If Birds Can Sit on 72,000 Volt Power Lines, Why Can't You?（原声文稿）

Right now, somewhere near you, a bird is sitting on a power line with enough voltage to kill a person many times over, and it's completely fine.

It might even be up there grooming itself, or it could even be sleeping perfectly safe.

Now listen to this. The wire beneath its feet is not insulated.

There's no rubber coating on it, no plastic, no protective sheath of any kind.

So when a bird is up there on a high voltage power line, it's basically hanging on to bare metal wire with an electric potential of thousands of volts and carrying alternating current back and forth humming along with enough electrical energy to power a small neighborhood.

And here's the thing, that bird does not have so much as a scorch mark on its claws. Basically doesn't notice the electricity at all. Now we're always taught growing up be careful for electricity.

Don't grab a live wire So this seems weird to us that birds can hang out up there now to understand why it's okay for them You need to understand something about how electricity actually works And once you do the answer turns out to be one of the most elegant ideas in all of physics today We're gonna answer the question.

How do birds sit safely on high voltage lines without being electrocuted?

So to understand what's going on here, first we have to talk about electricity and we have to learn about the difference between AC and DC and I'm gonna do this through a few everyday analogies. So what is electricity anyway?

Basically electricity in its simplest form is the movement of electrons through a conductor and the easiest way to really understand that visually because you can't really see electrons is to think about the flow of water.

I want you to first imagine a water tank that's elevated on one of those tall towers. Now it's connected to a pipe that's going into a lower tank on the ground.

So you have a high tank, a pipe that's carrying water down to a lower tank.

So water flows from the high tank to the low tank, right?

You take that for granted, but it's doing it because of the difference in height of the water tanks, basically, which creates pressure.

Ultimately, it's coming from the gravitational force acting on the water.

Basically, to work in the everyday language, what we say is gravity is exerting a gravitational potential, a potential difference in gravitational potential energy between the two heights.

And that leads to a pressure which forces the water in the downward direction.

Now in electricity, even though it's electrons and it's obviously different, the voltage is what's acting sort of like the pressure. So when you think of voltage, I want you to think of the word pressure.

So think about the difference in electrical potential instead of gravitational potential electrical potential between two points Now electric current is what is actually flowing the flow of electrons through the wire Equivalent to kind of like the water flowing through the pipe So that's voltage and that's current finally we have resistance Resistance is anything that restricts the flow of current or the flow of water in a pipe For instance, if the pipe gets narrow, so there's a smaller cross -sectional area, then you're going to restrict the flow of water and you would be increasing the resistance. So very important to summarize.

When you have a high tank of water and a low tank of water, the water that's way up high has a high gravitational potential.

When you get to the lower level, we have a lower gravitational potential.

And the difference in the potential is what's really driving the water down, because effectively, that's the force that we call pressure that's pushing it in that downward direction all coming from gravity.

Now in the case of electricity, the pressure is replaced by voltage.

That is ultimately coming from the electric field.

In a battery, it comes from ions and electrons which are charged and pushing on the charged electrons.

So we call it an electric potential which we rename in terms of circuits, a voltage.

So the voltage, when you hear the word voltage, think pressure, think force, right? But what is actually flowing? Well in water, it's the water flowing, that's the current. In an electric circuit, it's the current that's flowing the electrons.

So voltage is the push or the pressure, and electrons flowing and moving, that's the electric current. So we say current flows.

We never say voltage flows because voltage is a pressure.

It isn't moving, it's just the difference in pressure is what drives the current to flow.

And And when the current flows, if it encounters obstacles in a circuit, they're called resistors or other elements that restrict the current flow, then that's going to tend to restrict the flow of current just like a narrow bend or a narrow part of a pipe.

And then finally, there's the famous Ohm's law, which relates voltage, the push, current, what's flowing, and resistance, the obstacles. V equals I times R, right?

So the voltage across any part of a circuit is equal to the current moving through that part multiplied by the resistance.

And if you solve that equation for current i equals v over r then what you're calculating is the current flow is equal to the voltage divided by the resistance.

If you increase the voltage, you increase the push and you increase the current, right?

If you increase the resistance, the denominator i equals v over r, then you increasing the resistance and then the current flow goes down.

So now that we have that out of the way, the critical rule here is this.

Current only flows in a circuit when there is a difference in voltage.

That's kind of like being a difference in pressure if you're thinking about water between two points. And if there has to be obviously a path connecting those two points.

If there is no voltage difference between two points in a circuit, that would be called a voltage drop.

If the voltage drop is zero, then you get no flow of current because it would kind of be like a section of pipe without any pressure drop.

