The Poynting Vector in a DC Circuit
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- Опубліковано 27 тра 2024
- Energy in a circuit flows in the electric and magnetic fields around the wires. Here's a fully-worked example of how.
Veritasium posted a followup video to the one mentioned here, which I'd highly recommend. It looks more into the dynamics whereas this video focuses on the statics.
• How Electricity Actual...
Mathematica notebook for calculating the Poynting vector in a circuit: drive.google.com/file/d/1yf_R...
PDF version:
drive.google.com/file/d/14yTF...
Veritasium's video: • The Big Misconception ...
ElectroBOOM's video: • How Wrong Is VERITASIU...
Animations were made with Manim: docs.manim.community/en/stabl...
I tried to keep this video reasonably short but I know lots of people are going to have lots of questions, so here's a long FAQ to cover some things that I missed in the video:
Q: In the wire between the capacitor plates, why does the energy come from far away?
A: This is a quirk of using infinitely big plates, as we just assume the plates have a voltage difference without making it clear where the source is. We could picture the plates as very big disks, and at their edges are a ring of batteries connecting them providing the voltage difference.
Q: Doesn't that formula for the magnetic field only work for an infinitely long wire?
A: Yes, I had to sacrifice exactness for simplicity. The calculations for the square-shaped circuit make no such approximation, although they do assume infinitely thin wires, and this assumption needs to be treated as an approximation when calculating the total power. Give me a break, I'm trying my hardest!
Q: If electric and magnetic fields are produced by charges and currents, then surely it is correct to say that energy is carried in the wires?
A: This is really a question of semantics and there are many arguments to be made either way. Fields can exist without charges and currents in the form of electromagnetic radiation, although you could argue that the radiation had to come from particles originally. Special relativity and quantum mechanics makes these philosophical questions very difficult.
Q: I heard that charges move through a circuit very slowly. If that's true, then how do charges build up on the surfaces of wires?
A: Individual charges don't have to move very far because there are many more positive and negative charges filling the wires. The battery pushes in charges at one end and everything shifts over very slightly, resulting in more charges coming out at the other end and on the surface. Think of it like squeezing water into a teabag; the water doesn't flow through very quickly, but a decent amount still oozes out the sides. Man, that's a disgusting analogy.
Q: Why do the wires in the two examples in the video behave in such different ways?
A: In the parallel plates example, the wire is resisting and taking up the entire voltage drop across the circuit. That means energy is flowing into the wire and being turned into heat. For the battery and lightbulb, the wires are ideal and so no energy flows into them. Real wires with low resistance are somewhere between these, with the Poynting vector directed mostly parallel to the wires but also slightly into them.
Q: So... ideal wires are kind of weird?
A: Yes. If there is a voltage source across an ideal wire, the current is theoretically infinite, which doesn't make sense. The calculations only make sense if at some point in the circuit there is something to take up the drop in voltage between the terminals of the battery, e.g. a light bulb. But this isn't hard to see for yourself: if you attach a conducting wire to both ends of a battery, you'll find that it heats up very quickly. A lot of current is flowing through because of the low resistance. Be careful if you try this at home, because it's a very fast way to burn yourself.
Q: Circuits use measured quantities like resistance, but electrodynamics also has measured quantities like resistivity and permittivity. Where do these quantities come from?
A: The electric properties of a material are determined by the material's atomic structure, and studying this requires a looooot of quantum mechanics. This area of physics is called 'solid state' or 'condensed matter' physics, and it provides quantum-mechanical explanations for things like electrical properties, colour, transparency, elasticity and brittleness.
00:00 Introduction
01:20 A wire between plates
05:58 A simple circuit
09:41 Electrodynamics versus circuits
13:23 Conclusion
"Let's be honest, if your name is poynting and you are going to have something named after you, it better well be a vector" is the funniest joke I've heard this year. Love it and the video
Indeed, it took me long time to realize, the Pointing vector is written Poynting vector!
This joke can be modified to describe Joseph's work as well. "If your name is Newton and you are going to make videos, they better be about Physics and Mathematics."
There is something everyone is missing in this Poynting vector topic (Derek, Electro-Boom, many others including you I'm afraid): you all assume that the EM energy is following the poynting vector; this is not true or at least not necessary !
If you look to the Maxwell equations and you try to make an energy balance you end up to the Poynting theorem. However this theorem doesnt define straitforwardly the energy path !
