[cool pontiac (OP)]

I have a pretty good understanding of electricity but was confused watching a you tube video of a commercial product that deters squirrels from getting into your bird feeder. it seems to do this by electrifying the pole that holds the feeder.  You see the squirrel leap onto the pole and then get shocked and then he gets the heck out of dodge. How does the squirrel get shocked if he was in mid air. he didnt really complete a circuit between 2 conductors?  is there an solid state cicuit (I know about van de graff generator) that is building up 'charge' on the pole or something like that?
  squirrel after getting shocked ---->  [O8)]

[Geezer]

Beats me. Maybe they faked it. Birds land on high voltage power lines all the time and they don't get shocked.

I believe the squirrel deterrent you describe could only work if it has an exposed conductor wrapped around the support pole for the birdfeeder. The exposed conductor would have to be insulated from the pole of course. Then when the squirrel tried to climb the pole it would close the circuit between the pole (ground) and the exposed conductor (hot) and get a zap. It could be rigged to be slightly uncomfortable but quite harmless for the squirrel.

[RD]

Electric fences to control livestock only deliver a shock when the circuit is completed via earth.
If you’re not touching the ground when you touch the wire you don’t get a shock …

See 

@  1:32

A mesh containing both electrodes would avoid squirrel having to touch the ground,
like the grid on electric bug zappers ... http://en.wikipedia.org/wiki/Bug_zapper

Them thar pesky critters will find a way ...

[:0]

[LeeE]

I reckon Geezer's probably got it right.

[Geezer]

Dave, I'm not sure 1) and 2) will work if the squirrel jumps onto the pole (which it seems likely to do). Perhaps 2) would at really high frequencies, but that might tend to barbecue the squirrel or the homeowner's dog if it happened to whizz on the pole.

As RD points out, electric fences only work when the target is grounded. Sometimes that can be a problem around here. The ground can get so dry that the current flow is insufficient to deter livestock with ambitions. That reminds me, there was a news item here last night about three cows who broke into their owner's home, ate a huge sack of dog food, destroyed all the furniture then crapped all over everything.

[Geezer]

Dave, you can jump onto a Van de Graaff generator and, because there is no low impedance path between you and ground, you won't feel a thing. Same thing if you jump onto a 100KV power line.

[RD]

... you won't feel a thing.

You will feel your hair standing on end ...

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http://www.physics.brown.edu/physics/demopages/Demo/em/demo/5b1010.htm

[Geezer]

Thanks for finding that RD. I anticipated some wise guy scientist would point that out.

Allow me to revise my statement.

"You can jump onto a Van de Graaff generator and you won't get an electric shock."

[RD]

"You can jump onto a Van de Graaff generator and you won't get an electric shock."

The Chinese will be disappointed ...
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http://www.telegraph.co.uk/travel/picturegalleries/signlanguage/7359582/Sign-Language-week-90.html?image=1

[Geezer]

"You can jump onto a Van de Graaff generator and you won't get an electric shock."

The Chinese will be disappointed ...
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http://www.telegraph.co.uk/travel/picturegalleries/signlanguage/7359582/Sign-Language-week-90.html?image=1

Well Lovey, as I always say, if you can't get a nice cup of tea, you'll just have to make do with a nice electric shock.

[graham.d]

There is a lot of dangerous advice in this thread :-)

The human body has quite a significant capacitance to free space and more when in proximity to a ground (in the form of "the ground" for example). Typically it is assumed as about 100pF. If you jump onto a 100kV line it would almost certainly kill you as you would have an AC current flow of around 3mA at 50Hz which is quite sufficient. Birds don't get affected because they are small and have a low capacitance. People can work on live power lines but they work from within a cage all at the same oscillating potential as the line. It is nonetheless a dangerous job. It used to be done, even operating from a hovering helicopter in the Everglades (Florida) until a few years ago when I believe the practice was stopped.