If there's no pressure drop across some volume of water, then there's no force to push it.

So no pressure drop, no water flow, no voltage drop, no current flow. So both of these conditions must be met.

This is the fundamental law that governs every circuit.

You have to have a voltage drop and a path connecting two points for current to flow.

and it's also the law that keeps this bird alive when they're sitting up on a power line.

Now when you have this kind of electricity, what we're talking about here in the beginning, which is a steady flow of electrons in only one direction, that's what you've probably heard referred to as DC or direct current.

It means it goes in one direction, never changes direction, just keeps going.

So for instance, a battery produces DC, current flow in only one direction.

The positive terminal is at a higher voltage the negative terminal is at the lower voltage, and electrons flow from one side to the other, through whatever circuit element, you know, wire or something is connected between them.

The bottom line is you have to have a potential difference or a voltage difference between the two sides of the battery, and that's what the battery is doing.

Ultimately, there's a chemical reaction in there involving electrons moving, and that's providing the voltage that's driving the circuit.

it, and when the battery runs down or is dead, what's happening is that chemical reaction is exhausted and so no more electrons flow.

That way I always start to talk about DC because that's where the discussion starts, but the power grid where the birds are does not use DC.

It uses something called AC called alternating current, and there's a very good engineering reason for that.

Basically, in alternating current, the word alternating means to move back and forth, right?

The voltage in those circuits does not hold steady in one direction like it does in direct current.

What it does instead is it cycles or oscillates back and forth, reversing polarity in direction 60 times per second in the United States, other frequencies around the world.

So the electrons in the wire basically oscillate in direction, literally reversing course back and forth rather than steady travel in one direction.

Now it might seem weird to even do that in less useful but actually alternating current has an enormous advantage over DC.

Basically one of the big advantages is the voltage can be transformed.

You ever heard of a transformer and not not the cartoon in terms of electricity?

So basically you can take alternating current at one voltage and then you can change it using a transformer to a completely different voltage with very high efficiency right with very little loss and with no moving parts, no mechanical thing moving.

You can't really do this very easily with DC, but it makes transmission of electricity over long distances much more viable.

Basically for the transformer to work, you have a primary winding of coil and a secondary winding or coil.

And you're driving on one side and then you have some voltage coming out the other side.

What's linking the coils, they're not physically touching, they're wrapped, but they're not literally connected to each other, they're just in close proximity.

the magnetic field coming from the electric current in one coil is inducing a voltage in the other coil.

And by adjusting the number of turns, how many turns the coils are, you can change the voltage to go up in voltage or down in voltage, and so the voltage is transformed. That's why it's called a transformer.

But basically, because of the physics of how it works, the law of induction, the voltage must be changing all the time to have a changing magnetic field to do the voltage transformation at all.

So for DC, it wouldn't work because in order to switch the voltage up and down, you need to have an alternating voltage changing the magnetic field all the time.

Now you might ask, why are you using AC for power lines and why does the voltage matter in any of this?

Basically, if you think about it, when electricity travels long distances, you know, dozens of miles or hundreds of miles over very long distances through wires, The wires themselves, even though they're made of metals that have low resistance, they basically waste energy as heat. That's the resistance of the wire. And the higher the current, the worse the loss.

So the trick here to transport a large amount of energy and electricity is we want to do it at a very high voltage and at the same time a very low current.

So it's the same total power but is distributed very differently.

Then, when you get down close to the neighborhoods, what you want to do is you want to take the voltage and bring it back down to a lower voltage at a higher current near the destination where it's actually needed.

And the only way you can do that, at least efficiently over long distances, is to use alternating current and transformers to change the voltage up and down. So basically, you see the power lines up there.

They're alternating current, the transformers, the cans that you see are transforming the voltage. but why are there usually three wires on those things?

That's called three phase power.

So when you see that cluster of wires strung between poles, we're talking about three phase, and we wanna talk about why we need that.

So you see the power grid doesn't use simple single phase AC.

That would be like a single sinusoidal electricity voltage.

For large scale power and distribution, it uses three phase alternating current.

Basically, instead of one wave of alternating voltage, which would work, a three -phase power uses three separate oscillating voltages in three separate wires. That's why when you look up there, you always see multiple wires up there, right?

Now they're all carrying sinusoidal voltage, but they're offset relative to each other by getting into the weeds here a third of a cycle. So you kind of have overlapping waves is kind of the way to look at it. to graph it, you'll have sort of these overlapping sine waves.

So for instance, if you picture a sine wave, a smooth repeating curve, right?

Then three phase power is basically three of those curves running simultaneously in different wires.