The classical Poynting vector is indeed a possible choice but if you add any arbitrary rotational field to the Poynting field, the resulting field will also verify the Poynting theorem. It results that in the Maxwell frame there is an infinity of possible pathes for the energy flow !
A long time ago, with my former PhD supervisor, we wrote an article on the subject. We proposed, in the quasi-static electric approximation (the capacitor case), an energy flow density only based on the voltage and the local electric field (no magnetic field involved). In this case you end up with an energy flow that is perpendicular to the Poynting field (parallel to the capacitor axis instead of radial to it).
Many researchers justify the choice of the Poynting vector because it is the only quantity that both fullfills the energy balance equation (the Poynting theorem) and is conservative through lorentz transformations (is a relativistic invariant) but this last point is irrelevant in the quasi-static domain (for coils and capacitors).
QED states that there is only a probability for a photon to be emitted somewhere and absorbed somewhere else and that there is no way to follow a photon inbetween these events. Said otherwise there is no experiment that is able to determine where the EM energy flows whitout interfering with the setup you try to measure.
To summarize, no one knows the actual trajectory for the flow of EM energy, this information is not imbedded in Maxwell equations !
Interesting point but the Poynting Theorem is about local energy conservation. Relationship between rate of change of energy density, work done by field on charges and charge on field, plus flux thru surface. The differential equation for local energy conservation in electromagnetic theory is a statement about what's happening at a POINT but when integrated the Poynting Vector is used to calculate total flux out of volume thru boundary surface. Poynting vector gives you the magnitude and DIRECTION at each and every POINT with magnitude units of power over area. So it's giving you energy "flow" and in which DIRECTION. Flow can be potential energy.
The local energy flux is not defined by the Poynting theorem. The local energy flow is unchanged by adding any rotationnal field to it (this is related to the gauge invariance). The same issue arises for the trajectory of a photon that is also undefined locally (there is no quantum dentity matrix attached to the photon in contrary to other particles). I agree that theses notions are not easy to grasp and require a good knowledge of differential operators.
2:05 " conventionally we denote the current 'I' for some reason". The reason is apparently because it stood for "Intensity of current" (in French). Wikipedia has further explanation at "Electric Current".
Great video!
You basically say exactly the same things as in Veritasium's video, but with a bit, just a bit, more math it becomes much more consistent and clear
Thanks, I thought how I can easily explain that Poynting Vector stuff and here you are: already nailed it!
12:46 - the core observation that you get a small "step" immediately after launching a wave down a transmission line is routinely seen in digital circuits and can be surprising to newbies. It's not explicitly mentioned anywhere in the wider discourse of the Veritasium video but electronics engineers might have caught wind of that.
Also the full t-line based analysis of the veritasium video is insanely complex since the structure of the problem excites the single ended and double ended modes of the two t-line arms, which really makes it hard to understand unless you're familiar with it.
if the problem were posited as one transmission line to the right instead of two on both sides, the analysis becomes much simpler and any EE undergraduate should be able to understand that.
No where near enough people show enough respect to introducing ideas using the proper nitty gritty mathematical explainations so the fact you did is kind of amazing. Yes it unfortunately gets lost on everyone unfamiliar with university level mathematics, but I still truly appeaciate it. I think the way that presenting maths is just simply a correct explaination without random unnecessary misleading possibilities for incorrectly generalizing handwavy explanations is beautiful.
Would you mind sharing your notebook as a pdf? I don’t have Mathematica but it looks like a very nice piece of work!
Sure, I've added a link to a pdf version in the video description
I think the random letters for variables is because a lot of the obvious ones were already used by other things in physics. When you're only using one letter each, you only have the constants and variables for the number of letters in the alphabet, the Greek alphabet, and maybe a few special things (and upper case letters, but that adds a lot of confusion if you aren't careful, and is harder to say than write).
The earlier equations got to use the obvious letters (f=ma), but letters in a language aren't evenly distributed, so there were many later ones where the obvious letter was already taken, so they had to use a different letter.
The letters are not usually random, and actually can show a bit of history behind the physics, like I coming from the French intensité du courant, intensity of current
Wow, your explanations really going into the *why* behind concepts. Great work, keep it up!
Amazing not to mention helpful and great animations. Thanks!
This is my favorite explanation yet.
