Naked Science Forum
Non Life Sciences => Technology => Topic started by: chris on 22/04/2009 15:40:42
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How do electric fences, for instance of the type used to confine animals to a certain part of a field, actually work? What voltage is used, is it AC or DC, and what prevents it from being lethal?
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They use bursts of AC at a few kV , using low voltage circuit and a step up transformer. You can hear them click every so often. At one time they used mechanical interruptors. Now they would be electronic. They are far from lethal because the maximum current is very low - try it one day if you're feeling 'ard. It only makes you jump*. I hate shocks, personally but I do know people who do it for fun.
The cattle get very smart about them. Often you don't even need to leave them switched on - they learn.
*Try using a bit of green grass as a go between.
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DON'T PEE ON IT!!!
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Presumably you need an inverter to produce the AC because usually they run off a car battery don't they?
Chris
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They used to use a 'ringing' circuit and an interruptor. When the circuit is broken by a mechanical timeswitch, an LC resonator will 'ring' as the energy stored in the capacitor dissipates and this low voltage, decaying, oscillation is what is transformed to a high voltage. * It is a very simple form of inverter circuit which does the job excellently. Nowadays, it's anyone's guess how they choose to produce the bursts of high volts - there are any number of switch mode modules which will do the job. I imagine health and safety need to give a system 'type approval'.
*Just like the old car ignition circuits. The primary of the spark coil resonates with the 'condenser' when the contact breaker opens and the 400V back emf spike(s) is (are) transformed to several kV to make the spark.
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DON'T PEE ON IT!!!
I once peed under an electric fence and decided to check after. Good thing I did not hit it. However, Myth Busters did a show once on peeing on high power rails. They found that the stream is disrupted too much to conduct electricity. It turns out if you are closer (as you would be when peeing on a fence)it works.
http://kwc.org/mythbusters/2004/06/return_of_mythbusters.html
"Third Rail: This wasn't a revisiting as much as it was a variation. The myth the first time around was that you could get shocked by peeing on an electrified third rail. Their slow-motion video from the first time showed that the stream breaks apart too much falling to the ground. This time around they built an electric fence with small charge, which would be closer to the person peeing. Once again, Adam volunteered as the guinea pig and peed directly onto the fence. It was only a couple seconds before he felt a little shock."
The cattle get very smart about them. Often you don't even need to leave them switched on - they learn.
How is it smart if they respect the fence even if it is turned off? [;)] I know what you mean - I just could not resist.
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Not as smart as US!
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When I used to receive radiofax news pictures on 119 KHz (thats going back a bit!) these electric fences were an annoying source of spots on the pictures and had to be airbrushed out (old style).
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How would the movement of the urine in the "stream" affect the time take before a shock was felt, or would it?
In other words, how fast do the ions in the urine move to conduct electricity and is this in the same ball-park as the velocity of the stream itself?
Chris
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Chris- it's not the speed of the ions which affects how fast you get the 'jolt'. The Electric field changes at near the speed of light. Even in a metal, the electrons only move at mms per second.
The ions could well be moving slower upwards than the stream is falling downwards but there will still be an ion or two available right at the end of J Thomas to cause a shock. (I don't like to think about that too much.)
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Sure, but what I was asking was more academic than practical - would the "shock" be detected more promptly in a static column of pee than a moving stream?
Chris
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I've read various values for the speed of a signal in a conductor : from 0.1c to 0.7c (http://van.physics.illinois.edu/qa/listing.php?id=3341).
So the pee stream velocity would be negligible in comparison with those speeds .