I understand basically that the current, or amps, are what cause harm, but based on Ohm’s law, V=IR, for the amps to be so low with such high voltage, the resistance must be very high. So why do you need such a high voltage in the first place? Wouldn’t that require a transformer to step up the voltage? And doesn’t such a high voltage create a safety risk, for instance if a conductor with lower resistance came into contact with the voltage source? If you want low amps as not to electrocute animals/people, would it not be easier to use lower voltage?
Just curious and trying to understand. Electricity is like magic to me.
In: Physics
Adding to what mostly Ndvorsky said (ignore M5K64, his answer is quite bad), the shock is very brief, so even though the voltage/current is high, the duration and energy are both low. The strength of stun guns, for instance, is not measured by voltage/current (aside from bad marketing) and instead in microcoulombs as that is a more valid measurement for such device. Similarly, the effect from a fence does not necessarily correlate with the instantaneous current/amp when you touch it — but instead, the designed microcoulombs that each discharge is able to deliver.
Because generally speaking, of course to a point, voltage isn’t what kills, current does.
Voltage IS what hurts, though. The whole point of electric fences, tasers, batons, etc is to hurt without killing.
Wattage tells you how much electricity there is, simply put.
Wattage is derived from voltage in volts times current in amps.
High voltage power lines of course have lots of current because they have to supply hundreds, thousands, millions of homes. They’re very high wattage overall.
But electric fences don’t really have much of a load, they don’t need much current. Conversely, electric motors like those found on machine tools need relatively low voltage (110, 220, etc are relatively low compared to what’s on an electric fence or on big transmission lines) but need LOTS of current to get the motor to move, sometimes needing a whole circuit in the house dedicated to that one machine.
If you think of electricity like a spring:
Current would be some of the physical properties of the spring. Its material, how big around the spring steel wire is that makes it up, the spacing of coils,
Voltage would be how much that spring is compressed or in tension.
Wattage therefore is derived from those two properties.
You have a correct understanding of ohm’s law. We need high voltage *because* the resistance of people and animals is so high. A lower voltage would not produce any meaningful current and then we wouldn’t feel it.
It may need a transformer or it uses an inductor (half a transformer) to boost the voltage using a DC-DC converter.
If a conductor with a lower resistance touches the wires then it could start a fire. A good shock device should have the ability to detect when it is touching a low resistance object and either limit current artificially or shut down.
>And doesn’t such a high voltage create a safety risk, for instance if a conductor with lower resistance came into contact with the voltage source?
Just pointing this out: An electric fence is not, by nature, a safe device. It’s designed to hurt people/animals. You can try to design something to hurt people in as non-permanently damaging a way as possible, but it’s never going to be as safe as not hurting them in the first place.
It’s just like a barbed wire fence. Something can’t be harmful enough to be a serious deterrent without also carrying some risk for serious injury.
Practical explanation from a sheep rancher: the voltage must be high because the resistance *is* high. Sheep in particular are well insulated, and the return path for the current (the ground) is often dry.
Hard dry hooves on dry ground and a wool covered animal make hard conditions to deliver a shock.
Here’s the electrician part of the explanation. Fence wires are long and cheap, made of high resistance steel. When transmitting power long distances, you lose power to the resistance of the wire. This power heats up the wire. P=I squared R. Notice how voltage is not in this equation.
Now P also = VI. So to deliver the same power over a long wire without losing as much to resistance of the wire, you increase the voltage and decrease the current. So even for easier to shock animals, you want high voltage to cover the miles of fence.
The reason the fence charger is safe is because the *energy* is limited. It is delivered by capacitor discharge. The capacitor is a little energy tank that can deliver high power for a short time. It fills up the capacitor and then delivers that energy to the fence. This is what makes the ticking sound you hear from a fence charger. My fence charger has an **18 joule** capacitor. So no matter what touches the wire, it is getting a maximum of 18 joules of energy delivered to it.
The biggest reason they’re not lethal is that they have a very short duration pulse. Even with very high voltage the total amount of *energy* is miniscule.
It is not voltage that kills, it is current that kills.
Static electricity, what you get from dragging your feet across carpeting or not using a dryer sheet, can be 10,000 volts or higher. It can also hurt when you zap someone or touch a doorknob. It is not dangerous because there is no amperage (current flow).
An electric fence is much the same. It is high voltage, but there is very little current available to flow. Think of it as a water pipe with 10,000 psi, but only a trickle of water is available.
A little less simple: you need to use P= I*V when dealing with transformers. You cannot have a higher P on the output than on the input.
If your input is 100v at .1A (10w) and your output is 10,000v, you cannot produce more than 1mA
Even this assumes a perfect system. In reality, you will have losses in the transformer.
Edit: to whoever down voted me: learn more about how transformers work and why Ohm’s law is not the correct formula to answer this question.
A candlelight is hot (think high voltage). Touch it and it will sting, but it the candle will die (but not you). It doesn’t have enough energy (ampere when talking about electricity).
Dive in to a pool that’s 45deg celcius. It will feel nice and warm, but if you stay in for a longer period, you will die.
An electric fence is there to alarm you, so thats why we need that high voltage.
It’s all to do with the length of the cable. Resistance is proportional to the length of the wire being used, so if you have a long cable like an electric fence the voltage needs to be high enough to overcome the resistance due to length for any sort of current to flow. Most consumer electronics are far too short to really worry about this, but in the case of an electric fence you do have to.
Yeah if you made the cable shorter you would run into arcing issues so you’d lower the voltage to compensate.
You walk across a wool carpet on a cold crisp day and as you reach for a doorknob a static electric spark might jump a quarter of an inch to zap you on your finger. It happens all the time and though harmless it hurts.
The resistance in air is so high that it takes 20,000 volts to jump through air, but the amount of current is trivially small and, though annoying, it’s harmless. It’s the same way with electric fences, and [Van de Graaff generators](https://en.wikipedia.org/wiki/Van_de_Graaff_generator).
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