Questions tagged [unit-conversion]

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14
votes
3answers
29k views

Physical dimensions for the Coulomb force in SI and CGS

The Coulomb force in SI units reads $\displaystyle F = \frac{1}{4\pi\varepsilon }\frac{Q_1\times Q_2}{R^{2}} $. On the other hand, in CGS this reads $\displaystyle F = \frac{Q_1\times Q_2}{R^{2}} $. ...
12
votes
2answers
5k views

Higgs Boson mass in Electron volts?

Im no physics genius here, I was just interested in the Higgs Boson so I was reading this article : How the Discovery of the Higgs Boson Could Break Physics I came across this Furthermore, all ...
9
votes
3answers
8k views

Coulomb's Law: why is $k = \dfrac{1}{4\pi\epsilon_0}$ [duplicate]

This was supposed to be a long question but something went wrong and everything I typed was lost. Here goes. Why is $k = \dfrac{1}{4\pi\epsilon_0}$ in Coulomb's law? Is this an experimental fact? ...
8
votes
2answers
2k views

Where does the joules come from (ideal gas constant)?

I have not done any physics in ages and have recently started studying it. The first chapter in my book deals with the ideal gas constant: $$pV=nRT$$ It is rewritten as: $$R=\frac{pV}{nT}$$ When I ...
7
votes
2answers
395 views

How to recover units?

Theorists frequently use convenient units like $\hbar=1$ or $m=2$ or whatever is useful to simplify the notation in the problem. And after all the calculations are done the units are recovered based ...
7
votes
2answers
3k views

Converting between brilliance, intensity, and flux

This one should be a bit of a softball, but I can't find it explicitly stated anywhere on the internet, and my basic unit analysis doesn't seem to work. Suppose you have a beam of synchrotron ...
7
votes
1answer
423 views

Magnetic susceptibility in 1/eV

In this paper the authors refer to transverse susceptibility $\chi_{ \perp}$ [meV $^{−1}$] I was taught that the magnetic susceptibility is dimensionless. How do I get $\chi$ in the above units??
6
votes
2answers
701 views

CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
6
votes
2answers
11k views

Relationship between measures of light (candela and lux)

I am trying to understand the relationship between two measures of light: lux and candela. It is probably easy for those with longer experience, but here it goes: I have learnt (here for instance) ...
6
votes
1answer
34k views

How is formula for converting pressure from mmHg to Pa derived?

Today my younger brother asked me from where does the 1 Pa = 0.00750061683 mmHg formula for mercury barometer come. He needs a way to derive it, or an academic source which can be cited. After ...
5
votes
4answers
839 views

Why do (kilo)watt-hours double-count units per time?

A watt is defined as 1 joule per second and is therefore a measurement of energy transfer over a period of time. A kilowatt-hour is a measurement of energy equal to one kilowatt sustained over a ...
5
votes
1answer
52k views

How much is 1 electron-volt (eV)?

I am interested in knowing how much is one eV of energy. Everywhere I found are the technical definitions. Can anybody please tell me how much is this much energy. I need something which I can feel. I ...
5
votes
3answers
807 views

What is a “measure equation” as mentioned by this TeX Users Group guide?

In this TeX Users Group (TUG) document, Typesetting mathematics for science and technology according to ISO 31/XI by Claudio Beccari, the author makes various typesetting recommendations including: ...
5
votes
3answers
11k 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 ...
4
votes
3answers
25k views

What is the relationship between the magnetic units oersted and tesla?

How are the units oersted and tesla related? For example, how would you express $20\:\mathrm{Oe}$ in tesla?
4
votes
3answers
7k views

What does 1714 mean in hydraulics?

