# Coaxial cable

1. Jul 24, 2012

### lufc88

http://en.wikipedia.org/wiki/Coaxial_cable
http://en.wikipedia.org/wiki/Poynting_vector
Could a coaxial cable be used for electric current instead of radio signals?

'For example, the Poynting vector within the dielectric insulator of a coaxial cable is nearly parallel to the wire axis (assuming no fields outside the cable and a wavelength longer than the diameter of the cable, including DC). Electrical energy is flowing entirely through the dielectric between the conductors. No energy flows in the conductors themselves, since the electric field strength is zero. No energy flows outside the cable, either, since there the magnetic fields of inner and outer conductors cancel to zero.'
What causes resistance in a coaxial cable is it like normal conductors or does some resistance happen in the dielectric?

Could you have a vacuum as the dielectric in a coaxial cable?

2. Jul 24, 2012

### Staff: Mentor

Coax is not volumetrically efficient for carrying DC current, because of the large difference in size (and therefore resistance) of the two conductors.

3. Jul 24, 2012

### davenn

yes you can, tho that can be a real problem as if the cable gets holed and a smal/large leak develops, then water will flow into the cable with ease.

Rather, large coax cables/waveguides get filled with compressed dry air or some other inert gas. That way if the cable is holed the air will continue to escape aout the hole and stop moisture from entering.

For lowest loss coax cables, the best easy dielectric is total air (gas), but that's impossible as you need something physical to keep the inner and outer conductors separated. This is often achieved by a thin spiral of teflon and ~ 90% or so of the dielectric is still air.

Dave

4. Jul 24, 2012

### lufc88

Is there any examples of an insulator, conductor, insulator arrangement

5. Jul 24, 2012

### Staff: Mentor

That would be only one wire...? What is the return path for the current?

6. Jul 24, 2012

### davenn

google is a wonderful thing :)

http://en.wikipedia.org/wiki/Coaxial_cable

It would appear that you didnt even read through this link that you yourself gave above

scroll down there are you will even see the spiral teflon dielectric that I spoke of

cheers
Dave

7. Jul 25, 2012

### lufc88

Thanks also does any one know of any more examples of an insulator surrounded by two conductors?

8. Jul 25, 2012

### jbriggs444

Unshielded Twisted Pair comes close to being that.

9. Jul 25, 2012

### davenn

there are no "more" examples, as none have and can be given so far :)
and so how are the 2 conductors insulated ?? they cant be you instead should have 2 conductors surrounded by insulation

no, you still have 2 conductors surrounded by insulation, whether there's a screen/shield and another insulator ( the sheath) is irrelevent.
In a cable, you cant have 2 conductors surrounding an insulator else the conductors will be in contact with each other

10. Jul 25, 2012

### lufc88

cheers everyone