# Single conductor inside hollow conductor

1. ## Single conductor inside hollow conductor

If and ac voltage is applied to a conductor inside of another hollow conductor will a voltage be induced in the hollow conductor.

2. ## Re: Single conductor inside hollow conductor

yes
where is the return path of the AC current?

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3. ## Re: Single conductor inside hollow conductor

Originally Posted by wwfeldman
yes
where is the return path of the AC current?
Not sure what your asking. Are you implying that current flow will be around the circumference of the hollow conductor.

4. ## Re: Single conductor inside hollow conductor

Yes - it is like a 1 turn air cored transformer - there is some coupling - but very weak

I note you say the inner conductor is energised with AC - end to end - the outer conductor will have energy inductively coupled to it - but not a great deal.

Coaxial wire ( very similar to what you describe ) is often wound around a core to make a functional transformer with high coupling .....

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5. ## Re: Single conductor inside hollow conductor

I'm missing a clear problem specification. Applying a voltage as such won't generate a current, thus no magnetic field and no induced voltage. Wwfeldmann assumes apparently that the voltage drives a current and asks about the return path.

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6. ## Re: Single conductor inside hollow conductor

"Single conductor inside hollow conductor" sounds like coax cable, in which case the shield is connected as the return path. But maybe your setup is different. In any case, there is capacitance between the conductors and inductive coupling.

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7. ## Re: Single conductor inside hollow conductor

The post was very clear:

ac voltage is applied to a conductor inside of another hollow conductor
i.e. voltage is applied to the ends of the inner conductor, therefore current flows in the inner conductor - repliers should read carefully.

8. ## Re: Single conductor inside hollow conductor

Is there any one type of coax that might be better to experiment with. The coax I have laying around is stranded wire on outside.

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Originally Posted by Kajunbee
Is there any one type of coax that might be better to experiment with. The coax I have laying around is stranded wire on outside.
No foil layer if that makes a difference.

9. ## Re: Single conductor inside hollow conductor

the outer conductor of coax is usually stranded and woven
if it was solid, like a pipe, it wouldn't bend.

when i asked where the return path was, i did assume a current
as you know, there is current only if there is a closed path
you connected a voltage source to one end of the central wire
did you connect the other end of the central wire back to the return of the source,
or did you leave it open?

if there was a return path, then there is current and there is an induced voltage in the outer conductor
since it is AC, there is also an induced current in the outer conductor

if there is no return path, the voltage in the central conductor still changes
and there is a current induced in the outer conductor and an EMF
(electro motive force) but no voltage

there is a distinct difference between EMF and Voltage, even though they look a lot alike and they have the same units

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10. ## Re: Single conductor inside hollow conductor

I think soft copper could be used. Something with just large enough diameter to pass the wire. It may become work hardened if you coil and uncoil it many times though.

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11. ## Re: Single conductor inside hollow conductor

there is a distinct difference between EMF and Voltage, even though they look a lot alike and they have the same units
can you tell me the distinct difference please? without using " back - emf "

12. ## Re: Single conductor inside hollow conductor

Would passing current through outer conductor provide better coupling to inner conductor. As opposed to passing current through inner conductor.

13. ## Re: Single conductor inside hollow conductor

@easy peasy:
potential (voltage) is the energy per unit charge based on static charge distributions
things where we use V = sum of kq/r or integral thereof for continuous charge distributions

emf is the energy per unit charge based "other" conditions, where there are no charges
such as by faraday's law where a changing magnetic flux induces an emf - but not a potential

reference Moore Six ideas that Shaped Physics Unit E Electric and Magnetic Fields are United page 84
i expect this is expanded on later in that section

reference Halliday and Resnick (and likely in Haliday Resnick and Walker) but i do not have the reference

reference Marion Classical Electromagnetic Radiation page 106
( i believe this is a first edition, as it doesn't say second, etc)

14. ## Re: Single conductor inside hollow conductor

Originally Posted by Kajunbee
Would passing current through outer conductor provide better coupling to inner conductor. As opposed to passing current through inner conductor.
Your setup is unclear, as mentioned above. The normal use of coax is to have current on both inner and outer conductor, with opposite direction. Signal path and return path, you know ....

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15. ## Re: Single conductor inside hollow conductor

Originally Posted by volker@muehlhaus
Your setup is unclear, as mentioned above. The normal use of coax is to have current on both inner and outer conductor, with opposite direction. Signal path and return path, you know ....
I was wondering if a power transformer could be design this way. But if I'm understanding Easy peasey correctly it wouldn't work. No matter how many turns of a coaxial like conductor you wind on a core.
To clarify the setup think of center conductor as primary and outer shield as secondary. Or vice versa.

16. ## Re: Single conductor inside hollow conductor

But if I'm understanding Easy peasey correctly it wouldn't work. No matter how many turns of a coaxial like conductor you wind on a core.
I understand post #4 quite differently.

Coaxial transformers are a well known solution for specific purposes in RF design, e.g. impedance matching in wideband amplifiers.

There are different "flavours" of coaxial transformers, e.g. with closed and open magnetic path. It would be helpful for the discussion if you describe the configuration of interest and the electrical function you want to achieve.

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17. ## Re: Single conductor inside hollow conductor

@wwfeldman, EMF is measured in volts, and the voltage of a system is the electro motive force, there is no edict that says that volts only relate to static charge conditions - the very term electro "motive" force implies the volts are generated thru some sort of motion - but to make any distinction is verging on a unique level of pedantry - surely ...

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FvM is correct - the answer to post#1 is yes - for AC

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18. ## Re: Single conductor inside hollow conductor

@easy peasy
emf and potential are both measured in volts
when the "voltage" is caused by charges, it is potential and the potential difference between two locations is the voltage we measure
when the "voltage is caused by "motion" as described by faraday's law, there is no potential difference, but there is an emf

call it pedantry if you like, the root cause is different

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19. ## Re: Single conductor inside hollow conductor

potential (voltage) is the energy per unit charge based on static charge distributions ...
Potential is related to an electric field. There is nothing about "static" charge distribution: of course if the charges are moving, you get an electromagnetic radiation.

Electromagnetic radiation can be considered independently of charge distribution.

Potential is related to the electric field (static or dynamics does not matter)- it is defined as an integral of the field from a given point to infinity.

Every vector field is associated with a scalar potential (say gravitational field).

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Applying a voltage as such won't generate a current, thus no magnetic field and no induced voltage. Wwfeldmann assumes apparently that the voltage drives a current and asks about the return path...
I do not understand.

A conductor surrounded by a cylindrical conductor forms a capacitor. The exact value does not matter.

Now you apply an AC to the centre conductor (assume sinusoidal for simplicity). I presume this potential is referenced with respect to the outer shield: there must be some reference for the potential)

It is known that an AC applied to a capacitor is associated with a current. This current will also be associated with a magnetic field.

Just a gedanken.

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when the "voltage is caused by "motion" as described by faraday's law, there is no potential difference, but there is an emf...
What is the similarity between an dry cell and a cycle dynamo? I presume both produce an EMF but no potential???

One of the basic tenets of Physics is that the underlying cause is a conjecture; it is the effect that can be observed and quantified.

If a voltage source and EMF (by the way it is described as a force but measured as a potential) cannot be distinguished by their effects, they are the same.

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20. ## Re: Single conductor inside hollow conductor

I was able to wrap 13 turns of coax around a pair of u shaped cores I have. I applied 3 vac across center conductor and measured 1.4 vac on shield wire. When I applied 3 volts to shield wire I measure 2.71 volts on center conductor. Would these differences in measured voltage be expected. The frequency was 60 hertz off my variac.

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