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# Questions tagged [unit-conversion]

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11 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; ...
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### 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, \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
44 views

### MW@kV conversion

First, I'm sure this question is way too easy for this forum but I didn't find a better one. I'm having an argument on the following and we'd like to settle it officially. On a website about Diesel-...
106 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 ...
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### 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 ...
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### Express the total mass of the galaxy in solar masses

I have the following problem: "The Sun has an orbital speed of about 220 km s−1 around the center of the Galaxy, whose distance is 28 000 light years. Estimate the total mass of the Galaxy in ...
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### How to convert $\rm kg/m^3$ to $\rm J/m^3$ for natural gas and biogas? [closed]

I want to convert from mass density to energy density: Natural gas has a density of $0.8\: \rm kg/m^3$. Biogas has $1.15\:\rm kg/m^3$. I want to convert them into $\rm J/m^3$ so that I can ...
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### Spectral irradiance unit conversion (Jansky)

I'm a bit confused between two units for spectral irradiance, specifically $W/sr/m^2/nm$ and $\mu Jy/"^2$, which is analogous to $W/sr/m^2/Hz$. It seems to me that I need some sort of spectral ...
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### Equivalences for mCi doses and cGy?

I was searching a lot and could only find dosages for curing cancer and allowed emission, but no Iridium 192 dose that could be given internally through brachy sources been found, lets say a 700 cGy ...
60 views

### When using Planck's constant, how do I know when to use electron volts or joules?

Planck's constant may be written as 6.63 x 10^-34 Js or 4.14 x 10^-15 eVs right? But how do I choose which one I should use to figure out an answer to a question?
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### For vibrations in continuous beam, what is the unit of eigenvalue?

I have been solving a fourth order euler bernoulli differential equation to solve for vibrations of a continuous cantilever beam. When I verified for them using Comsol eigenvalue solver, it gives me ...
51 views

### Units conversion - quanta/cm$^2$/s and photons/cm$^2$/s

Can you tell me if there is any difference between these 2 units? $\log \mathrm{quanta/cm^2/s}$ vs $\log \mathrm{photons/cm^2/s}$ Thanks
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### Order of magnitude radiation measures conversion?

Standard unit of ionizing radiation dose is Sievert. I can get a rough idea of how dangerous absorbing various amounts is from https://xkcd.com/radiation/ and other sources. There are many other ...
63 views

### Unit conversion between dynes and newtons

I am learning Electricity and Magnetism by myself. Can someone explain me the math behind the equation below? (Quoted from Edward Purcell's Electricity and Magnetism (Cambridge University Press, ...
117 views

### How many calories of energy does it take to heat up 1 cubic inch of water 1 degree Fahrenheit? [closed]

In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade--which is 1 percent of the difference between ...
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### Why have a conversion factor of $10 \:\rm mA / 10 \: mV$ instead of $1\:\rm A/1\:V$?

I’m working with a device that measures current by measuring voltage and then giving current with a conversion factor of $10 \:\rm mA / 10 \:mV$. Why write this instead of $1\:\rm A/1\:V$?
95 views

### Is 1 liter always equal to 1 cubic decimeter, independently of temperature, pressure, etc?

I recently found this conversion table for the unit conversion $\rm mmol/m^3 \ \leftrightarrow\ \rm mmol/L$ (millimoles per cubic meter to millimoles per liter) My physics is very rusty, but just to ...
150 views

### How to convert MeV to velocity? [closed]

For example, in the fission process: $${}^{235}\mathrm{U} + n \rightarrow {}^{140}\mathrm{Xe} + {}^{94}\mathrm{Sr} + 2n$$ the masses in atomic mass units are: \begin{align} M\left({}^{235}\mathrm{...
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### Grasping systems of units

Perhaps it's that I've grown up with the SI system of units, but there's something about other unit systems that doesn't sit well with me. For example, Coulomb's law in Gaussian units can be written ...
77 views

### Super confused over simple unit conversion from centimeters to cubic centimeters

I'm stuck in a really stupid question. It seems like the meaning of "centi" is chancing meaning. Because if: $r = 5 cm = 5 \cdot 10^{-2} m$ so $c = 10^{-2}$ But say i need to take the cubic root ...
254 views

### Dimensions of mass in terms of Length and Time

From Maxwell's Treaties on Electricity and Magnetism: For acceleration due to attraction of a mass m at a distance r is by the Newtonian Law m/(r^2). Suppose this attraction to act fro a very small ...
142 views

### A question about natural/geometrized units

I had a question about the following document- Natural units I understand the conversion factors. But if you look at the tables, they take an SI unit, say 1 kg, convert it into geometrized units, say ...
68 views

### Why multiply by $\frac{\pi}{4}$ when converting a light source's angular diameter to its solid angle?

Why does the formula $$\Omega =( \text{angular diameter} * \frac{\pi}{180^{\circ}})^2 * \frac{\pi}{4}$$ give a more accurate calculation of the solid angle of a light source from its angular diameter ...
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### Pressure unit computation (feet of mercury, but applies also to foot of water..) [closed]

Grr this question makes me crazy. Here you can see that ftHg = 13595.1 kg/m3 × 1 ft × ɡ0 = ≈ 4.063666×104 Pa. (= 40636.664 Pa) https://en.wikipedia.org/wiki/Conversion_of_units (Pressure or ...
452 views

3k views

### How to convert bars to meters head?

I am trying to convert 1 bar to meters to come up with a conversion factor. Thus, 1 bar = x meters head What is x? From my computations I got something like ...
389 views

I have information on the spectral irradiance incoming from the sun at the top of the atmosphere in units of $\rm mW\ m^{-2}\ nm^{-1}$. The photons hit a hypothetical surface on the earth with 0.3 ...
54 views

### Finding the generated magnetic field of a Hall generator

I've been using a Hall generator to generate a particular magnetic field by applying a current. I need to work out what field this corresponds to in Tesla. All I have to go on is what's in the photo ...
185 views

### Why does $\frac{1}{4\pi}$ appear in many formulas? [duplicate]

Why do most of the physical equations have $\frac{1}{4\pi}$ as constants? I have seen that many equations have $\frac{1}{4\pi}$ as constants like Coulomb, pendulum problems, etc. Can anyone tell me ...
160 views

### 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?
40 views

### Converting units of $G$ [closed]

I want to convert $G$ from $\rm m^3 \:kg^{-1} \:s^{-2}$ to the units $\rm km^3 \: kg^{-1} min^{-2}$. How can this be done? I want to know what $6.67\times 10^{-11} \:\rm m^3 \:kg^{-1} \:s^{-2}$ is ...
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### Dimension scaling of a model hamiltonian system?

I have a model 2dof double well potential Hamiltonian given as $$H = P1*P1/2 + P2*P2/2 + V(Q1,Q2) ,$$ where $$V(Q1,Q2) = -2*Q1^2 + Q1^4 - Q2^2 +Q2^4 +0.4*Q1*Q1*Q2*Q2 ,$$ there is no units are ...
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### Thermal Conductivity Conversion

I need to convert a thermal conduction unit W/m.K to Btu/ft.h.°F. I have 400 W/m.K and I ...
265 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 ...
I'm having trouble solving this Physics question with "made up" units. Here is the problem and my work so far. The Omicronians have told you that they use the units: \\{OP=7.29\frac{flurg}{grom^2}}...