New go-to reference for explaining to peers why noise (e.g. spectral power) is confined to the dielectric space between a trace and reference plane.
i appreciate your commentary and explanations. Hope you decide to make more videos in the future
Great explanation, I learned a lot!
Wonderful video that made me rethink my circuit physics
Brilliant video. Thank you very much for explaining these interesting ideas.
Wonderful video that made me rethink my circuit physics. A+ for this video, well done!.
The algorithm may have just picked you up. I know almost absolutely nothing about this stuff and UA-cam recommended this to me.
You made this fairly easy to understand what’s going on even if I can’t fully grasp the math. I really liked the visuals and hand gestures lol
this was an awesome video. youre right - learn it because its cool. i appreciate your attention to the value of fundamentals.
direction of the slap - i'm losing my mind that's so fucking funny
I'm glad this video got recommended to me even if it is late.
I tried to educate some people on how electricity works. How there is no energy in voltage current as I do believe Rick Hartley is correct, And here's information came fromRalph Morris. Who wrote the book fast circuits.
I tried to tell people that the speedometer in your vehicle does not measure energy. The speedometer is measuring a property of your vehicle in motion
Voltage and current are measurements of the electromagnetic energy.
I see from the PDF you linked that you know a lot of both Physics and Mathematica stuff. That is a big deal and is what makes it possible for you to be a good physicist. It completes your toolkit.
I‘m in my first year of physics and we‘ve just been introduced to the poynting vector. I didn‘t get what it was in my lectures because the concept of how physics is taught in university doesn’t work for me but know I got a slight sense of what it might be! :)
One question though about the E-field though:
How can there be current if it (the E-field) is perpendicular to the wire? Is it slightly bent into the direction of current? Does it abruptly switch direction inside the wire? I‘m sorry if this doesn’t make any sense, correct me about my english, I‘m not native and would love to learn. Thank you for such an amazing video!
This is a great question, because it's commonly taught that there's an electric field inside a wire in the direction of the current, but not how strong this electric field really is. In this example it's actually very weak, much weaker than the field outside. The relevant equation is E = rho * J where J is the current density and rho is the resistivity. Wires have very low resistance and so E is very small, and in an ideal wire with no resistance there's no electric field. As an analogy, imagine that the electrons are little balls moving through a tube. If the tube has no air, then there's no air resistance, and you don't need a force to push them through. They just keep moving because of inertia. Similarly, you don't need an electric field to push the electrons when there's no resistance.
If there is some resistance then there will be a field inside the wires (in the analogy, the balls slow down because of air drag and you need a force to keep pushing them). So what happens if we take the second example with the lightbulb and battery and add some resistance to the wires? Then there will also be a small electric field in the direction of the current, making the total electric field point mostly perpendicular but slightly parallel to the wires. The thing that actually produces this new electric field is an uneven charge distribution on the wires, changing gradually from positive at one battery terminal to negative at the other.
In the Mathematica code I assumed the charge distribution was uniform, meaning all of that gradient happens across the lightbulb instead, and there's no resistance in the wires. I wanted to try adding some resistance to the wires as well, but then I would also need to solve for the charge density which makes the calculations much more difficult!
It is a great video and much easier to understand as compared to Veritasium's video. Much appreciated!
Thats cool, i clearly understand what have you said. Thanks for the clearly picture
I wish I knew about this video when I took circuits/advanced optics. Ugh! Great video.
Great video! Thanks for not over-simplifying things like a lot of pop sci does. I have a question though, so for the square circuit, the poynting vector field followed the path of the energy in the wire, which makes intuitive sense to me. And you talk about how the divergence of the poynting vector is a measure of how much energy is leaving or entering a point on average. But in the first example the poynting vector was pointing perpendicular to the wire and the flow of energy. So, is the direction of the poynting vector sometimes distinct from the direction of the flow of energy and sometimes not? Or is there really no energy flow between the plates through the wire, and all of the energy is flowing into the wire from somewhere off to the side off screen?
my teacher taught us the cross product rhr as the “buddha’s palm” and even included a video from some old movie to demonstrate it. don’t think i’m forgetting that one anytime soon
0:26 Bro your content is awsum subbed
Wooow Amazing dear friend!!
"I" is the symbol for current because it stands for "intensity". Also, C was already taken for electric charge.
