"Veritasium" (YT) - "The Big Misconception About Electricity" ?

[bdunham7]

I struggle to reconcile the suggested explanation with cutting the wire about half-way along either loop.  Would the bulb still light up?  If so, would it light up for the time required for the "information" about the wire break to be 'observed' by the rest of the system?  If not, does it not violate the speed of light if instead it never lights up, allowing the passing of information at faster than c?  How do you reconcile the explanation with the knowledge that 'electricity' travels at about 2/3 to 1/2 the speed of light in copper?

If Ve is truly suggesting that the energy is coupled purely by the electromagnetic field I would have thought the coupling efficiency would be too poor to see any significant light from the bulb.  I appreciate the energy must be coupled through the cable somehow, but in any case, it must follow the length of the cable to avoid violating c, unless I am missing something pretty obvious.

Yes, the problems abound.  I didn't bother with the velocity factor issue because it isn't the main problem.  In a conductor the fields are concentrated along the wire, not through space--that's the whole point of having wires.  The coupling and initial response would be much weaker, likely not observable for some time quite a bit longer than the "1/c" value and certainly not enough to light up the bulb, not even a little bit. 

[nixxon]

Haven't watched the video yet, is the point basically: energy flows in the fields around/between wires?  Because that's pretty accurate.

Yes, this is the point of the video.

The premise is that if you place a bulb 1 m away from a battery, but make the wires take a very, very long path between the battery and bulb terminals, the bulb will still start to light up with a propagation delay of 1 meter/speed of light, since whatever path the wires take, the electromagnetic fields will ultimately couple the bulb to the battery over the shortest distance.

Regarding the alternative "D" answer (1/c seconds): The problem is that the distance between the voltage scource and the bulb was never stated in the video. (as far as I have noticed). Even if it had been stated, and the theory right, the answer would not be exact, due to inaccuracies in distance, medium of energy propagation etc. And where is the center of a lead acid battery energy scource? Is it at the mid point between the poles? And what if the switch in this circuit was ~half way to the moon? (1/2 light seconds away). Would the bulb still light up 1/c seconds later?

And at what exact moment does a bulb start glowing...

[ejeffrey]

I struggle to reconcile the suggested explanation with cutting the wire about half-way along either loop.  Would the bulb still light up?  If so, would it light up for the time required for the "information" about the wire break to be 'observed' by the rest of the system?

Yes.

How do you reconcile the explanation with the knowledge that 'electricity' travels at about 2/3 to 1/2 the speed of light in copper?

That's not true.  "electricity" travels at the speed of light in the dielectric where the fields exist.  For common dielectric insulation and geometries where most of the field is in the insulation that gives a velocity factor around 2/3.  But if most of the fields are in air/vacuum the propagation is at c.  The copper doesn't matter much because there are almost no fields in the copper.

[Cerebus]

I struggle to reconcile the suggested explanation with cutting the wire about half-way along either loop.  Would the bulb still light up?  If so, would it light up for the time required for the "information" about the wire break to be 'observed' by the rest of the system?  If not, does it not violate the speed of light if instead it never lights up, allowing the passing of information at faster than c?  How do you reconcile the explanation with the knowledge that 'electricity' travels at about 2/3 to 1/2 the speed of light in copper?

If Ve is truly suggesting that the energy is coupled purely by the electromagnetic field I would have thought the coupling efficiency would be too poor to see any significant light from the bulb.  I appreciate the energy must be coupled through the cable somehow, but in any case, it must follow the length of the cable to avoid violating c, unless I am missing something pretty obvious.

Yes, the problems abound.  I didn't bother with the velocity factor issue because it isn't the main problem.  In a conductor the fields are concentrated along the wire, not through space--that's the whole point of having wires.  The coupling and initial response would be much weaker, likely not observable for some time quite a bit longer than the "1/c" value and certainly not enough to light up the bulb, not even a little bit.

That indicates that you don't understand electric fields as well as you think you do.



Where is the electric field strongest?

• Between the two battery terminals (which are 1 cm apart).
• Along the copper wire (which is 6283 m long).

[Tomorokoshi]

Buildings have walls and halls.
People travel in the halls, no the walls.
Circuits have traces and spaces.
Energy travels in the spaces, not the traces.
- Ralph Morrison

[nixxon]

Buildings have walls and halls.
People travel in the halls, no the walls.
Circuits have traces and spaces.
Energy travels in the spaces, not the traces.
- Ralph Morrison

Thus people and energy travel in a similar way. That is if halls are similar to spaces, and walls are similar to traces.