Each one shifted 120 degrees from the last one right before it so that when one wave is at its peak, the other two are at intermediate and lower points in their cycle.

Now you're probably saying, why do they make it so complicated? Why do we need to do that? well basically it's more efficient.

Three phase power delivers electricity more smoothly and continuously than single phase power.

It can also carry significantly more power for the same amount of wire and it's really the standard for motors and industrial equipment. It's also more stable over long distances.

I'll tell you just real quick I want you to pretend that we have a pulley in front of me.

Just a piece of a rope going over a single You know circular pulley at the top right and comes down the other side if I held one hand behind my back So I only had one phase meaning my one hand and I wanted to raise the block on the pulley only with one hand What would I do I would pull it and pull probably grab it with my knees So it would fall and then I would reach back up pull it again pretend It doesn't fall reach back up pull it again So you see I'm gonna make it all the way to the top But you see I'm wasting a lot of time every time I let go and come back up I'm wasting time because I can only pull it once I grab it again.

Now pretend instead that there's actually three people and they're all standing in a circle around the single rope that I'm trying to lift this weight, right?

So a more efficient way to do it would have one person grab and pull down.

As soon as that person gets to the bottom, the next person has his hand positioned higher, grabs it and pulls it down.

And then while I'm moving this hand up, the third person grabs It's exactly the right time when this one gets close to the bottom. So there's some overlap, right?

They grab right before the previous person gets to the bottom.

And as it goes around, everyone's taking turns pulling this thing.

So from an outside point of view, you're going to lift the weight much more smoothly because there's never any herky -jerky motion when I'm letting go. It's like grab, grab, grab, grab, grab, grab. That is the best way to do it. Even with two hands, you can see it would be like much better.

It's easier to do it in three phases, especially for rotating equipment, any kind of motors, anything like that, because it's just going to run smoother.

It's going to be less vibration, more efficient, and easier to transmit the power. Now here's the crucial thing.

All of these three wires carrying three phases of AC, they're all at high voltage relative to the ground, but they're at different voltages relative to each other and at any given instant because their waves are offset in time.

remember the sine waves are just slightly overlapping and this is actually going to matter for our bird.

Before we finish up with the bird it's actually really important to talk about the scale of the voltages that we're talking about in these transmission lines because they're actually very different and very cool the way the system is designed.

You probably see those massive steel towers like in the countryside the really really big ones the tall ones they're carrying transmission lines, right?

The long -haul highways of the electrical grid.

Basically, this is where the bulk of the very high voltages.

Those lines operate at voltages between 69 ,000 volts and up to even 700, 765 ,000 volts.

Believe it or not, some of those lines in the United States are really big ones like between states and such. They can operate at around 500 ,000 volts as standard.

Now the wires are thick, uninsulated cables.

There's nothing around them, right? Like plastic sheathing or anything like that.

They're often bundled in groups and slung between towers that are even up to 150 feet in the air.

Those guys, the really big ones, their sole purpose is to move enormous quantities of electrical energy over hundreds of miles from power plants to the regional substations.

And then the extreme voltage is what makes that actually possible to do it efficiently without losing too much energy along the way.

Now, when you get to a substation, of course, that voltage, that super high voltage, is not really good for houses.

So the transformers at the substation step that voltage down significantly.

And what arrives at your neighborhood on those familiar wooden poles that you see in the entrance of the neighborhood is distribution level voltage.

Usually it's around 4 ,000 volts and 13 ,800 volts depending on the utility.

A really common standard in the United States is 7 ,200 volts on the primary distribution line. That's the uppermost wire on the neighborhood pole.

Now obviously this is still far more than enough to actually kill a person instantly and repeatedly.

It's just dramatically lower than what was on the transmission lines from the power plant.

Now I mentioned this before, but those metal cans which are on those power lines you see in the front of the neighborhood, those are the transformers.

You know a lot of people think they're batteries or some kind of switch.

They're actually transformers and they're doing the last critical voltage reduction in the entire system.

Now each of those canisters having the transformer is oil filled, filled up with oil sealed and contains two sets of wire coils.

Remember I mentioned the primary and the secondary coil and they're wrapped around an iron core because iron will concentrate the magnetic field and basically help everything become easier to change the voltage up and down.

The primary coil is connected to the 7 ,200 volts from the distribution line and the secondary coil is wound with a different number of turns and the turns ratio basically determines if the voltage is going to go up or it's going to go down of course going into a neighborhood we want to bring it down.

Now the output of that That secondary coil is what runs the three phase wires down to your house.

And those are actually two insulated wires, each carrying 120 volts.

And they're 180 degrees out of phase with each other, and there's also a bare neutral wire.