Great video and awesome simulation results! Congrats. Just one comment: in order to get this simulation results with S along the wires you have certainly considered the surface charge model which although known from long time, it is not really widespread, and it doesn't appear in hardly any textbook.
Good endeavor at not handwaving steps! I still got lost along the way due to the confusing variable names, but those are hardly your fault.
Edit: I would have also expected the math in the video to be just high school math, but I don’t think you normally study vector calculus until university? At least I didn’t so I was a little offput expecting the video to be ”something that could be derived right out of high school”.
Yeah, that’s why I did a little aside into the cross product. The divergence stuff is extra for those who do have the background, and for those who don’t I hope it at least demonstrates that a precise mathematical foundation does exist.
TLDR: Energy is transported both by electrons and through the fields. Electrons interact via the electromagnetic field, and the energy related to these interactions is carried by this field, allowing for energy transport both within and outside of conductors.
In the realm of quantum physics, matter is composed of fundamental particles, exhibiting a property known as wave-particle duality. The electron, one such particle, is not just a point-like entity but is more accurately represented by a quantum field, the electron field. Electrons carry an inherent property - electric charge, which is quantized. This charge is not merely a label, but the source of rich interactions mediated by another quantum field, the electromagnetic field. These fields are distinct, yet intimately connected. The electromagnetic field mediates the interaction between electrons, and this interaction is fundamentally governed by the amount of charge they possess. This relationship is captured by mathematical models such as Coulomb's law. However, in our daily language, we often speak in terms of voltage, which is tied to the electric field. This is an electrostatic approximation, and in time-varying, dynamic situations, it's more accurate to speak in terms of the broader electromagnetic field. When we discuss the flow of electric charge, we talk about current, which is typically confined to conductors like wires. But when we consider the flow of energy, we enter the realm of the electromagnetic field that surrounds and permeates these conductors. The Poynting vector is a tool that gives us the direction of this energy flow within the field. The energy within these systems is a conserved quantity and represents their capacity to do work. This is encapsulated in the definition of work, W=F⋅D⋅cos(θ), where F = force, D = displacement in meters, cos is the cosine trigonometric function, θ is the angle at which the force is applied, and work has units of joules (J), a measure of energy. The potential energy of an electron within a system is a function of its location and proximity to other charges, indicating its capacity to do work. The electromagnetic field is more than just a stage for these interactions. It's a player in its own right, mediating the forces that move electrons and is responsible for transporting energy in electromagnetic systems. Within this field, electric and magnetic components are inseparable aspects of the same entity, influencing each other such that changing electric fields coincide with magnetic fields. Energy is carried within this electromagnetic field, extending around and beyond conductors. In alternating current (AC) systems, the oscillating electric and magnetic fields can carry energy away from the wires, illustrating that energy is indeed transported via these fields. In contrast, in direct current (DC) systems, the electric and magnetic fields are static. The energy transfer still occurs via the electromagnetic field, but the energy remains within the system unless a change occurs, maintaining the principle of energy conservation. In conclusion, the dance of the electrons, mediated by the electromagnetic field, plays a central role in energy transport both within and outside conductors. The story of energy is told both by the electrons themselves and through the fields they interact with.
0:23 Awesome Thanks 🙏
I just want to mention this, during the my undergraduate studies for my physics degree one of my professors expressed how funny it was to see students in freshman physics decide to try and use their left hand for cross products and magnetic field cause they didn't want to put down their pencils. To be honest we have all been there.
A+ for this video, well done!
Awesome video, Im wondering why you didnt use the H-Field (with units of amps / meter)? Wouldn't it yield simpler calculations? I get the feeling that using the B-Field though paints a more accurate picture of the situation?
Thanks! The H field isn’t just a conversion factor, it has a very different value inside a material. It gives simpler forms for Maxwell’s equations by taking into account bound charges and magnetisation of the material. That’s mostly irrelevant for this video where everything is in a vacuum, and my memory is that E and B are the commonly used fields in high school physics, so I thought it wasn’t worth the additional confusion.
Although this is probably the best explanation I have seen so far then I still have lots of questions. Firstly all the fields and vectors mentioned are really no more real than the values we put to them. They are mathematical tools just as numbers in my opinion.
It is probably correct to say that the energy is transmitted with the help of Photons. As I understand it then a photon can be either following a wire and the electrons in the wire or it can be let free as happens when we have any radiation. Photons always travel as close to the speed of light as the material they travel in allows them to travel.
