As I understand it, voltage is the difference in charge between two points. The greater that difference, the greater the current because more electrons will flow between the two points.
I thought a high resistance might explain this, but apparently the metal in electric fences have very low resistance.
In: Physics
Current is how much electricity is available. The voltage is how hard you are pushing it (i.e. the “pressure”).
The voltage is high, but the **available** current is low. Meaning, you’ll feel a good shock if you touch it, but if your resistance is too low and you try to draw too much current, the voltage will quickly drop until it is too low to be felt.
This is why things as simple as weeds along the fence can drain too much of the voltage for the fence to work well.
If the available current had been higher, even drawing a lot of current would leave the voltage high – and deadly.
With low available current, the idea is the voltage is high enough to hurt, but will drop quickly enough to not be really dangerous.
Electric fences have a current limiter. So as soon as the current gets to dangerous levels, it drops the voltage or increases the resistance so the voltage drop across your body is lower.
They don’t have low current. It’s that simple, you’ve been lied to.
The secret behind electric fence, tasers, and that doorknob that was whopping 10 kV and inconveniently stung a little is TIME. Not low current. The current is high. The time it existed for is tiny. This limits the damage done and energy unloaded on you.
Now, with a static shock, it’s clear that it is a very short duration. But aren’t tasers and electric fences a constant shock? No. They just pulse on and off really fast with short durations and a duty cycle that is mostly off.
Now, you could describe the time average current as being low. Which despite the high peaks, the long times off results in a low time average current. However, that’s misleading to do that to current, but not voltage. The voltage peaks are high before contacting, but quickly decay so also exist for a very short time. If you time average the voltage, it’s also low. But doing that is a problem, as a low voltage can’t arc or anything and high voltage can exist steady state before making contact at a high value no issue, so you need the high peak voltages to explain that.
Hence, your confusion. A high peak voltage and a low time averaged current is what you have been told, but this is a highly misleading way to state it if you drop to clarifications and confuses anyone familiar with ohms law (rightfully so).
More voltage only means more current if the current source is capable of supplying that amount of current. On an electric fence or some other high voltage / low power source, the voltage will not stay that high once current actually starts flowing, so only a little current will actually flow. That’s in contrast to something like an power line, which has enough juice at the source to keep the voltage (and therefore current) high even when a bunch of current is actually flowing.
It’s kind of like how the tiny sparks thrown up by campfires can’t burn you, because although they’re hot, they’re so tiny that they’ll use up all their heat before they can heat your skin up enough for you to notice. A big block of metal at the same temperature as that spark, however, could burn you and keep on burning you for a long time, because it has more total heat.
You have the right idea, high resistance can limit the current flow. The high resistance in this case is in the power supply itself, it can put out a high voltage, but only at low current. Either the power supply actually has a high resistor in series with the output, or it has a switch that flips off or throttles the output power to limit the current.
There’s a high voltage difference between the fence and the ground, and (as long as nobody is touching it) essentially zero current. The resistance from the fence to the ground is very high, thanks to the air between them, and to the insulated mounts.
There’s practically no voltage difference between one part of the fence and another, because the fence material is an excellent conductor.
When something (a weed or an animal) touches the fence and the ground, it provides a lower-resistance path between them. Current will then flow along the fence to the point of contact, down to the ground, and back to the power source.
Imagine popping a pimple. You have to squeeze really hard (high voltage), but when it pops only a little bit of stuff comes out (current).
The voltage is kept high on the fence so that it can overcome enough resistance to shock you, but there’s just not a lot of current available to shock you that much.
Think of electricity like a river.
A wide and deep river will run slow. That is high current (volume) but low voltage (speed).
A shallow narrow river will run fast. That is low current (volume) but high voltage (speed).
The same river can can fast and slow areas. This is the equivalent of transformers stepping the voltage up or down.
Current and voltage are related to each other. You can trade them back and forth, which is what transformers do.
The big transmission lines run high voltage but low current. The speed of the electricity is very slow in those lines. This reduces losses due to heat, which happens in high current (speed) applications.
To be useful, that electricity needs to be stepped down in voltage from 12,500V to 120V, which increases the current (speed) of the electricity. This allows high current machines such as an electric dryer to function.
*For a given resistor*, voltage and current are proportional. If you lower the voltage of an electric fence then the current will be even lower. But different resistors can have different resistance.
Electric fences have a very high resistance between the fence and everything else, so the current is low even with a relatively high voltage.
You start with the wrong comprehension. The difference between charge is indeed the voltage, but a great difference just means a great voltage.
Imagine a bathtub full of water in a multi story building.
The voltage is the floor the bathtub is sitting on.
The current would be the flow of water once you pull the plug keeping the water in the tub.
The higher the tub the more potential energy it has, but if the pipe letting the water out is as small as a straw, it won’t have much energy coming out (current).
Electric fences are a bathtub on a very high floor with a pipe with barely bigger than the head of a pin.
Latest Answers