[bdunham7]

That indicates that you don't understand electric fields as well as you think you do.

Perhaps not, or perhaps I didn't state it very well.  Obviously the field strength in V/M across the battery terminals is the highest.   But...


Which light bulb lights up? 

[bdunham7]

Buildings have walls and halls.
People travel in the halls, no the walls.
Circuits have traces and spaces.
Energy travels in the spaces, not the traces.
- Ralph Morrison

People do like their fields and such, but not everything is a transmission line.  Electrons actually do travel.  An electron beam in a CRT is actual electrons moving through a wire until they get to the cathode where they jump off and go for a ride.  Those electrons travelled in the wire, not in the space outside of it.  The same goes for an electric arc--those are actual electrons that made the trip along the wire (perhaps slowly, but still...) and then did their thing by physically exiting the conductor and traversing the arc.

The fields around a conductor that interact with the charges in the conductor are closely coupled to the conductor.  In a DC or LF circuit they don't transmit the bulk of their power by simply radiating through space as is claimed in the video, even if the fields in space can be calculated or even measured.

[IanB]

The problem is that the distance between the voltage scource and the bulb was never stated in the video. (as far as I have noticed).

See attachment.

[Simon311]

I am going to take the risk of looking like a complete idiot, here we go.
At first, I thought he was wrong. Then I realized, he might be right, but not for the reason that he explained. I think that in his diagram, one might argue that the fields have already travelled up the entire conductor length when he had hooked up the circuit. This also applies to any kind of capacitive or inductive effects. So, by the time he's closing the switch, the fields were only "waiting" to jump that final distance.
If there were two switches however, each placed immediately after the battery terminals, I think Derek would take the big L and have to wait his full second or more.

[nixxon]

The problem is that the distance between the voltage scource and the bulb was never stated in the video. (as far as I have noticed).

See attachment.

Ouch, I missed that part of the illustration. But if we drill into the illustration; At what exact point is an energy source, like a battery? And at what exact point is "the load"? And how come the bulb is the load?

[nixxon]

The problem is that the distance between the voltage scource and the bulb was never stated in the video. (as far as I have noticed).

See attachment.

Ouch, I missed that part of the illustration. But if we drill into the illustration; At what exact point is an energy source, like a battery, located? And at what exact point is "the load" located? And how come the bulb is the only load shown in the illustration?

[SiliconWizard]

What happens the moment the circuit is closed is a fairly interesting question. In particular, if you think about electrons "moving along" a conductor, can you describe what happens at all points along the conductor? What about the return path?

[TimFox]

Two long parallel wires form a transmission line.  (Think of the formerly ubiquitous 300 ohm flat "twin lead" used for old-fashioned TV antennas.)
In theory, you can place a high impedance voltmeter/oscilloscope between the two wires at any point along the line (differential, of course) and a pair of ammeters in series with each wire (harder to do in practice) and see what happens anywhere along the line.  In fact, I used to do this in practical situations by inserting short loops into large constructions to allow the use of a CRO current probe, along with an active differential voltage probe.
As a physicist, I like to treat complex issues by recasting them into the simplest situation that exhibits the phenomenon in question, leaving the elaboration of simple situations into more complex practical situations for the engineers.

[rfeecs]

Veritasium's video is exactly right.  Much better than Science Asylum who gets some things wrong.

The two wires act as a transmission line, so flick the switch and the current (equal and opposite in each conductor) equals (voltage / impedance).  So you get near instantaneous current through the lamp.  The step in voltage travels down the line and is reflected back and forth and eventually stabilizes.

Feynman has a lecture on the Poynting vector and energy flow:  https://www.feynmanlectures.caltech.edu/II_27.html

He gets a bit comical:

Finally, in order to really convince you that this theory is obviously nuts, we will take one more example—an example in which an electric charge and a magnet are at rest near each other—both sitting quite still...
...How absurd it gets!

[bdunham7]

Two long parallel wires form a transmission line. 

Two wires a meter apart in space will have a very low capacitance per unit length, no dielectric conductance and "normal" resistance and inductance, so the characteristic impedance is going to be very high.  I'm not sure how that affects this example because I don't see how it really is a transmission line even though it physically fits the description, since we aren't applying the voltage across the line at any one point.

[rfeecs]

Two long parallel wires form a transmission line. 