I would suppose that a photon can only be let free when it has been there long enough to form a full sign wave.
As it is we also know that any pulse can be mathematically formed by a base frequency and harmonics. I believe that to be true in real life too. I believe that when we switch on a DC current then to create the initial potential movement a lot of high harmonics are created and as their wave lengths are very short they will be free to move in all directions. I believe these harmonics are really the ones that Veritasium see at the lamp initially because I think most of us agree that at (supposed ) DC the actual power has to follow the wire.
I say supposed DC as I really will classify DC as a very slow AC. I am yet to know one that has stayed constant for ever. Batteries die. There fore I believe we can look at all currents as AC currents and all changes there fore contain harmonics. That is why it is impossible to have a 100% square wave. There is always a rise time and a fall time.
What I really would like to know is what is a Photon? It is an amount of energy having a frequency but what is it really. It can move with an electron but it can also move without an electron. It's generation appears to have some thing to do with the electron but it is only an amount of energy released from the electron. It can be trapped in a transmission wire as a standing wave. It influences the electrons so that we can measure it's existence and it can heat up the cable by moving the electrons and molecules against the resistance in the cable. It has polarities as seen with polarised light But what is a Photon?
So, in "classical" electrodynamics (i.e. Maxwell's equations) there's no such thing as photons; there are only charges, currents, electric fields and magnetic fields. Photons arise in quantum mechanics, and I'm not super experienced with QM, but I can give my vague understanding. In the most "ideal" model of QM, which I guess would be quantum field theory, everything is a field and particles like photons are just excited states of those fields. My understanding is that the "harmonic" properties of the electromagnetic field are what becomes quantised when you move to QM, which is why photons have quantised frequency and polarisation.
In theory if you were to simulate these fields on a massive scale with a vast number of excitations, it should behave very similarly to classical electrodynamics. What we call classical electric and magnetic fields are really just the average of an absurd number of particles constantly appearing, interacting and disappearing. But this is extremely impractical, so for areas like optics and electrical engineering there's typically no need for the quantum model. If it ever becomes important to include the quantum effects of atoms, then you can work in a sort of 'hybrid' model where atoms have quantised energy levels but the electric and magnetic fields still follow Maxwell's equations and there are no photons.
These sorts of approximations slip in all over the place when trying to describe quantum mechanics intuitively, which is annoying because it means a lot of intuitive descriptions of photons are contradictory! But at the same time, those approximations are precise enough in their respective contexts that I think they can be considered "reality" in a sense.
@@josephnewton In my opinion the word Photon may not have been mentioned in Classical Physics but it exists just the same. Photon is nothing else but a fancy word for a sine wave or more exact the energy of a sine wave. As I understand it it originated from the amount of light or energy given out when an electron jumps from one position in an atom to a lower energy level. This energy is usually given out as light or at least an electromagnetic emission. it has later been established that all EME are photons just with different frequencies. As the energy of photons are Planks constant multiplied by the frequency of the EME it is there fore also the energy of one sine wave. (Planks constant is the smallest possible energy of radiation divided by the frequency. E=h*f or h=E/f) That makes sense to me as the smallest part of a radiation you can have and still know all the data of an emission has to be one full sine wave.
Although I have not done it I would not be surprised if that could be derived from Maxwell's equations and Coulomb's law.
When you talk about having to measure on a massive scale then this is really no different to when you measure a 50 or 60Hz main voltage. You do never measure it momentarily as it wouldn't give you any sensible result. You could get zero volt or you could get peak voltage or anywhere in between.
In my opinion looking at it from my practical experience as an electronic engineer having mainly worked as a technician then the photon is never a particle. It is always a fluctuation. The field it is fluctuating in is if we look at it on an oscilloscope nothing else than the zero level (which I suppose could be called the zero field) and there fore really nothing I would say. It is the philosophical question about what zero is.
I really think that the Physicians are making it much more difficult to understand than it really is by trying to make it 100% correct in the wave calculations. To correctly calculate the power of a 240V 50Hz current flow you should really multiply the point current with the point voltage which involves integration to get the result but we usually don't need it that accurate.
As an add on after some more study I find that every photon has the same energy value = 4.1333*10^-15eV or 6.626*10^-34J if a photon is one cycle. If a photon is the energy in one second then it is the frequency multiplied by the energy value of one cycle. To me that points more to that the knocking free an electron from a cold body has more to do with the frequency than the amount of energy.
