# Questions tagged [unit-conversion]

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### 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 $\varepsilon$ ...
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? ...
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?
3answers
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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) ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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....
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, ...
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 ...
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 ...