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I touched a tree that was touching an electric fence and got an electric shock. How was this possible if wood is an insulator?

The tree wasn't wet either, and it was a pretty strong shock too.

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    $\begingroup$ A dry cow hide is a pretty good insulator, and that's why an electric fence produces rather high voltages. In general, when you see an electric fence, stay away from anything that could touch it. $\endgroup$ – CuriousOne Dec 20 '14 at 11:18
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    $\begingroup$ I've built enough electric fence I thought I should add that if you have to touch the fence, use the back of your hand, so if you're shocked you'll pull away instead of grabbing the wire. A wallet makes a good tool to push down a fence so you can step over. Also, electric fences typically pulse every 2 seconds, and if they have a short you can hear a pop. If you have an AM radio you should be able to hear the static near the fence. $\endgroup$ – user137 Dec 21 '14 at 8:00
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Trees are not as good an insulator as you might think. This source suggests a typical conductivity of living tree sap is 0.01 S/m with a relative permittivity of 80. So not an insulator, though a poor conductor. Typical advice when using electric fencing is that you do not use wooden posts! Presumably because wet wood is also conductive to some extent.

In any case, all that is required is that the tree acquired an electric potential and that you were more resistive than the path between the fence and you through the tree. It's the "volts that jolt". The current flow through the tree and you, would have been very small.

I would expect that the jolt would be maximised if you touched the tree near where it touched the fence or at least at the same height as where it touched the fence - thus minimising the resistance along the path to you.

EDIT: Oven dried wood has a conductivity of $\sim 10^{-15}$ S/m (i.e. 13 orders of magnitude lower), so it would be fair enough to call that an insulator for most practical purposes.

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    $\begingroup$ In school we were taught it was an insulator :/ $\endgroup$ – Ray Kay Dec 20 '14 at 11:37
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    $\begingroup$ @RayKay I think there would be a big difference between a living tree full of (ionic) sap and dead wood. $\endgroup$ – Rob Jeffries Dec 20 '14 at 11:45
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    $\begingroup$ @RayKay Really, there's no such thing as an insulator. There are things that conduct well, things that conduct not so well and things that hardly conduct at all. Things that conduct well are called "conductors" and things that conduct badly are called "insulators" but even insulators conduct a bit. That's why, for example, you need bigger insulators on a high-voltage power line than on a lead to a 1.5V battery. $\endgroup$ – David Richerby Dec 20 '14 at 15:26
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    $\begingroup$ @DavidRicherby: conductor, semiconductor and isolator do have pretty rigorous definitions. (Strictly speaking, wood meets none of those, since it's a composit too complex to allow determining a particular band structure.) At any rate, isolators in the band-gap definition are, for most practical purposes, isolators in the naïve sense as well. The reason you need bigger isolators on a high-voltage line is not that the isolators themselves aren't good enough, but that you need to keep exposed conductor surfaces apart far enough to prevent arc discharge through the air. $\endgroup$ – leftaroundabout Dec 20 '14 at 18:00
  • $\begingroup$ Re: "It's the 'volts that jolt'. The current flow through the tree and you, would have been very small": I don't follow this reasoning. The voltage drop across the OP is proportional to the current flow through the OP. If the tree had had such high resistance as to cause "very small" current flow, then most of the voltage drop would have been across the tree, not across the OP. $\endgroup$ – ruakh Dec 20 '14 at 20:07
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"Wood is an insulator" is a very broad statement. In fact it's wrong. "Wood is not a good conductor" would be a more accurate statement, but moisture in a living tree would easily carry enough current to give you a shock and have enough resistance not to trip out the breaker/discharge the battery powering the fence.

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  • $\begingroup$ Furthermore, there may be vast differences in the tree from one spot to another. A layer of bark, relatively loose from the tree but in contact with the wire, could be expected to deliver a big jolt. A relatively bark-free tree, where the contact between wire and tree is near the "meat" of the tree, would be expected to carry most of the jolt to ground. $\endgroup$ – Hot Licks Dec 21 '14 at 19:15
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Although dry wood is a relative "good" insulator, a tree is not a good insulator because of the sap/water inside it. Most likely you touched the tree at about the same height as the fence. If the tree has about a 10" diameter, you might have the equivalent of 2" of insulation (and 8" of water), that means most of the voltage will be dropped across you (specially if you are barefooted).

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There is no such perfect insulator. Depending on conductivity materials are loosely classified as conductors, semiconductors and insulators. As Potential difference between two points in the medium increases the material begin to conduct at some point. The nature of the material also crucial here. If it contain ions then charges easily flows. Hence the materials which are normally non conductor may turn into conductor. Air is not a good conductor. But during lightning even air conductos. Wearing shoes and gloves touching the live electric wire is dangerous as they may conduct depending on voltage.

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protected by Qmechanic Dec 20 '14 at 14:19

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