Just over half way thru the video when you work out the Divergence of the Poynting Vector. The sum you do appears wrong.
With out writing every detail
Div(S) = B*(curl(E)) - E*(curl(B))
Not bothering with constants, just general method.
Sub-in Maxwell Equations you get
B*(-dB/dt) - E*(J + dE/dt)
Which has general form
d/dt(-B^2) + (d/dt(-E^2) - E*J
The derivatives are partial derivatives and
d/dt(E^2) = 2EdE/dt
Same for B field.
So you end up with
Div(S) = -d/dt(energy density) - E*J
Correct, I assumed B and E are static throughout the video, so dB/dt and dE/dt are zero. I’m aware that isn’t how we work with electricity in real life and there’s an extra term, but I’m trying to emphasise that even in static situations it still makes sense to describe energy flow using fields, and it’s not just an AC phenomenon.
Yes good point THANK YOU for your reply and thank you for your videos I find them extremely interesting and learn alot from them.
6:56 Pardon me if this is a commonly asked question, but isn't the wire neutral?
Kudos to Joseph for spelling out this model so patiently. However there are some holes.
At 5:50 There's a statement: "so yes it is indeed true that energy in electric circuit is transmitted by the electric and magnetic fields". That's not the logical conclusion from the calculations performed so far. All that has been demonstrated is that there are electric and magnetic fields that can be geometrically/algebraically combined so as to produce a number that equals the P = V * I accounting for power. The calculation does _not_ prove that the fields actually carry the energy. It is at least equally compelling to account for the conveyance of energy by simply integrating the electric field gradient along the wire from beginning to end (V), and the quantity of charges moving "down" that gradient (I), losing potential energy to the heating interactions with molecules in the resistive parts of the circuit (R).
It is no surprise that there are electric and magnetic fields associated with this activity, and that they are proportional to voltage and current. And those fields get "charged" from the circuit when it is turned on, and "discharge" back into the circuit when turned off. That does not mean that the pair of fields is the means of conveying energy during steady state operation.
My comment here does not prove the statement false, it just claims that the statement doesn't follow from the premises provided.
Correct. The energy is supplied by the fields but this fact does not follow completely from the statements made. Electron flow through a wire takes a long time and requires work to be done by the field. That is the causal relationship. The wire provides the electrons that support the field and field moves the electrons slowly, converting energy into radiation which is a disturbance in the near field. Photons are emitted as IR (hairdryer, toaster, incandescent light bulb). A high frequency AC signal oscillates the charges in the wires producing heat and radio waves. There may be no net displacement of the electrons. They still get worked back and forth, overcoming resistance, while the field is subject to the effects of capacitance and inductance. Thus information can be transferred quickly while the electrons, not. An ideal antenna would have no resistance. Or something like that :)
@@DanielL143 I'm happy you more or less agree with my comment, but I'd like to make one revision to your notes. It is true that the electrons move slowly (drift velocity), but the speed with which that motion propagates from one end of the wire to the other is at a large fraction of speed-of-light. So though the electron velocity is slow, the speed with which they can have an action at any point in the circuit is very fast.
@@Graham_WidemanA valid and key point for clarification. The 'impetus' or change in state, especially during the transient response approaches 'C and this enables communication as we know it. Would it be accurate to say that changes in the field can approach the speed of light while the movement of matter (electrons through a solid material in the case of a wire) is much slower. Do electrons in a vacuum tube move faster under the influence of a field? If current is the rate of change of charge, does the movement of the charge carriers affect Power and therefore the rate of energy transfer? The ultimate point that I am trying to understand relates to the speed of transfer of energy vs the speed of transfer of information? Can you help & thanks.!
@@DanielL143 Although I am not sure I do believe that electrons in a vacuum tube are moving faster than in a wire. Thinking about a picture tube (as used in old TV sets or oscilloscopes) then I believe they are also the main energy carrier. We know from particle accelerators that matter (electrons or protons etc.) can be speeded up to a high speed. I also know from my experience with old picture tube TV's that if the high voltage was too low then the picture would be too big for the screen. That correspond with that the magnetic influence from the deflection coils have a longer time to deflect the electrons as they fly slower.