Agricultural electric fences vary in design but usually deliver a pulse, or several pulses, at something like 10kV. It helps a lot if you are not earthed when you grab one but you would almost certainly notice the effect. As it is non-lethal I suggest siome of those confidently predicting no effect should try it. By all means put some wellie boots on first.

Van der Graaf devices have almost no current supply capability so the current is very low. If you use them with capacitors (quaintly called Leyden jars) they can be lethal. You will find that health and safety no longer permit Leyden jars to be used with these devices in schools, even if a good way to deal with disruptive pupils.

[graham.d]

The phrase is "it's volts that jolts but mils that kills" - the "mils" being milliamps. Depends on where on the body it flows too.

[Geezer]

Graham,

I don't believe it's the size of birds that protects them. It's the electric field around the conductors that does. The bird gets charged to the same potential as the power line before it even makes contact with it. The same would be true for a human, although I must admit I've never actually tried jumping onto a high tension line.

Birds will have a smaller capacitance because of their smaller surface area but it is not negligible, so surely they would have to feel something? If birds experienced any kind of jolt, even if non-fatal, I don't think they would keep doing it.

I've seen some rather large hawks on HT lines. They have quite a large capacitance, particularly when they extend their wings. HT lines don't bother them in the slightest. Also, I would think it will take proportionately less current to electrocute smaller animals, so even a reduction in capacitance might not help a bird if there was a possibility of electric shock.

[Bored chemist]

"The phrase is "it's volts that jolts but mils that kills""
And the phrase is wrong.

[Geezer]

"The phrase is "it's volts that jolts but mils that kills""
And the phrase is wrong.

Correct. The "jolt" is caused by current, and it's really the energy dissipated in critical parts of the body that kills.

If it was simply a case of current, static electricity would have wiped us all out long ago.

[RD]

I've seen some rather large hawks on HT lines. They have quite a large capacitance, particularly when they extend their wings.
 HT lines don't bother them in the slightest.

wise guy strikes again ... http://www.thenakedscientists.com/forum/index.php?topic=11580.msg141560#msg141560

[Geezer]

I've seen some rather large hawks on HT lines. They have quite a large capacitance, particularly when they extend their wings.
 HT lines don't bother them in the slightest.

wise guy strikes again ... http://www.thenakedscientists.com/forum/index.php?topic=11580.msg141560#msg141560

Quite right Wise Guy er RD. If they get too close to another conductor, or the neutral conductor, very bad things will happen.

The debate here is about a single conductor, presumably sufficiently distant from other conductors (or ground) that the only path for conduction is through free space.

[graham.d]

The "volts that jolts ..." thing is not really a scientific statement but is something in common parlance. It really is a reminder that it is the power dissipated that is dangerous. It is often surprising to youngsters learning about electricity that it only needs a few milliamps at mains voltages to be lethal. However, a high voltage pulse from an induction coil (we have all had it done to us when young - "hey grab hold of this metal tube while I flick this switch") is quite unpleasant but non-lethal because the total power is very low.

Geezer, you are right about the field effect in that small objects (like a bird) will have the "other plates" of their capacitor to the source (the wire) as well as "free space". An extreme example would be an ant where very little of the ants capacitance would be to free space. The capacitance of a bird (a crow, say)would be about 1/10th that of a human too, so that reduces the effect as well. For a human though, his size would be of the same order as the line spacings of typical power lines (which would increase his capacitance). I'm not wholly sure what the ratio of capacitance to free space vs capacitance to ground would be on an infinite length, single line, but I think it would still be significant and deadly at 100kV/50Hz. Perhaps if the person approached the line with his body parallel to it, it would be better. We need a volunteer :-)

[Geezer]

Maybe we could talk Neil into it  []

Yes, I suppose if you were dangling from the wire, things might get a bit nasty. If you were to crawl along it, it would be a bit safer.

I found something that said birds won't land on 100KV lines, but I could not find any data to back it up. Perhaps they sense the field as they approach?

Next time I'm planning on jumping onto a HT line I'll make sure it's DC before I leap.