Questions tagged [unit-conversion]

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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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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 ...
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2answers
46 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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11 views

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 ...
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1answer
242 views

What are the actual electromagnetic units in the foot-pound-second (FPS) system?

So I've been reading this Wikipedia article, and I came across the cited note that someone has derived the electromagnetic units for the FPS system. However, I can't find that publication so does ...
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1answer
40 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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2answers
163 views

How to Convert Dimension of $R$ constant of Air (J/Kg.K)?

Example) (P=nRT/V) in this equation P=nRT/V, The final dimension is achieved should be a function of pressure, But I can not understand this dimension in final (mol.J/Kg.Liter). when use in R ...
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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 ...
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1answer
237 views

Units in Plasma Physics

What are the units people use in plasma physics? I am having hard time converting an equation from an old paper to SI units. I saw temperature being defined in $eV$. I assume this is $eV/k_B$ with $...
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2answers
55 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, ...
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2answers
1k views

Classification of plane angle as fundamental or derived

I recently started with my high school studies and the chapter I am stuck at is units and measurements. I was told about two types of physical quantities and my teacher gave me the following ...
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2answers
98 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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1answer
164 views

Mass due to the ionization energy of the hydrogen atom from $E = mc^2$

Using $ E = mc^2$ I want to calculate a hypothetical mass from the first ionization energy of the hydrogen atom. The ionization energy is given in units of $\rm kJ/mol$ which could seemingly be ...
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2answers
43 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 $1\:\rm A/1\:V$?
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1answer
93 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 ...
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1answer
113 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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30 views

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 ...
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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 ...
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4answers
231 views

Proper way to write the units to indicate that they include an offset?

Past a certain point of complexity, I get rather confused with physical units, so I am asking a physicist for help. I have a code that represents temperature, with a resolution of [0.5 °C], whose ...
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2answers
161 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 ...
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1answer
78 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 ...
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2answers
184 views

Conversion of compound temperature units

In general one can convert from one unit to another (of the same dimension) by using (($U$,$A$) and ($V$,$B$) being compatible unit pairs in the following) $xU=(x\ast f_{U\rightarrow A}+o_{U\...
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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 ...
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1answer
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 ...
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1answer
79 views

Correct form of Lorentz force equation [duplicate]

When I was reading the first volume of the theoretical minimum by Leonard Susskind, I encountered upon this: But when I checked Griffiths and all my other electricity and magnetism books, it showed $...
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3answers
8k views

How can you calculate (or convert) the Wh of a capacitor whose energy is given in Farads

When trying to compare the energy in a battery to the energy in a capacitor, the units don't match up. How can one compare a battery whose Ah are 10 and Voltage is 3 (for a total of 30 Wh) to a ...
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3answers
2k views

Confusion on electron volt and Kelvin?

If an element A has a binding energy of X eV with element B, can it be said that it is equivalent to X*11600K as 1 eV approx equals 11600K. I do not think this is correct as temp. is linked to the ...
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2answers
92 views

Can we set cosmological constant equal to one?

People often say that the cosmological constant is too small. $\Lambda=10^{-120}$ in Planck units. Can we set $\Lambda=\hbar=c=1$ ? If so what would this give for $G$, the gravitational constant in ...
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1answer
111 views

Is there a general algorithm for conversion of units?

I'm not exactly sure where the best place to put this, as it's more of a general question about dimensional analysis. I decided I was tired of having to convert units all of the time, and was not ...
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3answers
964 views

If a propane torch is rated at 200,000 BTUs, what does this translate to in terms of joules/second?

If a propane torch is rated at 200,000 BTUs, what does this translate to in terms of joules/second? Not exactly a physics question per say, but it falls in the realm. How do I convert/understand ...
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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 ...
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2answers
86 views

Conversion from Planck unit to SI [closed]

Good evening, I'm reading the paper Prehawking radiation by William G. Unruh where it says: ...a time scale of order of $m^{3}$ in Planck units, or $10^{53}$ ages of the current universe for a ...
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3answers
66 views

What are the units of time when planck's constant is equal to 1?

If I express a Hamiltonian $H$ in units of Hz by dividing the energy terms in the Hamiltonian by hbar $\tilde{H}=\dfrac{H}{\hbar}$ which means you set $\hbar =1$. Then what are the units of time? Also ...
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2answers
282 views

Proper units for physical quantities when $\hbar$=$1$

How to deal with the units of quantities if $\hbar=\tfrac{h}{2\pi}=1$? For example, the energy $E=\hbar\omega$: If I have chosen $\hbar=1$, how do I use the units to properly differentiate between ...
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1answer
64 views

Conversion between mass units

I am enrolled in a course related to physics, and converting between mass units is so tricky for me. I always treat in my work only in SI units and maybe MKS units, at level of weights, I find it easy....
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1answer
35 views

Issue in conversions from force to mass units

I have a problem converting force to mass units, in this example for 9.81kgf: To get the mass, dividing by gravity, I get 1kg ...
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1answer
35 views

How do I convert $W/(mK)$, $W/m^2$ and $W/(m^2K)$ to the same “dimensionality” and unit?

How do I convert $W/(mK)$, $W/m^2$ and $W/(m^2K)$ to the same "dimensionality" and unit? $W/mk$ is thermal conductivity. $W/m^2$ is heat flow density related to one unit. $W/m^2k$ is heat transfer ...
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1answer
87 views

I'm confused about electric flux?

In a previous physics class, I learned that the electric flux was $\vec{E}\cdot\vec{A}$ (dot product), and hence the unit is $Nm^2/C$. But in my electromagnetics book, it says the unit is Coulomb, and ...
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1answer
192 views

Intuition about the congruence of readings of Celsius and Fahrenheit scales at -40°C [closed]

At -40°C temperature, the Fahrenheit scale also shows -40°F. But how is this ever possible? They are two different scales! What's the intuition behind this?
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63 views

Geometrized units in Wald

I am trying to follow a derivation in this paper from Wald Specifically, at the end of the paper, just under Eq. 4.7 there is given the equation $$ Q/m \leq 2 B_0 m$$ where $Q$ is the system ...
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1answer
2k views

How to convert cm$^{-1}$ to eV?

I know that $1 \:\mathrm{cm}^{-1} = 8065.54429 \:\mathrm{eV}$. I know that wave-number has unit $\:\mathrm{cm}^{-1}$ and that $E=hc\times \mathrm{wavenumber}$. But how do I arrive to the number $...
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1answer
37 views

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 ...
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1answer
181 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 ...
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3answers
378 views

Magnetic $H$ field unit in cgs system

I'm studying an old E&M textbook which uses cgs unit system. I'm re-writing the formulas in SI unit. The book says $\vec{H}=\vec{B}-4\pi\vec{M}$. So I guessed that $H$ should have same unit as $...
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4answers
82 views

Converting part of a compound unit

I am working with several different datasets with different units. I want to convert the distances to mm for comparison. The units I'm dealing with are $\mathrm{kg/m^2/s}$, $\mathrm{mm/day}$, and $\...
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1answer
81 views

Need help in calculating the pull on string in kgs when moving weight on horizontal bar

need your help. I realized today, if you do not practice it becomes hard to understand simple concepts even you were once a science student :( I have attached a picture to help make the problem ...
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1answer
352 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 ...
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0answers
50 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 ...
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1answer
39 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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1answer
134 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?