# Questions tagged [unit-conversion]

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### How to convert $\rm kg/m^3$ to $\rm J/m^3$ for natural gas and biogas?

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 ...
0answers
8 views

### 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 ...
0answers
18 views

### 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 ...
2answers
47 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?
0answers
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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 ...
1answer
41 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
1answer
64 views

### 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 ...
2answers
59 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, ...
2answers
99 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 ...
2answers
44 views

### 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 $1\:\rm A/1\:V$?
1answer
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### 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 ...
1answer
114 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 ...
2answers
74 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 ...
2answers
177 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 ...
1answer
81 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 ...
1answer
55 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 ...
1answer
79 views

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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 ...
3answers
379 views

1answer
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### 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 ...
1answer
353 views

### How can I convert spectral irradiance to spectral radiance [closed]

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 ...
0answers
51 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 ...
1answer
182 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 ...
1answer
135 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?
1answer
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### 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 ...
0answers
29 views

### 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 ...
1answer
31 views

### 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 ...
0answers
240 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 ...
1answer
522 views

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### Converting volume to dimensions

Not sure if this is better posted in mathematics, but I'd like to know the formula used to convert cubic meters to dimensions (eg: 1m height x 1m width x 1m length). I understand that you can turn ...
1answer
45 views

### $k_e$ (constant of Coulomb force) is sometimes seen in calculations for atomic radii and sometimes not

One of our formula for radius of atom was $\frac{n^2h^2}{4\pi^2me^2}$. And another one was, $\frac{n^2h^2}{4\pi^2me^2K}$. For hydrogen, Why $k_e$ is sometimes included and sometimes excluded ?
1answer
113 views

### Finding a substitution of $\mu_0$ to convert from SI to c.g.s in EM laws [closed]

In c.g.s, we set $$k=\frac 1{4\pi\epsilon_0}=1$$ which gives $$\epsilon_0=\frac 1{4\pi}$$ This conversion works, for example, in Gauss' law: in SI \vec{\nabla}\cdot\vec{E}=\frac\rho{\epsilon_0}\$...
3answers
1k views

### Why is 1 newton meter=.225 lbs force?

This is a general question regarding force (mass x acceleration). I understand 1N =1kg mass x 1 m per second sq) all this basically tells me is that the force is proportionally increased and ...