Permitivity of Free Space

Tanker571 · 2004 October 10th, 23:46

From what does the permitivity of free space derive? What does it mean physically and mathematically? My teacher could not tell me when I asked him, so I now ask you. Thanks!

editor · 2004 October 11th, 00:58

Permittivity of free space is measured in Farads per meter, permeability of free space is measured in Henrys per meter. So, without getting too obscure, Permittivity relates to the effective capacitance and permeability refers to the effective inductance.
The interesting thing is that the speed of electromagnetism in free space, i. e. speed of light, is determined by these two.

1/C^2 = permittivity x permeability.

= (8.85 x 10^-12 )(1.26 x 10^-6)
C = 2.9946 x 10^8 m/s

[Edited on 10-11-2004 by editor]

[Edited on 10-12-2004 by editor]

nmondal · 2004 October 15th, 10:20

It\'s nothing serious, just an mathematical construct. if we do not have that stuff, we will end up having a different value of *C*. To have the value of C, as we know, we put that value into permitivity. take that as akind of reverse calculation. If we took the value of C as 1, then we will end up having a different permitivity of the free space.

gravity wave · 2004 October 15th, 15:08

Quote:

Originally posted by nmondal
It\'s nothing serious, just an mathematical construct. if we do not have that stuff, we will end up having a different value of *C*. To have the value of C, as we know, we put that value into permitivity. take that as akind of reverse calculation...

Not quite, nmondal... not even close!

Determining the value of *c* purely from the electromagnetic properties of free space was the crowning achievement of James Clerk Maxwell.
It was Maxwell\'s equations that showed precisely that the value of the speed of light could be calculated from the ALREADY KNOWN quantities of permittivity and permeability of free space!
In fact, it was due to that derivation that he determined in advance that light was in fact an electromagnetic wave!

P.S. Editor had the correct answer. Re-writing history is the wrong answer.

Gravity Wave

-\"The works of the Lord are great, studied by all who have pleasure therein\". --- Inscribed in the archway of the doorway to James Clerk Maxwell\'s Cavendish Laboratory -Cambridge--

[Edited on 10-15-2004 by gravity wave]

Sir_Zerp · 2004 October 15th, 16:01

Quote:

Originally posted by editor
Permittivity of free space is measured in Farads per meter, permeability of free space is measured in Henrys per meter. So, without getting too obscure, Permittivity relates to the
1/C^2 = permittivity x permeability.

This is neat stuff. Basic question about units. The thread said space, but meters x meters is area.

If you are refering to space, shouldn\'t it be meters x meters x meters for volume?

keebler_giant · 2004 October 15th, 16:11

\"Free space\" isn\'t referring to actual space, it is referring to the vacuum.

[Edited on 10-15-2004 by keebler_giant]

nmondal · 2004 October 17th, 00:36

Quote:

Originally posted by gravity wave

Quote:

Originally posted by nmondal
It\'s nothing serious, just an mathematical construct. if we do not have that stuff, we will end up having a different value of *C*. To have the value of C, as we know, we put that value into permitivity. take that as akind of reverse calculation...

Not quite, nmondal... not even close!

Determining the value of *c* purely from the electromagnetic properties of free space was the crowning achievement of James Clerk Maxwell.
It was Maxwell\'s equations that showed precisely that the value of the speed of light could be calculated from the ALREADY KNOWN quantities of permittivity and permeability of free space!
In fact, it was due to that derivation that he determined in advance that light was in fact an electromagnetic wave!

P.S. Editor had the correct answer. Re-writing history is the wrong answer.

Gravity Wave

-\"The works of the Lord are great, studied by all who have pleasure therein\". --- Inscribed in the archway of the doorway to James Clerk Maxwell\'s Cavendish Laboratory -Cambridge--

[Edited on 10-15-2004 by gravity wave]

I do perfectly agree. I also agree on the equation. What I actually meant to say is that, *measure* is something of an comparison. We have a nice equation, but what is the cause, and what is the effect? If we take C=1, then the value of permitivity [and of cource permeability] has to undergo changes. [Am I right?] People already *measured* those quantities, and only for historical reasons, we think the other way round. :) Else, we would have a nice measurment system where, C=1, and then we could have different *Epsilon-0* and *mue-0*, to make up the C=1 part. Would it change the laws? I really do not think so. Because C has to constant , irrespective of anything, right?
Again, remember those are just *Constants* and are ther foir our own *will*. If we change them, we still have a nice looking *logical* system, albeit different from what we use. :D

les.doherty · 2004 November 18th, 23:04

Although these phenomena are constant in free space - whatever that is - they change in the presence of matter. What is the change in c, epsilon and mu under the influence of gravity & EM fields? This is the question that needs to be answered before we can make any real progress in fundamental physics!

editor · 2004 November 19th, 16:30

What exactly are you asking ?
Free space is the vacuum.
C does not change when gravity is present. em fields don\'t change C, they are C, in a sense.
Gravity changes the geodesic of 4-dimensional spacetime but not the speed of C.

[Edited on 11-19-2004 by editor]

les.doherty · 2004 December 2nd, 06:03

Obviously it is not free space, but even an \'inert\' gas such as helium has a refractive index, which means that mu and/or epsilon are changed by the presence of matter. This means that C is changed and also the impedance. The question is: \"Does a gravitational field influence the speed of light in a vacuum\"?
or in experimental terms \"Is the speed of light in a vacuum different at the noth pole than at the equator or on the moon\"?
Since we can now measure the speed of light very accurately, has anyone done these experiments?

editor · 2004 December 2nd, 15:46

According to Feynman light always takes the path of least time. If that is a geodesic path, that is, spacetime bent by gravity, that\'s the way it goes.

keebler_giant · 2004 December 2nd, 16:20

Quote:

According to Feynman light always takes the path of least time.

I thought it was more like light takes every path, but in the limit that hbar->0 this becomes the path of least time.

Sir_Zerp · 2004 December 2nd, 21:15

I have a more basic question. What the heck is free space?

I think you think, you know the answer but come on.

In real space we got solar wind, stray hydrogen atoms, legacy photons from the big bang.

Around earth we go 50 and 60 hz radio waves (photons) in every direction. Another name for high tension power lines are antennas.

Oh you mean, a man made artificial vacuum on earth. Nope, we got neutrinos out the wazoo. There’s a whole unseen uncharged universe out there that we can’t directly interface with, but it doesn’t mean it’s not there.

So, the term free space is a very loaded term.

Hmmm... what is the permitivity of a glass of neutrinos?

Norman Albers · 2006 February 13th, 17:48

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