After reading this answer: http://www.answers.com/Q/What_does_1714_mean_in_hydraulics, I still do not have a good idea of what 1714 represents. In fact, in the equations I was working with I saw a ...
4
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2answers
2k views

Fundamental units

Is it right that all units in physics can be defined in terms of only mass, length and time? Why is it so? Is there some principle that explains it or is it just observational fact?
4
votes
4answers
865 views

How to express an energy in natural units

If I have some angular frequency e.g. $\omega [\text{rad } \text{s}^{-1}]$, I can easily express this as an energy as $E = \hbar \omega [\text{ J}]$. Now suppose I am working in Natural (Planck) ...
4
votes
3answers
572 views

What am I doing wrong when I try to make this unit conversion?

I am trying to reconcile data that I have found in one publication (Allen 1969) with data that I found in another publication (George 2003) that synthesized this data. The data is root respiration ...
4
votes
4answers
3k views

How accurate are constants in cgs units?

I am just curious that if constants in cgs units change the answer of an equation. For example, Coulomb's constant, in SI units it equals to $8.98...\times 10^9 \,\mathrm{N\,m^2\,C^{-2}}$. However in ...
4
votes
3answers
1k views

What is the missing proportionality constant in the magnetic levitation formula?

The formula for magnetic levitation is $$ B \frac{dB}{dz} = \frac{ \rho g }{\chi} $$ but as always, I have a hard time figuring the units in SI. The left hand side is $\mathrm{T^2 /m}$, while $\chi$ ...
4
votes
1answer
983 views

Avogadro's number

Could I get an explanation of Avogadro's number and how it relates to determining the mass of a substance? My chemistry textbook only serves to confuse me and the Wikipedia article is aimed towards a ...
4
votes
1answer
536 views

Dangerous dose of I-131?

I was searching a lot and could only find dosages for curing cancer and allowed emission, but no Iodine-131 dose that could be connected with increased thyroid cancer risk (like, 10mSv is the ...
4
votes
1answer
1k views

How to convert Richter magnitude scale to approximate TNT?

I know the Richter magnitude scale is often used for measuring the strength of earthquakes. At the same time, explosive/destructive releases of energy are often quoted in equivalent amounts of TNT. Is ...
3
votes
2answers
600 views

Newton's Law of Graviation: Why $G$ and not e.g. $\dfrac{1}{4\pi G_0}$?

I've been wondering, in Coulomb's Law, $k_e = \dfrac{1}{4\pi\epsilon_0}$. Therefore, why do we use $G$ in Newton's Law of Gravitation? What if the constant is more like Coulomb's Law, e.g. $G = \dfrac{...
3
votes
1answer
926 views

Why does the vacuum permeability have the value of $\pi$ in it?

The vacuum permeability, or the capability of the vacuum to permit magnetic field lines, contains the value of $\pi$. Why? What does this have to do with the ratio of a circle's circumference to its ...
3
votes
1answer
146 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 ...
3
votes
1answer
133 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}$...
3
votes
1answer
589 views

Does the expression of the orbital magnetic dipole moment have $c$?

The orbital magnetic dipole moment of a particle with mass $m$ and charge $q$ can be shown to be related to the orbital angular momentum through the equation $$\displaystyle \boldsymbol\mu_L=\frac{q}{...
3
votes
1answer
535 views

Help on unit conversion problem

This is a problem from school. I will show my attempt. The question: "The gas constant for dry air R is 287 $\frac{m^2}{s^2*K}$. Assuming the temperature is 330 K and the pressure is 1050 hPa, what ...
3
votes
1answer
2k views

CGS Units to SI Units

This may be a stupid question but I am having trouble getting the same result in CGS units as I would if I used SI units for a unitless calculation. I have to calculate $E_\mathrm c=\frac{m_\mathrm ...
2
votes
7answers
42k views

Celsius to Fahrenheit confusion: why there is no 1 to x ratio?

