Should have gone to school?

[Deux]

notice anything wrong in that picture?

[Curlytop]

Deux set the following eddies spiralling through the space-time continuum:

Qb lbh zrna gur gbgny nofrapr bs nal cngu jurer ryrpgevp pheerag pbhyq sybj?

[Graham.]

That's not the whole story, remember the gold leaf electroscope at school?

Current will flow into the bird until it attains the same potential as the wire. The charge they take will depend on their size. If the wire is carriing AC, the current flowing into and out of the bird will alternate.

I always assumed that's why you rarly see birds perching on the actual phase wires, and when you do, they (the wires) are probebly dead.

x-post uk.d-i-y

[Yellow]

Question - If the bird is in free space and then lands on the wire, where is this current flowing through the bird, flowing too?

You could aways test the theory by touching the wire with the bird sitting on it, making sure you are well earthed first of course. :-)

[John Williamson]

The current flows in and out of the capacitance presented by the bird, due to its mass. A few nanofarads at most, which at 50Hz gives a negligible current. The bird is more likely to sense the alternating magnetic field surrounding the wire, as experiments have shown that some, if not all, birds can sense magnetic fields.

The more likely explanation is that live cables are warmer than the surrounding air, and the birds don't like having hot feet, or that the magnetic field causes them discomfort or disorientation.

[Dave Liquorice]

Could be the magnetic field but also the size of the wire, even our lowly single phase 11kV feed has wires about 1/2" dia. Pylon supported grid cables are up to 2" or so in dia... Birds certainly perch on our powerline at times but normally in a flock. Sat on a wire in free space is a very exposed position for a tasty snack.

[John Rumm]

True, but the current flow to charge and reverse charge the bird will be negligible - it will have a tiny capacitance.

[Graham]

Internode Ltd -

|

e(dot)co(dot)uk But. Won't the bird experience the effect of the arc that Will strike between it and the conductor as it approaches? I should think that would be a deterrent. |

[Martin Brown]

Main reason is probably that the big EHT mains cables are too large a diameter for most bird to perch onto. You often see birds perched on the HT 40kV lines which are more suited to their feet.

The bird only feels a current from charging and discharging its almost negligible self capacitance - treating a bird here as a uniform conduction sphere of radius about 0.05m for convenience. I doubt it even feels it at all - might enjoy having warm feet in winter though.

[F Murtz]

Big bats have a problem,I used to get dead bats in the front of my place,we had a lillypilly tree which bats like and I thought it must poison them. It turned out, when they left the tree their wingspan was big enough to span the two phases on the aerials in the street.

[Martin Brown]

On 01/09/2012 09:29, F Murtz wrote: > Martin Brown wrote: >> On 31/08/2012 22:35, Graham. wrote: >>> On Fri, 31 Aug 2012 22:01:10 +0100, Curlytop >>> wrote: >>>

That is surprising. I once saw a flock of seagulls get into big trouble with a 40kV thin wire line they didn't see and flew into it en mass and at speed. It was pretty horrible with many injured seagulls flapping around on the ground and an unlucky few hung up by a wing on the wires.

Arguably the poor things should have gone to specsavers in this case.

RSPCA had to come and do them in. Not sure how they resolved the ones on the hot wire I think NEDL had to come and knock them off with some sort of isolated broom handle. I didn't hang around for that bit.

It wasn't the electricity that harmed them though it was physically hitting a strong thin wire at speed.

[Man at B&Q]

Almost every day outside our house. In the evenings at certain times of year there are literally hundreds.

Hardly "rarely".

MBQ

[Tim Streater]

Into the bird and charging it up, thereby raising its potential. As others have said, the capacitance is negligible so the actual amount of charge is too.

[Martin]

Haven't you ever seen men jumping onto HV cables from helicopters? They have a lot more capacitance than a small bird. The birds will be fine

[Ste]

There are lots of interesting questions like that. It's not actually necessary to complete a circuit with wires.

The presence of free space is sufficient to conduct electromagnetic energy - that's why you can watch television, without having two wires connected between the screen and your eyes. So too, it is why you can receive a television signal using an aerial, which by definition does not have a wired circuit to the transmitter.

Birds can and do perch on live high-voltage conductors. What happens when they do so, is that they are "charged" more or less up to the potential of the conductor. A very small amount of current will flow into the bird, according to its electrical characteristics - and in the case of AC, that current will continue to flow according to the regular change in voltage (with DC, current would flow as the bird charged, and then would drop to nothing).

In terms of "where does the current passing into the bird flow to", the straightforward answer is nowhere, if the bird is insulated from the conductor of the opposing voltage. In reality, there is no perfect insulator, and a very tiny amount of current will flow around the circuit and through the bird, but the bird is (usually) sufficiently insulated that no harmful levels of power will pass.

I often find that the hydraulic analogy is very apt to demonstrate this sort of thing. (Technically, and contrary to common understanding, fluid also requires a circuit in order to flow effectively, just as electricity - no fluid will flow between two points in space, if there is no pressure difference between those points or no flow path between those points). If you use a pump to pressure a fluid in a dead-end pipe (the pipe being the hydraulic equivalent to an electrical insulator), then even though there is no immediate way for the fluid to escape, a small amount of fluid will move away from the pump towards the end of the pipe furthest from the pump, in order to equalise pressures throughout the fluid on that side of the pump. If you then tap into this pipe along its length, adding another tee piece (the bird), then fluid will also flow into the tee piece until it is also charged to the same pressure as the whole pipe.

If the bird flies off the wire at the appropriate time (i.e. when the AC voltage is not zero), for a period of time it will actually remain charged at the high voltage, until inductance re-equalises the potential difference between the bird and its surroundings (or until it resettles on an earthed conductor).

If one were "well-earthed", touching a live conductor is the last thing one would want to do.

[polygonum]

Musicians know this. You often see the conductor kept well away from them, often in a sort of metal 'cage'.

[polygonum]

Is that so? What charge does a bird flying through dry air build-up? My guess is that in some conditions a bird's potential might be higher than that of the conductor they land on. Hence there would be an outflow rather than an inflow. (Mind, are we talking electron flow, conventional current flow?)

[F Murtz]

Our fruit bats have a wingspan of almost a metre (African ones up to 6 feet)

[Yellow]

No shit sherlock.

[Nick Odell]

I've just had an idea for a renewable energy scheme....

Nick