If the energy was carried by EM forces to the screen they would be very fast and as far as I know would not be deflected by either static or magnetic forces and we know that it does so it must be the actual electron hitting the fluorescent on the screen that makes it light up.
Veritasium's thought experiment is elementary by definition. And Veritasium was 100% correct
So how does the lightbulb light up? Normally the flowing electrons bump into the atoms of the wire, so heat is generated and, in case of a lightbulb, energy is radiated in EM-Waves (visible light) and thermal energy. I don't get how the energy of the EM-Waves of the flowing electrons in the circuit is necessary for the bulb to light up.
That’s correct, electrical energy gets converted into heat and light by electrons bumping around inside the lightbulb, but that doesn’t explain how the electrical energy gets to the lightbulb in the first place. The argument is that the propagation of energy from the battery to resisting components in the circuit is described by the EM fields, and then the conversion of energy into heat and light is described by atomic forces inside the resisting components.
@@josephnewton But this description flies in the face of how Veritasium insisted that we have been lied to and that all of the energy is in the fields. The movement of the electrons in the wires was discounted as unimportant and they even used the "slow" average drift velocity as some type of justification... as though an intuitive sense of drift velocity is sufficient to either discard or explain energy transfer. But now inside the bulb we are now re-allowed to reconsider particle motion, energy exchange and heat. What justifies this overall treatment? What magic takes the energy from the field only at the bulb to only cause electrons in the bulb to move about and heat up and become energy that is transforms to light... but which magic just doesn't apply to the current in the wire?
@@cadeperkins the magic of a thought experiment intended to illustrate principles.
you're tripping yourself up over a lightbulb which was simplified to absurdity specifically so it wouldn't distract from the underlying physical principles.
thought experiment lightbulb = simple, idealized, easy-to-imagine output device comprehensible to a 6-year-old; intended so you have an obvious thing to imagine happening when the energy reaches it.
real lightbulb = annoyingly complex device relying on quirks of electromagnetism, thermodynamics, and the chemical inactivity of a noble gas to even be functional for any length of time.
if you refuse to use the first lightbulb because it doesn't behave in a way you find believable, i don't advise you look into Einstein's thought experiments either.
@@cadeperkinsthe bulb is a high resistance element than the wire.hence only bulb lights up
@@directajith Yes, but that's not my point. I was highlighting how there are two description of energy transfer--kinetic transfer via the electrons and also the EM fields, but they are only selectively applied to certain features of the circuit. Veritasium was fairly bold in asserting that "lies" have been used to teach how energy propagates in a circuit, that it is all about the fields. The video here has a heavy focus on the fields also. But when questions about the heat and light in the bulb arise, once again the description reverts to particle motion, etc. My intent was to point out that despite all the detail and knowledge, the tendency is still to compartmentalize the description to whatever is intuitively convenient. A complete description will necessarily include both energy transfer in the EM fields and also that of the electron configuration and energy exchange between the particles (both electrons and atoms). Also, unlike free propagating EM waves (i.e. light), the fields here have a source in the charged particles--the moving electrons, so any energy in the fields must necessarily come from the charged particles in the first place. The "lie" is now becoming that they are separate concepts and that only one applies.
When talking about current, please specify whether it's "conventional" (holes), or electron current.
and when talking about left and righthand rules, I'm confused over the magnetic direction the fingers are pointing.
I have read explanations of electromagnetic effects where the author speaks of positive and negative poles of a *magnet.*
I didn't understand what they meant.
Electric current is by definition conventional current. Whether the charge carriers are positive moving in one direction or negative moving in the opposite direction is irrelevant for this system.
1/c seconds makes sense. It's 1m/(c m/s) = dist the wires are apart 1m / speed of light. That's not weird. What is wierd is that Veritasium is suggesting that if the separation laterally is increase to say 60 light seconds apart the electricity will take 60 sec to turn on the light. I have trouble believing that.
So from a philosophical point of view we will never know if our physics theories are 100% correct or just good enough approximations?
Perhaps, but I think the important point is that many theories of physics aren’t even aiming to be 100% correct. They’re trying to strike a balance between precision, elegance and practicality.
@@josephnewton Suddenly I get why lots of physics classes have circular definitions. Its because they are aproximations of the real thing and to explain that would require physics which is way more advanced that students can handle at the point.