Why is it not possible to know the ratio of 1°C to how many Fahrenheit and use that to convert from or to Celsius/Fahrenheit? What is really happening? Fahrenheit increases linearly and so does ...
2
votes
2answers
832 views

Checking units for equation with degree symbol

Using the following equation: $$ U = \left(\frac{B \times L \times \sin(\theta)}{C}\right)^{1/3} $$ I can calcukate the velocity of a flow traveling down a slope. I would like to check that the ...
2
votes
3answers
2k views

Unit conversion help (school) [closed]

Can someone please help me explaining how to convert units in problems like the one below? I've done many things, but I am very unsure of all my methods. Could you tell me what steps should I take? $$...
2
votes
2answers
6k views

Approximation to the number of seconds in a year?

Is there any mathematical formula which shows that there are approximately $\pi \times 10^7$ seconds in one year. I understand that the pi is probably due to the earth's circular orbit, but am not ...
2
votes
3answers
745 views

Astronomical Constant in Astronomical units?

I'm doing a computer simulation of the solar system and I'm having trouble working with big numbers (implementation specific problem). So what would be the Newtonian gravitational constant $G$ in ...
2
votes
2answers
98 views

Convert $\text{mm}^2$ to $\text{m}^2$

Stuck on a very very basic concept. I have $1750\text{ mm}^2$ and need to get into $\text{m}^2$. I figured $1750\text{ mm}^2 = (1750\text{ mm}) \cdot (1750\text{ mm})$, but I know this isnt right.
2
votes
1answer
207 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 ...
2
votes
2answers
105 views

Why does physical quantity misinterpretation result in near-identical torque results?

I am going to use an equation $$\text{torque} = \frac{\text{power}\times 5252}{\text{RPM}}$$ derived on Wikipedia. Suppose that $\text{power} = 100\ \mathrm{hp}$ and $\text{RPM} = 5252\ \mathrm{rpm}$ ...
2
votes
6answers
4k views

Confused about unit of kilowatt hours

So I am a little confused on how to deal with the Kilowatt hours unit of power, I have only ever used Kilowatts and I have to design a residential fuel cell used as a backup generator for one day. ...
2
votes
2answers
1k 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 $...
2
votes
4answers
23k views

Temperature in CGS (Gaussian) units

I've been struggling with conversion from Gaussian to SI units for sometime, trying to figure out how derived units in CGS (current, charge etc) relate to the SI units. But I couldn't find any ...
2
votes
1answer
10k views

How to convert Bq/g to Sievert (Dose), and how to get values Sv/Bq?

How to convert Bq/g (Activity) to Sievert/g(Dose)? How can I get values in (Sv/Bq)? Please help, it is really confusing.
2
votes
2answers
2k views

How do we convert between the maximum value of the adhesive force (in kilograms or in Newton) of a magnet and its magnetic field strength (in tesla)?

On the following website they indicate how powerful is a magnet by giving the theoretical maximum value of its adhesive force in kilograms or in Newton (its adhesive force at a distance of zero). ...
2
votes
1answer
473 views

What is the de Sitter temperature as of now?

It is claimed that de Sitter temperature is $$T=\frac{1}{2\pi}H,$$ where $H$ is the Hubble constant. I presume it is expressed in natural units with which I am not familiar. So what it will be in ...
2
votes
1answer
99 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 ...
2
votes
1answer
456 views

Confusing about the unit of Fermi velocity

The unit 'eVA(angstrom)' is used in some ARPES papers for the Fermi velocity. How can I convert it to the conventional unit ‘m/s’?
2
votes
1answer
626 views

Clarification on factors of $c$ in the Lorentz' force

I was told that Lorentz' force is given by $${\bf F}= q{\bf v} \times {\bf B}.$$ But I have read that it is given by $${\bf F}= \frac{q}{c}{\bf v} \times {\bf B}.$$ Why have these two relations ...
2
votes
2answers
229 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\...
2
votes
4answers
3k views

How to express a mechanical force field in the units of electric fields?

Electrostatic fields have a unit $\frac{V}{m}$ (volts per meter) and the mechanical force fields (including gravitational field) have a unit $\frac{m}{s^2}$. If we exclude gravitational force from ...

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