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, two seconds, 1/c seconds, or none of the above. You have to make some
simplifying assumptions about this circuit, 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 to your home, 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. Now, at your house, 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, and so you can toast your bread. 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, 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, then your thumb points 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 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. 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, 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 could 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. - It only worked for about a month, it never worked properly. - There are all kinds of distortions when they try to send signals. - Enormous amounts of distortion. They could work it at
a few words per minute. - What they found was sending signals 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. So, what is the answer 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. Now, the bulb won't receive 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. Now, I asked several
experts about this question, and got kind of different answers, but we all agreed on these main points. So, I'm gonna put their
analysis in the description in case you want to learn more
about this particular setup. 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. - I agree, I think you're
gonna get called out. (laughing) I think that's right. - I think it's just kinda wild that this is one of those things that we use everyday, that almost nobody thinks about 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, you can think about that every time you flick on a light switch. And if you want to take your
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Electromagnetic waves, Poyning vector -- it is all very fine, and true, but they obfuscate a more basic fact: the "flow of power" is an equally unobvious concept even in simple mechanical systems!
Once upon a time, there were power companies that instead of electricity distributed power literally by pumping pressurized water through a network of pipes. In many ways water flow is exactly the same thing as the flow of electrons, only without generation of the electric and magnetic fields around the conductors. The flow is the current, the pressure difference is the potential difference.
If water was being pumped in a closed loop, like electricity, exactly the same arguments as in the video can be applied to show that the direction of the flow of water does not tell which direction the energy flows in in the system.
By looking at the flow of water in a single pipe, there is no way to deduce which way the hydraulic energy went! To see how the energy is being transferred in the system, we have to zoom out, and see simultaneously the flow and the pressure difference that it crosses -- just like in the case of electricity, where power is current times the potential difference. If we can simultaneously see the two conductors of the closed circuit -- one carrying the current from the source to the load, and another returning the current back, then we can see both the flow and the potential difference, and only that allows us to determine how much energy passes through the pair and in which direction. It makes no difference whether we consider the flow of water or the flow of electrons.
Thus the difference between the system level "flow of power" and the localized "flow of stuff" through the circuit is already present in simple, mechanical systems. This is not specific to electricity -- electricity simply adds many beautiful and confusing phenomena on top of this more basic conceptual difficulty.
The top answer in r/physics nails it, I think:
https://reddit.com/r/Physics/comments/qxyz3m/_/hld9g8f/?context=1
Quote: "As soon as you close the switch, current flows from the battery, producing a magnetic field around the wire. This changing magnetic field will induce a current in the wire on the light bulb side (Faraday's law of induction) after the speed of light propagation delay across the gap: (1m/c) seconds (assuming a vacuum)."
I think it's a bit of a sneaky/trick question designed for an undergrad physics exam.
This reminds me of the saying that all models are wrong but some are useful. Analyzing power flow in terms of the Poynting vector is very useful. Thinking about fields surrounding wires is very useful. Thinking about circuits in terms of the flow of charges is also very useful, particularly if you want to design a semiconductor.
And, if you want to drive engagement in your YouTube channel, stirring up controversy is very useful. But that's not necessarily the best way to improve people's understanding.
Disclaimer: I am not an engineer
How can this be true? Information fundamentally cannot travel faster than the speed of light right?
If the light is able to turn on faster than 1 second, then that would be information propagation faster than the speed of light, which shouldn't be possible..
What am I missing here?
This video illustrates why RF electronics will continue to be black magic to some people.
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This video is misleading. Though there will be a non-zero amount current traveling through the bulb after >1/c seconds, it would not be enough to see the bulb light with your naked eye. The current through the bulb will slowly increase as the current travels down the line and all the way back around to the bulb, taking essentially 1s to reach full brightness.
His explanation of electrons not providing energy is also misleading. You absolutely 100% need electrons to be flowing through an electrical element in order to transmit electrical power to it. The fact that AC transformers can connect 2 circuits through thin air or even vacuum does not change this. If you swapped out the wires on the bulb side with a non-conducting material the bulb will not light, even though the same amount of flux is reaching the bulb.
You can literally take a computer on a high frequency clock, swap out different lengths of wire, and measure the difference in delay between two components without either component moving. Solid state phased array antennas operate entirely on the principle that you can change the signal speed between your source and your emitters on a fixed circuit. You don't get to just entirely throw out the effect of current proportion down a wire just because fields permeate a vacuum. The conclusions he comes to are measurably wrong, or at least intentionally misleading.
If this is right and it’s the fields that carry the energy, would that not mean if you were to just leave a bulb beside the wire without attaching it, it would light up as well ?
It’s a very pedantic sort of gotcha video which kinda grinds my gears. I’m just MechE so I’m admittedly out of my depth here but I think the critical piece of information that is needlessly obfuscated in a sense is that the information front does not actually need to travel out all the way around the circuit. It merely needs to travel across the 1 meter gap from the switch to the bulb and then you have no causality issues.
1/c is just a sneaky cancellation of distance units because the problem is setup to have a distance of only 1m
Out to the moon and back is a red herring thrown in there to distract. How “wide” the circuit is doesn’t matter. How “long” it is is the more important issue. In this example they played the game of making the width the longer dimension and the length the shorter dimension
It’s really the (distance between switch and detector)/c
IE if you would move the bulb to the moon the overall length of the circuit would remain the same but now the time to turn on will have increased to the (distance to the moon)/c even though the circuit topology is unchanged.
He’s isn’t wrong and in fact is explaining an important concept it just could have been tackled very differently.