Tear This Guys Theory Apart

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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
I saw this on another forum and thought the theory minded members would get a kick from this. Personally it is a bit over my head...

 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I haven't watched the video, but I do remember learning that the energy actually flows in the electric and magnetic fields, and not in the electron flow.

Always seemed more like a matter of semantics to me, so I never really put too much thought into it.
 

EC Dan

Member
Location
Florida
Occupation
E&C Manager
I did have the understanding that waves were the real source of energy and electrons actually move very slowly in a wire, however I'm not sure I understand the solution to the question he originally posits. I would think the fields still need to propagate along the length of the wire at some fraction of the speed of light, resulting in a delay of ~1 second in a light-second long wire, however the result is actually 1/c (1 being the spatial distance of the light bulb from the battery in meters).
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer

EC Dan

Member
Location
Florida
Occupation
E&C Manager
The fraction to which you refer is called the "velocity factor" -- the actual propagation speed in a cable, referenced to the speed of light in free space -- and is rarely specified for anything but RF cables.
But does the premise that the fields "jump the gap" between the battery and the bulb to light up the bulb, as proposed in the video, make sense? I understand the electric and magnetic fields of the battery and wires immediately surrounding the battery would extend across the 1 meter gap, but is that really what is lighting the bulb initially? I have a hard time picturing the Poynting vector in this case. The delay should be t = x / (c * VOP), with x being the length of the wire, not the spatial distance between the battery and bulb.

I think this image shows it a bit more clearly. The plane P is massive in the case proposed in the video (3E8 m^2) but still only has to propagate 1 m from battery to bulb. I guess I just never really considered that circuit geometry plays any role in energy transfer but this implies that it does, at least initially.

440px-Poynting_vectors_of_DC_circuit.svg.png
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Cutting the fog with a simple alternate viewpoint: Forget the fields and look at the loop as a transmission line. Assume DC for simplicity.

The distributed capacitance of the wire allows real electron current to start flowing immediately whether there is a closed or open circuit at the far end of the line.
The amount of "instant' current flow will depend on the series impedance of the bulb and the characteristic impedance of the transmission line.
Only after several time constants of fhe line will the current stabilize at its steady state value, based on the actual resistance of the wires rather than the transmission line impedance.

The fields around the wires near the battery show us in which direction the energy is flowing, but the Poynting vectors point in and out along each of the two wires of the transmission line and not between the two!
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
With a photon, the energy is carried entirely in the electric and magnetic fields, since the photon has zero rest mass.
In the case of the electrons in the wire the energy can be calculated and analyzed perfectly well using the mass and velocity of the electrons that make up the current.
His "everything is in the fields" approach gives no explanation at all of how a resistor converts electrical energy into thermal energy. The electrons bump into atoms in the resistor material and lose kinetic energy.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
It's kind of basic electromagnetic theory. For technical students (as opposed to engineering students who normally take an electromagnetic fields and waves class) its usually explained with the water in the hose analogy. Assume I have a long garden hose. If I turn on the water at the spigot, it will take a while for the hose to fill up with water, which means it will take a while for water to come out the far end. People expect the same thing with electric wires.

However, if the hose is already filled with water, as soon as I turn on the spigot, water will start flowing out the far end. Technical students are told the same thing happens with the wires, since the wires are already full of electrons. Therefore, if I have a really long wire connected to a light bulb through a switch, as soon as I turn on the switch, the light bulb will light up.

But I do have to take issue with his comment at about 6 minutes where he surrounds the top wire with positive charges, and the bottom wire with negative charges. I think what he really means is that the magnetic fields are in opposite directions.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
But does the premise that the fields "jump the gap" between the battery and the bulb to light up the bulb, as proposed in the video, make sense? ...
Not a clue. I quit wasting my time on YouTube crackpot hypotheses proclaiming "The Big Misconception".

When somebody says "All y'all got it wrong, despite a hundred years of analysis, experimentation & implementation", I turn the page. Maybe I might be interested if it said, "Here's an alternative perspective on the existing body of theory & data ... ".
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The whole 'everything is fields the electrons don't count' misses a key point: when you have charges moving in a wire you create fields, and those fields are strictly tied to the motion of the charges.

In the limit of small circuits compared to the wavelength being considered, the charge flow description of electricity is perfectly valid.

It is only when circuits are large that you need to think about fields in space and the speed of light. But even then, if your fields are interacting with conductors, if you look closely you will see charges flowing to correspond to the fields.

For the example circuit in the video, you are forced to use the field description. The wavelength of the signal is much shorter than the size of the circuit. But wait you say: the circuit is DC! Nope: the DC got switched, and that switching transient is AC.

IMHO what actually happens to the light would be lots more complex then just turning on 1/c seconds later. But whatever happens it would start at that time.

Jon
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Not a clue. I quit wasting my time on YouTube crackpot hypotheses proclaiming "The Big Misconception".

When somebody says "All y'all got it wrong, despite a hundred years of analysis, experimentation & implementation", I turn the page. Maybe I might be interested if it said, "Here's an alternative perspective on the existing body of theory & data ... ".
Ah, yes, the lure of special knowledge. What they don't want you to know, what most people think is wrong, etc. I know people to whom a preface like that is all it takes to convince them that what follows is the truth. It makes them feel special.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
When somebody says "All y'all got it wrong, despite a hundred years of analysis, experimentation & implementation", I turn the page. Maybe I might be interested if it said, "Here's an alternative perspective on the existing body of theory & data ... ".

Agreed. Especially since what he is saying is strictly _true_ and known to be true given the body of data.

What is said is analogous to someone saying "Newtonian physics is wrong wrong wrong, you have to use Einstein's Relativity." Well, that is exactly true and misses the point. Newtonian physics is a close enough approximation for most everything, but fails spectacularly out of its domain. No special private knowledge needed, you can find it in the appropriate textbook.

Now if all you know is Newtonian physics, and are unwilling to see it as an approximation, then you won't know what to do when the approximation falls apart.

I like this guy's take on the video:

Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
So is the 1 m in the answer 1m/c the distance from the battery to the light bulb, or the distance between the parallel wires?

I.e. if the light-second of wire on either side was laid out as, say, a diamond of perimeter 1 light-second, but the distance from the battery to the bulb is still 1 m, is the answer unchanged?

Cheers, Wayne
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The part of the answer that says the initial influence of closing the switch will hit the light 1m/c later remains unchanged if the space between battery and light remains 1m.

The response curve of how the voltage hitting the light changes over time after the switch closes will change as the geometry of the wire changes.

Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Going back to the original geometry, let's say the steady-state power flow is 1 watt. How does the power flow into the light bulb vary over time after the switch is (instantaneously) closed at time t=0?

So far we know the power flow into the light bulb is 0 up until t = 1m / c. When does it reach 1 watt, and what is the shape of the curve in between t = 1m / c and that time?

Cheers, Wayne
 
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