Now if you measure either of those hot wires to the neutral, you'll get the 120 volts. That's the familiar house voltage that we talk about all the time.

That's what powers your lights and all of your wall outlets.

But, if you measure across the two hot wires, not to the neutral, but the two hot wires, you'll get 240 volts.

That's what powers your dryer, your electric range or stove if you have that, and your air conditioner. Much, much higher loads.

Now, with all that out of the way, how can birds be up there hanging on to an un, you know, shielded wire, just bare metal, with so much electricity going on there and not get totally destroyed and scorched and catch on fire?

basically what's really happening is when a bird lands they have a single power line and a bird sitting on a single power line has both feet grabbing on to the same wire a few centimeters apart.

Now that wire is at some particular voltage here in the United States it's usually near a neighborhood at 7 ,200 volts on some secondary neighborhood distribution line.

So there is electric current flowing through the wire and there is a potential difference of voltage between one end of the wire, way back at the substation and over where the transformer is on the pole.

But that bird's left foot is at 7 ,200 volts, and its right foot is also at 7 ,200 volts because it's on the same wire a few centimeters apart.

So the voltage difference between the two feet of the bird is basically almost exactly the same.

And remember what we said at the beginning, if there's no voltage difference, no voltage drop, then no current flow.

Remember Ohm's law, I, the current is V over R, but if the voltage drop, that's what V is, the voltage drop is zero over such a small distance in a almost perfectly conducting wire, it's not perfect but pretty close, then the voltage drop is zero and zero divided by anything is going to be basically zero.

So there's no current flow through the bird because there's no voltage drop across the bird.

And so if there's no electric current flowing through the bird's body and then he's not going to be electrocuted because it's really not the voltage that kills you.

It's the movement of the electrons that's destroying you internally and what we call electrocution.

So the bird is sitting at a high voltage but is experiencing no electrical stress whatsoever because there's no voltage drop across his body or through his body.

And so no current is detouring around and up through the bird.

It's all just staying internal to the wire and so it's because it's not detouring and going around the bird, electricity is always going to take the path of least resistance through the perfect electrical conductor there and not through the bird and there's no voltage drop directly across the bird or very negligible so he's not electrocuted. Now here's where things go sideways.

The moment any animal, bird, raccoon, anything, bear, we've seen videos of bears, that simultaneously contacts two conductors at different voltages, like two wires that are close to each other on that on that pole strung up, then the circuit closes between two different voltages.

Remember the wires are carrying the same sine wave but shifted in time and so the voltage at any given point in time between the two wires is very different.

So if you actually go out and connect across two of the wires then you will obviously be very electrocuted because in that case the body becomes the path between those two voltage levels and then current flows, then we get electrocuted.

So for small songbirds whose feet are only a few centimeters apart on a single wire, this is almost never an issue and they think they're just perfectly safe, no problem at all.

But what if it's a large raptor or an eagle or a hawk or an owl or a vulture or some other large mammal that climbs up there with have a wingspan or a stretch wide enough basically on that utility pole they land awkwardly let's say and they simultaneously just clip it with their wing they touch one while the foot touches the other wire well then you're bridging two different conductors with very different voltages you're gonna have a bad day.

Now the train lineman working on a live distribution line can only do it safely if he's insulated from ground and does not bridge two different voltages simultaneously so obviously they're gonna take their time doing that.

So I know I meander through the weeds talking about distribution and transformers and all the stuff because it's fun to kind of go a roundabout way to get to our answer.

But basically, to wrap it all up, the bird is not lucky.

It's not protected by some special property of feathers or bird biology or any special coating on the wires.

It's protected by the most fundamental rule in electrical engineering.

And that is that electric current flows only when there's a voltage difference or a voltage drop and a complete path between the two places, between the two points, right?

If you're sitting on a single wire, it has neither one of those conditions.

As an analogy, it would be as safe if you were a bird, don't do this yourself, but if you're a bird, it would be as safe as it would be sitting on a park bench, which is placed on top of Mount Everest.

The entire body on top of a mountain would be thousands of feet above the ground.

So obviously, if you jump off, you're gonna have a bad day but if your entire body is at that high potential then you have no risk of falling off the mountain unless you're just clumsy and jump off the edge right so it's very similar the electric potential being the same for the bird being okay on the wire and being on top of a mountain you have a very high gravitational potential but as long as you're just staying there not moving then you're you're not going to jump and experience the fall to the bottom.

So the bottom line is the bird just happened to find the one place in the system where the voltage difference is zero and so not electrocuted.

And as you look at a bird sleeping on a power line and wonder about all of this stuff, always remember to stay curious.

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