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#### 关于电能的重大误解

This video was sponsored by Caséta by Lutron.

Imagine you have a giant circuit

consisting of a battery, a switch, a light bulb,

and two wires which are each 300,000 kilometers long.

That is the distance light travels in one second.

So, they would reach out half way to the moon

and then come back to be connected to the light bulb,

which is one meter away.

Now, the question is,

after I close this switch,

how long would it take for the bulb to light up.

Is it half a second,

one second,
1秒
two seconds,
2秒
1/c seconds,
1/c秒
or none of the above.

You have to make some simplifying assumptions

like the wires have to have no resistance,

otherwise this wouldn’t work

and the light bulb has to turn on immediately

when current passes through it.

But I want you to commit to an answer

and put it down in the comments

so you can’t say,

oh yeah I knew that was the answer,

when I tell you the answer later on.

This question actually relates to how electrical energy

get from a power plant to your home.

Unlike a battery,

the electricity in the grid

comes in the form of alternating current, or AC,

which means electrons in the power lines

are just wiggling back and forth.

They never actually go anywhere.

So, if the charges don’t come from the power plant

how does the electrical energy actually reach you?

When I used to teach this subject,

I would say that power lines

are like this flexible plastic tubing

and the electrons inside are like this chain.

So, what a power station does,

is it pushes and pulls the electrons back and forth

60 times a second.

you can plug in a device, like a toaster,

which essentially means

allowing the electrons to run through it.

So when the power station pushes and pulls the electrons,

well, they encounter resistance in the toaster element,

and they dissipate their energy as heat,

Now, this is a great story,

I think it’s easy to visualize,

and I think my students understood it.

The only problem is, it’s wrong.

For one thing,

there is no continuous conducting wire

that runs all the way from a power station to your house.

No, there are physical gaps,

there are breaks in the line,

like in transformers

where one coil of wire is wrapped on one side,

a different coil of wire is wrapped on the other side.

So, electrons cannot possibly flow

from one the other.

Plus, if it’s the electrons

that are carrying the energy

from the power station to your device,

then when those same electrons

flow back to the power station,

why are they not also carrying energy

back from your house to the power station?

If the flow of current is two ways,

then why does energy only flow in one direction?

These are the lies you were taught about electricity,

that electrons themselves have potential energy,

that they are pushed or pulled

through a continuous conducting loop

and that they dissipate their energy in the device.

My claim in this video

is that all of that is false.

So, how does it actually work?

In the 1860’s and 70’s,

there was a huge breakthrough

in our understanding of the universe

when Scottish physicist, James Clerk Maxwell,

realized that light is made up

of oscillating electric and magnetic fields.

The fields are oscillating perpendicular to each other

and they are in phase,

meaning when one is at its maximum,

so is the other wave.

Now, he works out the equations

that govern the behavior of electric and magnetic fields

and hence, these waves,

those are now called Maxwell’s equations.

But in 1883,

one of Maxwell’s former students, John Henry Poynting,

is thinking about conversation of energy.

If energy is conserved locally in every tiny bit of space,

well, then you should be able to trace the path

that energy flows from one place to another.

So, think about the energy that comes to us from the sun,

during those eight minutes when the light is traveling,

the energy is stored and being transmitted

in the electric and magnetic fields of the light.

Now, Poynting works out an equation

to describe energy flux,

that is, how much electromagnetic energy

is passing through an area per second.

This is known as the Poynting vector

and it’s given the symbol S.

And the formula is really pretty simple,

it’s just a constant one over mu naught,

which is the permeability of free space

times E X B.

Now, E X B,
E乘以B
is the cross product

of the electric and magnetic fields.

Now, the cross product is just a particular way

of multiplying two vectors together,

where you multiply their perpendicular magnitudes

and to find the direction,

you put your fingers in the direction of the first vector,

which in this case is the electric field,

and curl them in the direction of the second vector,

the magnetic fields,

in the direction of the resulting vector,

the energy flux.

So, what this shows us about light

is that the energy is flowing perpendicular

to both the electronic an the magnetic fields.

And it’s in the same direction as the light is traveling,

so this makes a lot of sense.

Light carries energy from its source

out to its destination.

But the kicker is this,

Poynting’s equation doesn’t just work for light,

it works anytime there are electric

and magnetic fields coinciding.

Anytime you have electric and magnetic fields together,

there is a flow of energy

and you can calculate using Poynting’s vector.

To illustrate this,

let’s consider a simple circuit

with a battery and a light bulb.

The battery by itself has an electric field

but since no charges are moving,

there is no magnetic field

so the battery doesn’t lose energy.

When the battery is connected into the circuit,

its electric field extends through the circuit

at the speed of light.

This electric field pushes electrons around

so they accumulate on some of the surfaces of the conductors

making them negatively charged,

and are depleted elsewhere

leaving their surfaces positively charged.

These surface charges

create a small electric field inside the wires,

causing electrons to drift

preferentially in one direction.

Note that this drift velocity is extremely slow

around a tenth of a millimeter per second.

But this is current,

well, conventional current

is defined to flow opposite the motion of electrons,

but this is what’s making it happen.

The charge on the surfaces of the conductors

also creates an eclectic field outside the wires

and the current inside the wires

creates a magnetic field outside the wires.

So, now there is a combination

of electric and magnetic fields

in this space around the circuit.

So, according to Poynting’s theory,

energy should be flowing

and we can work out the direction of this energy flow

using the right hand rule.

Around the battery for example,

the electric field is down

and the magnetic field is into the screen.

So, you find the energy flux is to the right

away from the battery.

In fact, all around the battery,

you’ll find the energy is radially outwards.

Energy is going out through the sides of the battery

into the fields.