Theories are descriptions of observations. A thing's name is not the thing itself. These descriptions are often models and equations that predict future behavior. Precision costs complexity of calculation and measurement with the most precise model of reality being reality itself. Science usually defines a goal of 5 sigma for a claim which is an error of about 1/3.5million. Not 100%, but close enough for most.
7:15 - Emergency Broadcast System?
Well, for some reason you only get a shock once you've touched the live electrical wire.
Derek's entire point revolves around treating the wires as antennas. So yeah, the transient sure will send something across, but he's wrong that it "turns on". The most frustrating thing about his video, including his followup, is he never talks about antennas.
In concept, a hypothetical lightbulb that illuminates upon ANY current no matter how small it is, could be lit up within 3 nanoseconds, but it is nowhere near significant enough to light up a real light bulb. If it were, you've done a horrible job at selecting a lightbulb, because it will burn up as soon as the fields fully settle on their steady state condition, and the full power is delivered to the bulb.
@@carultch yes... walking through impractical situations is kinda the *point* of thought experiments. complaining that the lightbulb is unrealistic is as childish as whining about a relativistic train or perfectly aligned mirrors in an Einsteinian thought experiment.
@@Errenium I'm aware of that. And I never said I was complaining. I was just giving a reality check to this thought experiment, and addressing the OP's point.
The Ponything Vector
Awesome video! Electricity and Magnetism is my favorite subject I’d love more videos on it. A question I’ve got is what do you think about the statement that “electric fields are created by electric charges and electric currents create/cause magnetic fields” I’ve been trying to fully understand electrodynamics and a video by atoms and sporks ua-cam.com/video/uZnXhRgztEg/v-deo.html titled “No, changing electric fields don’t cause magnetic fields” has flipped my intuition especially with the introduction of “retarded potentials.” Do you think that general statement regarding Maxwell’s equations is misleading and or incorrect? Thank You again for making this video it’s so clear and concise :)
Thanks a lot! I think the only potentially misleading thing about "changing electric fields cause magnetic fields" is the word 'cause'. It might be more accurate to say "changing electric fields coincide with magnetic fields". We like to think of it as cause and effect because that's frequently how we work through the equations, but maybe a better picture is that electric and magnetic fields together form a single object that is uniquely determined by a set of equations.
If I recall correctly (I'd have to go over my uni notes to remember the details), you can replace the 'force' fields E and B with 'potential' fields V and A much in the same way you can replace gravitational force with gravitational potential, and these give an equivalent formulation of electrodynamics where retarded time pops right out of the maths. In fact, this formulation extends to quantum mechanics where the E and B formulation doesn't, for instance with the Aharonov-Bohm effect, so it is certainly more correct.
But then again, quantum mechanics being correct doesn't make classical mechanics wrong per say, it's just less accurate in specific circumstances. We still use Newtonian mechanics to build bridges, so I think it shouldn't be frowned upon to use Maxwell's equations to do electrodynamics!
iam watching this after one year. so don't dear to call yourself late.
Why do you sound like CDawg, 1:1 ?
1:10 "Don't be afraid to pause if you don't understand something..."
Me watching in x2 speed "Can't stop, won't stop"
“Don’t use your left hand. It won’t work.”
Yes, Veritasium is being harmful by being awfully misleading
To be fair, all vectors poynt in one direction or the other...
Energy density has very general form 1/2(E^2 + B^2)
At 5:22 i was confidently screaming watt's, and then so dissapointed when he said power, and not until i almost finished the video when i remember that the unit's for power is in fact, watt's....im an idiot
I noticed the asylum science joke .. So I guess the pointing joke is nothing new
No one on these Veritasium-type channels (like Veritasium and this one) are people who have ever done a serious physics degree and who make simple-minded over-generalizations about everything. It's making the public both stupid and completely deluded about what physics actually is.
I am pretty sure that Derek has a phd in physics.
@@JS-vj1il A fake one by mail order maybe.
Wow. Two claims with zero supporting evidence and one simple-minded, over-generalized opinion. Both Dr. Derek Muller from Verisatium and Dr. Joesph Newton have PhDs in physics. Still, despite your bad grammar and arrogant demeanor, maybe you have some insight into what physics actually is. Or maybe you're part of the stupid, deluded public?
j * S = U/d => S = E / j => E = U/d, B = j * S.
E * B = U/d * j * S = 1
Been on the beer? makes no sense!
@@user-yb9ol8sz7o 😁