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Veritasium "How Electricity Actually Works"

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Sredni:

--- Quote from: rfeecs on May 02, 2022, 05:23:18 pm ---
--- Quote from: Sredni on May 02, 2022, 03:27:30 pm ---I have a mass of 1 kg in position P at 0 meters over sea level.
I take this mass 1000 meters away to drop it from a cliff into a hole deep 10 meters.
The potential energy of the mass is converted into kinetic energy and then this is uses to generate heat.  Let's say I 'generated' 1 joule of energy.

Has this energy traveled along the 1000 meters path?


--- End quote ---

If we take the Poynting style conservation of energy argument, we know that energy is created in the volume of space around where you 'generated' it.  Energy was 'dissipated'  in the volume of space around where you dropped it.  So we can say energy flowed through the space between.  But we can't say exactly the path that the energy took.

Edit:
However, if we decide that the potential energy is located where the mass of the rock is located (like we do by saying that the fields have energy), then energy flowed along with the rock.  How much potential energy the rock contains is a problem without knowing the baseline zero potential energy of the system.

I hate analogies.  Water analogy, rock analogy, whatever.  :rant:

--- End quote ---

It was not meant to be an analogy.
I am considering the mechanical system only.

In more detail: one perfectly flat frictionless path goes from point A (the source) to point B (the load). We can put the weight on the path and with an infinitesimal push we make it travel D meters to point B.
Now, at point B we have a cusps that leads to lets' say 20 different holes of different depths, from 1 to 20 meters. Chance determines what the final path will be. But once the weight falls into one of the holes, all of its gravitational potential energy from height 0m to the depth of the hole gets converted into heat (to simplify things).

At the bottom of each hole there is a path (horizontal and perfectly frictionless) that leads back to the source.
At the source the weight is lifted by a machine to sea level and the cycle repeats.

Not an analogy, I repeat. It is a mechanical system.
Does the energy travel through the one forward path at sea level?
If so: HOW MUCH ENERGY does travel along the path?
Remember, I do not know which hole the weight will fall into until the weight falls into it.

If we cannot get an agreement on this mechanical system, how can we get an agreement on the electromagnetic system where the depth of the hole is determined by the charges themselves (by creating a surface distribution that obeys the constitutive relation in the wires and resistor)?

rfeecs:

--- Quote from: Sredni on May 02, 2022, 07:59:18 pm ---
If so: HOW MUCH ENERGY does travel along the path?


--- End quote ---

If we assume that potential energy is located in the rock, and arbitrarily say sea level is zero energy, then zero energy travels along the surface path, and negative energy travels back along the underground path.

Between the two paths (passing through a plane perpendicular to the paths), there is a net energy flow from A to B.  The amount of energy is the energy converted to heat.  So it depends on which hole it goes down.

bsfeechannel:

--- Quote from: T3sl4co1l on May 02, 2022, 02:29:25 pm ---The fact that most of the system's energy is stored in the magnetic field around the wires, hints further that "energy flows outside the wires", but this is an interpretation, and one can take either point; they're equivalent, once everything's quiescent, i.e. you can solve from one given the other (and material properties, boundary conditions, all that).

--- End quote ---

One thing that people are not considering is how the hydraulic analogy for electricity--although it helps people to have an initial grasp of what is going on in the wires--is ingrained in the collective minds of engineers and hobbyists preventing them from really understanding the phenomenon at hand.

Derek briefly approaches that in the misconception section of his video.

One of the problems with analogies is that you have to have a deep understanding of model's (in this case hydraulic) system. Derek seems to have done his homework. He points out that unlike a hydraulic system (where the water from the pump to the load has high pressure and low velocity and the water in the return pipe has lower pressure and higher velocity), with an electric circuit, there's no difference in current density or drift speed for the  electrons going in and coming out of the battery.

What is making the electrons drift is not some kind of pressure. It is just a portion of the electric field generated by the battery that does not contribute to the transfer of energy from the battery to the load. So electrons in the wire are not being pushed by each other like in a fluid. They're just parading in response to an external cause (the electric field) exactly like in the load.

TimFox:
As I have posted elsewhere, inside an ohmic conductor (tautologically defined as one that obeys Ohm's Law), the physics equation (analogous to I = V/R) relates the current density vector J in A/m2 to the E-field or voltage gradient E in V/m by the conductivity s in  (\$\Omega\$m)-1 .
For an isotropic conductor, s is a scalar, but for an anisotropic conductor (such as crystalline graphite), s is a tensor.  (In textbooks, this is usually written as a lower-case sigma.)

     J = s E

Just as with a resistor from Mouser, this can be considered as the voltage (gradient) produced by the current (density), or vice-versa.

Of course, if there be skin effect, E is a function of position within the conductor.
Non-ohmic conductors (PN diodes, vacuum diodes, etc.) have their own defining equations.

[corrected for typo about skin effect]

T3sl4co1l:

--- Quote from: bsfeechannel on May 02, 2022, 08:52:53 pm ---One of the problems with analogies is that you have to have a deep understanding of model's (in this case hydraulic) system. Derek seems to have done his homework. He points out that unlike a hydraulic system (where the water from the pump to the load has high pressure and low velocity and the water in the return pipe has lower pressure and higher velocity),
--- End quote ---

Are you sure about that? :)

Evidently the return line is smaller diameter; which is fine, that works too.  This perhaps highlights possible confusion over current density vs. total flow as well?

The most direct analogy for magnetic or electric fields in a hydraulic system, I think, would be the expansion or deflection of the pipes themselves?  But this isn't so much a general effect as dependent on geometry, mounting etc.  Much as mechanical deflection of electrical cables, it's a higher order effect you can all but ignore until very high levels.  Neh.

But really, it comes down to wave mechanics, which should be no surprise.  So the more unusual aspects, of inertial flow, acoustic waves, etc., will be similarly lost on those who simply aren't familiar with them.  (But, at that level, also even worse, because Navier-Stokes is hella nonlinear.  Maxwell's is largely linear in practice; that we should be so lucky as EEs!)



--- Quote ---What is making the electrons drift is not some kind of pressure. It is just a portion of the electric field generated by the battery that does not contribute to the transfer of energy from the battery to the load. So electrons in the wire are not being pushed by each other like in a fluid. They're just parading in response to an external cause (the electric field) exactly like in the load.

--- End quote ---

But like I said at the top of this thread -- electrons can be considered to push each other, given a suitable definition of "push".  It's hardly a stretch of physics to say electrons repel, and no one needs QED to describe that (really? really?..).  This remains true in the charge-balanced condition of a conductor, it's just over a much shorter range (except for the slight remaining charge imbalance, evident at the conductors' surface, and the field in space between them), and the absence therefore gives us a complementary "pull" as well.

Tim

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