Two wires a meter apart in space will have a very low capacitance per unit length, no dielectric conductance and "normal" resistance and inductance, so the characteristic impedance is going to be very high.  I'm not sure how that affects this example because I don't see how it really is a transmission line even though it physically fits the description, since we aren't applying the voltage across the line at any one point.

Look up "ladder line":
https://en.wikipedia.org/wiki/Twin-lead

Ladder line may also be manufactured or DIY-constructed as "open wire line" consisting of two parallel wires featuring widely spaced plastic or ceramic insulating bars and having a characteristic impedance of 600 ohms or more.

[HendriXML]

I don't think energy transfer behaves different from a signal. I my view they're the same thing: voltage of a signal is mostly measured by receiving a (small) amount of energy.

I can accept the theory, but I have a hard time with how it is applied to this thought experiment.

As said an oscilloscope seems like the tool to give an answer in practice.

[bdunham7]

Veritasium's video is exactly right.  Much better than Science Asylum who gets some things wrong.

The two wires act as a transmission line, so flick the switch and the current (equal and opposite in each conductor) equals (voltage / impedance).  So you get near instantaneous current through the lamp.  The step in voltage travels down the line and is reflected back and forth and eventually stabilizes.

I agree that there is at least theoretically an initial response, the question is the magnitude and whether that can justify the assertions in the video.  I think the previous poster who suggested considering the initial response as if the ends were not terminated has the best idea for analyzing the initial conditions.  And I'm not sure that the initial currents will be equal and opposite, could you explain that one?

[wilfred]

And at what exact moment does a bulb start glowing...

IIRC he stated the bulb is an ideal bulb that glows instantly. I also didn't take the 1/c to mean 1meter/c. But if 1/c is a measure of time you do need to deal with the units somehow. So I took it to be the length of the wires rather than the separation of the bulb from the battery.

[bdunham7]

Look up "ladder line"

I know what it is, 300R flat antenna cables were common many years ago and I still have them roaming around my house.  But if I hook up a 9 volt battery to one end and a bulb to the other, I don't call that a transmission line.  And even if I tried to analyze the initial conditions of connection this way, it is still a 300R characteristic impedance while the resistance of the loop is likely to be much lower.

[IanB]

I also didn't take the 1/c to mean 1meter/c. But if 1/c is a measure of time you do need to deal with the units somehow. So I took it to be the length of the wires rather than the separation of the bulb from the battery.

I'm pretty sure he was intending us to understand 1 meter distance divided by the speed of light.

[bdunham7]

IIRC he stated the bulb is an ideal bulb that glows instantly. I also didn't take the 1/c to mean 1meter/c. But if 1/c is a measure of time you do need to deal with the units somehow. So I took it to be the length of the wires rather than the separation of the bulb from the battery.

No, he meant the time it takes light to cross 1 meter. 

[mc172]

Do mind it's worth bringing some skepticism to Derek's videos these days -- three reasons:
1. Just because, of course; try not to take things at face value, but understand what relationships or motivations might underlie a claim.
2. YouTube revenue.  He's quite open about this, tuning everything from content to thumbnail to optimize viewership.  This isn't necessarily a bad thing -- greater viewership and a good explanation introduce more people to a technical subject.  But it does affect how the subject is presented, more sensationalized perhaps, creating drama from academic disagreement, etc.  (And also not that this has specifically happened -- just that it's something to beware of.)  And of course, the major downside of popular science presentation, the explanations can be oversimplified, and the content very shallow, so it may not even be all that useful if you want to get into the subject.  (But that's an audience problem -- it's an introductory video, you're simply looking in the wrong place if you want depth.  Can't have everything, unfortunately.)
3. Corporate sponsorship, when applicable.  The criticism of his recent driverless car video is particularly apt.  Look for similar patterns in, well, anything you consume, of course: we can especially place blame in this case when the channel's byline is "an element of truth", but in general, anywhere you see noncritical presentation or acceptance of facts, especially when the presenter may have a vested interest in the subject (sponsorship is a fine example!), keep your guard up.  Let alone possible omitted facts -- these can be hard to spot without broad knowledge in a subject, and so are an common strategy.

Tim

Spot on!

[bdunham7]

From another (audio) forum, quoting the video we are talking about:

"Not being a teckie, I didn't know how electricity travels until I just saw a YouTube video explaining it in detail. At long last, it makes perfect sense why people use risers for their audio cables.
All the years I spent thinking it travelled in the cable, it turns out it travels outside of them! Who'd a thought?"