SI Units lists Kelvin as an SI unit, and Celcius as a derived SI unit. This leads me to believe that ...
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88 views

Sommerfeld Parameter Confusion

In almost every reference site I can find, the Sommerfeld parameter $\eta$ is explicitly stated as $$\eta=\frac{Z_1Z_2e^2}{\hbar \nu}=\frac{Z_1Z_2e^2}{\hbar }\sqrt{\frac{\mu}{2E}}$$ Where $Z_1$ and ...
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Are there any scales other than temperature that have different zero points? [closed]

For most physical measurements, zero is the same regardless of the units used for the measure: $0 \mathrm{mi} = 0 \mathrm{km}$ $0 \mathrm{s} = 0 \mathrm{hr}$ but for absolute temperatures, ...
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107 views

Do we actually measure distances in light years?

The cosmic distance ladder has a wide range of length scales, which are quite difficult to measure and to conceptualize. These distances are commonly quoted, particularly in less technical articles, ...
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1answer
87 views

Difference between kilogram-force and kilogram

I know that a 1 kilogram-force is the force of 1 kilogram acted upon by 1 standard unit of gravity (9.80665 m/s^2). However, in torque descriptions I find that some use $kgf*cm$ while others use ...
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1answer
45 views

Calculating charge density

When calculating uniform charge density, I yielded a solution with units: $$VC^2 / Nm^2$$ The answer is telling me that this is equivalent to: $$C/m^2$$ where: $$m : meter$$ $$C : Coulomb$$ $$V : ...
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Converting $F^2$ to $C^2$

I'm trying to convert Fahrenheit squared $F^2$ into Celsius squared $C^2$. I know how to convert a value $x$ in $F$ into $C$ with: $\frac{5}{9}(x - 32)$ I also know how to convert a value $x$ in ...
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1answer
68 views

What does 400kW(e) mean?

I am trying to read a document and it says a power grid is 400kW(e) what does this mean?
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1answer
44 views

Torque wrench units

My torque wrench has these markings on it: (da. Nm.) and on the line below M. KGS. I can tell from comparing the scale to poundf-foot on the other side that the scale units are kilograms-meter ...
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Unit consistency

Does $^\circ\mathrm{C}^2$ have any physical consistency? I know that $\mathrm{K}^2$ is a valid unit even though I don't know in which context it can be used. But I feel strange about ...
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Using the boltzmann equation to find neutrino decoupling temperature

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...
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2answers
61 views

Usage of singular or plural SI base units when written in both symbol as well as name [closed]

I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name. I have framed this question under two parts, namely, Part (a) and Part ...
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1answer
32 views

Unit definition concerning light and metre [closed]

A stupid question. I see metre is officially defined based on the speed of light: The meter is the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of ...
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Black Hole Entropy Calculation

I was watching "Leonard Susskind on The World As Hologram" ( youtube ). At one point he describes the way Bekenstein calculates the entropy of a black hole. Paraphrasing: Take a minimally sized black ...
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Confusion in understanding wave number

The wave number is the number of complete wave cycles in a meter. So, $$K = \frac{1m}{\lambda}$$ and also, $$K = \frac{2\pi}{\lambda}$$ so according to both of the above equation how is $$2\pi ...
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288 views

100°C = 100 K =?

I'm in first year. Our class is in lesson " Heat and Thermodynamics". While solving a numerical problem of a reversible engine he told us that 100 degree Celsius is equal to 100 kelvin. I inquired but ...