# Coulomb's Law and Gauss Law

Why use k = 1/[4(pi)(epislon)] and epsilon = 8.8 * 10^(-12) and subsequently, k = 9 * 10^9

It could simply be k = 9 * 10^9, and different k for different medium instead of different permittivity for different medium.

What I mean is why does Pi, (I can handle the 4) comes into the equation, one reason I can think of is that it comes if you use Gauss Law to derive Coulomb's law but I am looking for something more convincing.

The Permittivity of free space is a constant experimentally derived. Why would one derive that in the first place? If the force between two charges was directly proportional to some constant K. One would experimentally see that it is 9 * 10^9. Where was the idea of breaking it down to 1/[4(pi)(epislon)] entertained?

Many problems in this area of physics will have a 4pi in the numerator. So a 4pi in the denomenator will simplify things. Perhaps that is the reason.

Many problems in this area of physics will have a 4pi in the numerator. So a 4pi in the denomenator will simplify things. Perhaps that is the reason.

That is the reason I thought of seconds after I clicked the Submit button, it does seem a likely reason but I will wait and see if someone knows something better.

Simon Bridge
Homework Helper
Your epsilon is usually $\epsilon_o$ - the permitivity of free space. We can modify the basic equation for different media by including the relative permitivity which is different for different materials.

It is possible to adopt a system of units where you don't have these constants - however, doing so hides some of the physics. For instance, there is an equivalent number for magnetism $\mu_o$ which is the permiability of free space. Together they make the speed of light: $c^2=1/\epsilon_o\mu_o$

The idea is to expose the interaction of fundamental properties and the relationships between physical models.

The pi is more interesting - it comes from the symmetry of the situation the equation describes: spherical.

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the speed of light: $c^2=1/\epsilon_o\mu_o[/itex I cannot resist posting this comment: Is not [itex]c^2=1/\epsilon_o\mu_o$

a beautiful formula.

It shows the permitivity on which the electric field depends and the permeability on which the magnetic field depends. And that is what light is - an electromagnetic wave. (At least in classical Physics)

Your epsilon is usually $\epsilon_o$ - the permitivity of free space. We can modify the basic equation for different media by including the relative permitivity which is different for different materials.

You can use K here too. epsilon(naught)/epsilon = K/K(naught)

It is possible to adopt a system of units where you don't have these constants - however, doing so hides some of the physics. For instance, there is an equivalent number for magnetism $\mu_o$ which is the permiability of free space. Together they make the speed of light: $c^2=1/\epsilon_o\mu_o$

No it doesn't really hide it.

k = 1/[4(pi)(epislon)]

Let l = mu/4(Pi) (I know l doesn't exist but bear with me.)

(I really need to learn how to type those formulae.)

then

epsilon = 1/[4(pi)(k)]

mu = 4(Pi)l

$\epsilon_o\mu_o = kl$

You can now put this back in your formula.

The idea is to expose the interaction of fundamental properties and the relationships between physical models.

Like I said before. It is just a ratio. Whatever ratio epsilon is in, K is in inverse of that ratio. So relationships between models isn't affected.

The pi is more interesting - it comes from the symmetry of the situation the equation describes: spherical.

a beautiful formula

I completely agree. But my point is that permittivity and permeability are experimentally derived.

Why not use 9 * 10^9 and 10^(-7) directly. As I proved earlier they do not deform the equation in any major way.

bump.

Born2bwire
Gold Member
I completely agree. But my point is that permittivity and permeability are experimentally derived.

Why not use 9 * 10^9 and 10^(-7) directly. As I proved earlier they do not deform the equation in any major way.

Permittivity and permeability are not experimentally derived, they are defined constants. Permeability is defined as 4\pi e-7 and permittivity is defined using c and the permeability of freespace. The value for the permittivity and permeability is purely a choice of your unit system. As previously stated, we can use a Gaussian system where the permeability and permittivity of free space are unity. In addition, we can move the 4\pi from the factor k to Maxwell's equations (as we do in Gaussian units). Placing the 4\pi off of the Maxwell equations is called rationalized units.

We do not use a numerical value for k directly since k is dependent upon the permittivity of the background. It is not a constant across all possible problems.

Simon Bridge