Along the wires, again,

you can use the right hand rule

to find the energy is flowing to the right.

This is true for the fields along the top wire

and the bottom wire.

But at the filament,

the Poynting vector is directed in toward the light bulb.

So, the light bulb is getting energy from the field.

If you do the cross product,

you find the energy is coming in from all around the bulb.

It takes many paths from the battery to the bulb,

but in all cases,

the energy is transmitted

by the electric and magnetic fields.

– People seem to think that you’re pumping electrons
-人们可能认为你在抽取电子
and that you’re buying electrons or something,

which is just so wrong. (laughs)

For most people,

and I think to this day, it’s quite counterintuitive

to think that the energy is flowing through the space

around the conductor,

but the energy is,

which is traveling through the field,

yeah, is going quite fast.

– So, there are a few things to notice here.
-因此 需要注意的是
Even though the electrons go two ways

away from the battery and towards it,

by using the Poynting vector,

you find that the energy flux only goes one way

from the battery to the bulb.

This also shows it’s the fields

and not the electrons that carry the energy.

– How far do the electrons go

in this little thing you’re talking about,

they barely move,

they probably don’t move at all.

– Now, what happens if in place of a battery,
-那么 如果我们在电池里
we use an alternating current source?

Well then, the direction of current

reverses every half cycle.

But this means that both the electric and magnetic fields

flip at the same time,

so at any instant,

the Poynting vector still points in the same direction,

from the source to the bulb.

So the exact same analysis we used for DC

still works for AC.

And this explains how energy is able to flow

from power plants to home in power lines.

Inside the wires,

electrons just oscillate back and forth.

Their motion is greatly exaggerated here.

But they do not carry the energy.

Outside the wires,

oscillating eclectic and magnetic fields

travel from the power station to your home.

You can use the Poynting vector to check

that the energy flux is going in one direction.

You might think this is just an academic discussion

that you couldn’t see the energy as transmitted

either by fields or by the current in the wire.

But that is not the case,

and people learned this the hard way

when they started laying undersea telegraph cables.

The first Trans Atlantic cable was laid in 1858.
1858年铺设了第一条跨大西洋电缆
– It only worked for about a month,
-它只工作了大约一个月
it never worked properly.

– There are all kinds of distortions
-当人们试图发射信号时
when they try to send signals.

– Enormous amounts of distortion.
-巨大数量的失真
They could work it at a few words per minute.

– What they found was sending signals
-他们发现 传输的信号
over such a long distance under the sea,

the pulses became distorted and lengthened.

It was hard to differentiate dots from dashes.

To account for the failure,

there was a debate among scientists.

William Thomson, the future Lord Kelvin,

thought electrical signals moved through submarine cables

like water flowing through a rubber tube.

But others like Heaviside and Fitzgerald,

argued it was the fields around the wires

that carried the energy and information.

And ultimately,

this view proved correct.

To insulate and protect the submarine cable,

the central copper conductor

had been coated in an insulator

and then encased in an iron sheath.

The iron was only meant to strengthen the cable,

but as a good conductor,

it interfered with a propagation of electromagnetic fields

because it increased the capacitance of the line.

This is why today, most power lines are suspended high up.

Even the damp earth acts as a conductor,

so you want a large insulating gap of air

to separate the wires from the ground.

to our giant circuit light bulb question?

Well, after I close the switch,

the light bulb will turn on almost instantaneously,

in roughly 1/C seconds.

So, the correct answer is D.

I think a lot of people imagine

that the electric field needs to travel

from the battery,

all the way down the wire

which is a light second long,

so it should take a second for the bulb to light up.

But what we’ve learned in this video

is it’s not really what’s happening in the wires

that matters,

it’s what happens around the wires.

And the electric and magnetic fields

can propagate out through space

to this light bulb,

which is only one meter away in a few nanoseconds.

And so, that is the limiting factor

for the light bulb turning on.

the entire voltage of the battery immediately,

it’ll be some fraction,

which depends on the impedance of these lines

and the impedance of the bulb.

and got kind of different answers,

but we all agreed on these main points.

So, I’m gonna put their analysis in the description

If I get called out on it

and people don’t think it’s real,

we can definitely invest the resources

and string up some lines,

and make our own power lines in the desert.

– I think you’re gonna get called out on it.
-我认为你需要说出来
– I agree, I think you’re gonna get called out.
-我同意 我想你需要说出来
(laughing)
[笑]
I think that’s right.
-我觉得对
– I think it’s just kinda wild
-我觉得有点奇怪
that this is one of those things

that we use everyday,

or knows the right answer to.

These traveling electromagnetic waves around power lines

are really what’s delivering your power.

Hey, now that you understand

how electrical energy actually flows,

every time you flick on a light switch.

And if you want to take your switches to the next level,

the sponsor of this video, Caseta by Lutron,

including switches, remotes, and plug-in smart dimmers.

And since one switch can control many regular bulbs,

you can effectively make all those bulbs smart

just by replacing the switch.

Then, you can turn your lights on and off

or you can use another device

Caseta works with more leading smart home brands

than any other smart lighting control system.

One of the things I like is setting timers.

The lights in my office for example,

turn on by themselves every evening.

And this feature gives you peace of mind

will always come home to a well-lit house.

And once you’re already in bed,

you can check which lights you forgot to turn off

and do that from your phone.

Installation is easy.

Make sure you turn off power to the switch first

and then disconnect the existing wires

and connect Caseda’s smart switch.

If you need any help,

they’re just a click or a call away.

lutron.com/veritasium.
lutron.com/veritasium.
I will put that link down in the description.

So, I want to thank Lutron Electronics

and I want to thank you for watching.

nis