# The Naked Scientists Forum

### Author Topic: Is there a magnetic field around ray of electrons in vacuum?  (Read 4146 times)

#### lightarrow

• Neilep Level Member
• Posts: 4586
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##### Re: Is there a magnetic field around ray of electrons in vacuum?
« Reply #25 on: 05/09/2015 08:24:26 »
Quote from: lightarrow, PmbPhy
[Permittivity is] not a real, physical property of vacuum
I can charge up 2 metal plates, measure the area of the plates, and the distance between them. I can apply a known electric charge, and measure the voltage between the plates. That represents measurement of real, physical properties. From these real, physical properties I can calculate the permittivity of the dielectric (yet another physical property).

I can repeat the experiment with a glass dielectric, an air dielectric and vacuum as a dielectric. These are all real, physical properties measurable in the lab.

The permittivity I get for a vacuum is just as real as the result I get for glass. It is just as real as the answer I get for air (and extremely close to the answer I get for air).

So I don't understand why the properties of glass and air should be real, and the result of the same experiment in a vacuum should be, in some sense, "unreal"? Please clarify.
The fact voltage in air (or other material medium) is different from voltage in vacuum is ascribed to electric properties of the first: its molecules polarize in presence of an electric field.
The fact you find relations between electric quantities in the vacuum is ascribed to properties of the field sources (charges, currents).

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lightarrow
« Last Edit: 05/09/2015 08:29:29 by lightarrow »

#### mathew_orman

• Sr. Member
• Posts: 132
##### Re: Is there a magnetic field around ray of electrons in vacuum?
« Reply #26 on: 05/09/2015 11:16:41 »
Vacuum capacitor has no dielectric material...
Empty space between the plates has no property of any kind ...
Ones again if there is no physical influence on matter placed in-between the capacitor plates then no dielectric property can be defined, but to satisfy the mathematical model a fudge number is assigned and it is a starting point of influence... Any single atom between plates influence both displacement current and self capacitance  and mutual capacitance of the plates...
« Last Edit: 05/09/2015 12:12:29 by mathew_orman »

#### The Naked Scientists Forum

##### Re: Is there a magnetic field around ray of electrons in vacuum?
« Reply #26 on: 05/09/2015 11:16:41 »