Questions tagged [unit-conversion]

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12
votes
3answers
24k views

Coulomb force in SI and cgs

Coulomb force in SI is $ F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}} $ while in CGS $ F = \frac{Q1*Q2}{R^{2}} $ why is it? I mean doesn't it any make difference in dimension? since $ \varepsilon $ ...
8
votes
3answers
7k 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? ...
4
votes
1answer
515 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?
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? $$...
3
votes
2answers
542 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
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?
1
vote
1answer
1k views

Photometer: measured Irradiance L converted to photon rate

I am conducting an experiment in which the power meter reading of $410\,nm$ narrow bandpass stimulus is noted to be 30 $\frac{\mu W}{cm^2}$ at a distance of 1 inch away from the light source. I wish ...
0
votes
2answers
226 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 F=q\left(\...
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) ...
2
votes
3answers
19k 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?
1
vote
3answers
3k 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 ...
3
votes
1answer
679 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 ...
2
votes
3answers
569 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 ...
1
vote
1answer
490 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 ...
1
vote
2answers
589 views

Bohr's model and positronium

I was trying to get an order of magnitude estimate for the radii and energies of positronium using the Bohr's model. I did find a few places where they have used the reduced mass to replace the ...
0
votes
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....
0
votes
1answer
165 views

Is this a correct addition to a pipe friction loss calculation?

I've been using this online calculator to work out the difference in pressure and flow between two ends of a pipe: Pipe Friction Calculation for Fluid Flow in a Pipe So far most of it makes sense, ...
0
votes
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 ...
-3
votes
1answer
620 views

How does one convert spectral radiance from $\mathrm{(W\: cm^{-2} \: sr^{-1} )/ nm}$ to $\mathrm{ (W \:cm^{-2} \: sr^{-1} )/ cm^{-1}}$? [closed]

How does one convert spectral radiance from $\mathrm{(W\: cm^{-2} \: sr^{-1} )/ nm}$ to $\mathrm{ (W \:cm^{-2} \: sr^{-1} )/ cm^{-1}}$? The first one is the radiance represented by wavelength and ...