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Author Topic: Is there a magnetic field around ray of electrons in vacuum?  (Read 4406 times)

Offline mathew_orman

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Is there a magnetic field around ray of electrons in vacuum just like around the CRT beam?
« Last Edit: 04/09/2015 12:36:29 by chiralSPO »


 

Offline lightarrow

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Re: Electrons and magnetic field
« Reply #1 on: 31/08/2015 10:20:49 »
Is there a magnetic field around ray of electrons in vacuum just like around the CRT beam?
Why there should be a difference? Isn't a CRT beam made of electrons in the void?

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Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #2 on: 31/08/2015 10:52:26 »
The question was about field and not about differences in description of ray of electrons...
 

Offline alancalverd

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Re: Electrons and magnetic field
« Reply #3 on: 31/08/2015 12:36:23 »
Yes
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #4 on: 31/08/2015 13:25:13 »
So the ray has self inductance and can produce electromagnetic waves?
Two rays in parallel would have mutual inductance?
 

Offline chiralSPO

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Re: Electrons and magnetic field
« Reply #5 on: 31/08/2015 13:27:28 »
Any net motion of charge will have an associated magnetic field. A cathode ray is no different from a wire with current in this regard.
 

Offline PmbPhy

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Re: Electrons and magnetic field
« Reply #6 on: 31/08/2015 13:58:39 »
Quote from: mathew_orman
... can produce electromagnetic waves?
Only when the current is time varying so as to create an electromagnetic wave.
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #7 on: 31/08/2015 14:52:46 »
It is easy to modulate the ray using gate electrode but would it radiate and act as a dipole antenna?
 

Offline alancalverd

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Re: Electrons and magnetic field
« Reply #8 on: 31/08/2015 17:09:16 »
If you are careful and diligent, you could end up designing a betatron.
 

Offline lightarrow

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Re: Electrons and magnetic field
« Reply #9 on: 01/09/2015 13:34:42 »
The question was about field and not about differences in description of ray of electrons...
If we knew that all dogs barks, you say that dog A barks and ask me if dog B barks too and I reply that it's still a dog, why do you ask me that the question was about barking?  :)

I ask you again: why should electrons moving in the void of a CRT should behave differently from electrons moving in another void region of space? If they generate a magnetic field in the first case, why shouldn't they do in the second? It's the fact that a charge moves, that makes it generate a magnetic field, nothing else than this.

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« Last Edit: 01/09/2015 13:38:13 by lightarrow »
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #10 on: 01/09/2015 14:53:25 »
There are many differences  like resistance skin effect and etc.
 

Online evan_au

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Re: Electrons and magnetic field
« Reply #11 on: 01/09/2015 15:46:43 »
If you want to efficiently generate an electromagnetic wave in space, a linear vacuum tube (like an old-fashioned TV CRT) is not necessarily the best way to do it.

A vacuum is basically a pretty good insulator, so you need very high voltages to produce a low current. You can help it along by heating the cathode, but you end up with a very high impedance AC source. This is not a good impedance match for free space, so you need to transform this high impedance source to match the low impedance of free-space radiation. Traditional vacuum tune transmitters used transformers to match the high source impedance to the low impedance of a conductive copper wire, which is matched to the impedance of free space at the intended operating frequency.

Mobile phones use semiconductor amplifiers, which are fairly low impedance devices, and a reasonable match to the antenna impedance. But they can only deal with fairly modest power levels.

For really high power transmitters (like TV transmitters), vacuum tube + transformer + copper antenna is still the best way to go.

One area where vacuum tubes are still used in residential applications is the cavity magnetron in microwave ovens. The magnetic field in the magnetron means that the electrons do not propagate in straight lines, but in helices. At GHz frequencies and higher, high-powered electromagnetic signals are not carried on wires, but more in "plumbing" - they propagate down pipes. In this application as an oscillator, the low efficiency of coupling to radiation is an advantage, as it keeps the oscillator running.

So I guess the next questions are:
  • What range of frequencies would you like to transmit?
  • At what power levels? and:
  • Is efficiency important in your application? 
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #12 on: 02/09/2015 08:32:17 »
The only property of empty space or vacuum is containment of matter...
And there is no such property as space resistance, impedance and etc...
Antenna theory creates many abstract quantities to satisfy the wave equations for empty space...
Examples of such are permittivity and permeability of vacuum, clear fudge number derived from speed of light...
The absence of influence is not a property of any kind...
The property of space can only be defined in presence of matter enclosed by that space or another words space between...
 

Offline PmbPhy

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Re: Electrons and magnetic field
« Reply #13 on: 02/09/2015 13:04:13 »
The only property of empty space or vacuum is containment of matter...
There are other properties of empty space such as the geometry of the space itself. Sometimes space is Euclidean and sometimes its not. Euclidean space can be altered by matter to create Non-Euclidean space.
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #14 on: 02/09/2015 13:39:15 »
So there is a way to measure empty space?
 

