Post by: thebrain13 on 01/04/2012 20:18:54
Post by: Phractality on 01/04/2012 21:22:33
Post by: thebrain13 on 01/04/2012 21:25:37
Post by: Phractality on 01/04/2012 21:40:12
interesting, so you are saying it is circular logic?Absolutely!
Post by: imatfaal on 02/04/2012 12:46:51
The speed of light can be defined as follows
As c is fundamental
the vacuum permeability and 
the vacuum permittivity are now defined in terms of each other and the speed of light. BUT - this is not the only way they can be defined and the other methods involve other fundamental units that are not dependant (I think) on the speed of light
Force between two wires based on the separation and the current flowing - this is Ampere's Law
and
Force between charged particles based on charge magnitude and separation - this is Coulomb's Law
Thus both
the vacuum permeability and the vacuum permittivity can be determinedindependently from speed of light.
Post by: Phractality on 02/04/2012 19:35:51
Not sure that is completely correct.I haven't fully analyzed the connection yet, but I think those formulas amount to definitions of the Coulomb in terms of
The speed of light can be defined as follows
As c is fundamental
Force between two wires based on the separation and the current flowing - this is Ampere's Law
and
Force between charged particles based on charge magnitude and separation - this is Coulomb's Law
Thus both
independently from speed of light.
and . Thus Coulomb's law is based on the speed of light.
Post by: JP on 02/04/2012 20:22:31
and , the only dimensionful units in the equations Matthew posted (and in Maxwell's equations + Lorentz force, which describe more fully how the electric and magnetic forces interact with charges) have units of: charge, distance and time.You certainly can redefine your unit of charge to eliminate either
or. To eliminate both, you need to redefine your measure of space or time, which is simply choosing a longer/shorter measuring stick or choosing to measure units of time that are longer/shorter than 1 second. Doing so doesn't change the speed of light, it just changes the units you're using to measure it.
Post by: Pmb on 02/04/2012 20:54:57
It's possible to calculate the speed of light by knowing the electric permittivity and the magnetic permittivity. the equation is c= 1/sqrt(magnetic permeability*electric permeability) It's neat that one can calculate the speed of light by knowing those two numbers, but I have no idea how it works. I was wondering if somebody has a good conceptual explanation about how those two things control the speed of an electromagnetic wave.The laws of physics, when expressed as a set of differential equations, are called Maxwell's Equations. Those equations can be manipulated resulting in wave equations, one equation for each component of the electric and magnetic fields. The speed of the waves in those equations is calculated to be c = 1/sqrt(magnetic permeability*electric permeability). The wave equation has a certain form and that form has the speed of the wave expressley written in the form of the equation.
How well do you understand vector calculus? If you're familiar with it then I can be more precise.
Pete
Post by: Phractality on 03/04/2012 02:22:55
Post by: JP on 03/04/2012 02:44:53
In fact it's a standard problem in undergraduate physics courses to take Maxwell's equations, which have been derived from experimental knowledge of how magnetic and electric fields interact with charges and show that they're also describing electric and magnetic fields that propagate as waves which move at the speed
The fact that any reference to the motion of the observer drops out means that this speed is constant and reference frame independent. There is no circular reasoning in there.
Post by: graham.d on 03/04/2012 11:23:53