Offline lightarrow

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Re: Electrons and magnetic field
« Reply #15 on: 02/09/2015 14:43:07 »
There are many differences  like resistance skin effect and etc.
But you were interested in "magnetic field".
I repeat: a beam of electrons (that is, many charges moving all together in the same direction with the same speed) generates a magnetic field around them. Period.
So, why should there be a difference if this beam is in the void between two galaxies or inside a CRT monitor? It's not a critic, it's a question, to understand what you have in mind.

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« Last Edit: 02/09/2015 14:45:03 by lightarrow »
 

Online evan_au

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Re: Electrons and magnetic field
« Reply #16 on: 02/09/2015 14:45:56 »
Quote from: mathew_orman
Antenna theory creates many abstract quantities to satisfy the wave equations for empty space... Examples of such are permittivity and permeability of vacuum, clear fudge number derived from speed of light...
I disagree that permittivity and permeability are purely abstract*.

Both parameters can be fairly readily measured in the lab. The large number of people wearing glasses or contact lenses would find that they wouldn't work if permittivity was purely abstract.

Optical fibres are very concrete (figuratively speaking), and the speed of light in this medium is determined by its permittivity and permeability. The speed of optical communications has real economic impacts.

The speed of light in a vacuum is just a special case of the speed of light in any transparent medium (including air, spectacles and the lens on the digital camera in your pocket).

*I recognise that permittivity and permeability may be just micrometer-scale averages of parameters that have more complex properties at nanometer scales.
« Last Edit: 04/09/2015 10:59:34 by evan_au »
 

Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #17 on: 03/09/2015 10:47:36 »
What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...
 

Offline lightarrow

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Re: Electrons and magnetic field
« Reply #18 on: 03/09/2015 13:16:39 »
What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...
Not only vacuum does have those properties, it has by definition!

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Offline mathew_orman

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Re: Electrons and magnetic field
« Reply #19 on: 04/09/2015 09:05:31 »
What definition is that?
 

Offline PmbPhy

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Re: Electrons and magnetic field
« Reply #20 on: 04/09/2015 09:33:00 »
Quote from: mathew_orman
What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...
He's referring to the permittivity and permeability of free space. Those quantities are not measured as a property of atomic structure since it's measured in a vacuum. I.e. the force between two charged particle in vacuum is

F = [1/4pi*e]qq'/r2

where "e" is the permittivity of free space. There is no atomic structure to speak of here. However I myself don't see this as a property of space.
 

Online evan_au

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Re: Electrons and magnetic field
« Reply #21 on: 04/09/2015 11:17:02 »
Quote from: mathew_orman
What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...
Air has a relative permittivity of εr=1.00058986, which is almost the same as a vacuum (εr=1).

This should not be surprising, because there is a lot of space (vacuum) between each molecule in the air. You could imagine air as >99.8% vacuum (ie if you condensed 1 liter of air to a liquid, it would take up <1.5ml).

If you connect a pump to a container of air, and halve the pressure of the air, εr will be around 1.0002, ie the molecules of oxygen, nitrogen & water will have a reduced impact on the permittivity of the vacuum.

As you reduce the air pressure towards zero, the relative permittivity of the gas will asymptotically approach the permittivity of a vacuum.

So the permittivity of a vacuum is not a fiction - and when we measure the permittivity of air, we are (mostly) measuring the permittivity of a vacuum.
 

Offline PmbPhy

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Re: Electrons and magnetic field
« Reply #22 on: 04/09/2015 12:35:54 »
Quote from: mathew_orman
What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...
Air has a relative permittivity of εr=1.00058986, which is almost the same as a vacuum (εr=1).

This should not be surprising, because there is a lot of space (vacuum) between each molecule in the air. You could imagine air as >99.8% vacuum (ie if you condensed 1 liter of air to a liquid, it would take up <1.5ml).

If you connect a pump to a container of air, and halve the pressure of the air, εr will be around 1.0002, ie the molecules of oxygen, nitrogen & water will have a reduced impact on the permittivity of the vacuum.

As you reduce the air pressure towards zero, the relative permittivity of the gas will asymptotically approach the permittivity of a vacuum.

So the permittivity of a vacuum is not a fiction - and when we measure the permittivity of air, we are (mostly) measuring the permittivity of a vacuum.
What gives you the impression that the permittivity of free space is a physical property of the vacuum? In free space all it is is a constant of proportionality, nothing more. In fact in Gaussian units Coulombs law is

F = qq'/r2

i.e. the constant isn't even there. See: https://en.wikipedia.org/wiki/Vacuum_permittivity
 

Offline lightarrow

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What definition is that?

ε0 = 107/4πc2.

Anyway it's not a real, physical property of vacuum, as PmbPhy wrote.

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« Last Edit: 04/09/2015 23:28:51 by lightarrow »
 

Online evan_au

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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.
 

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