The big misconception about electricity

[clpetersen]

I need the emoji for beating a dead horse (someplace please insert one for me, thx!)

I guess this is why I enjoyed the you tube presentation style but did not so much like the content.

Given what we have learned about electricity from the video:

1. Energy is transmitted (very quickly) by the electromagnetic fields surrounding the wire, not the electrons in the wire (correct)
2. Individual electrons don't really flow in AC current (just jiggle back and forth a bit inside the wire) and even in DC they take hours to move a few meters (correct)
3. and that Energy flow is one-way from the battery to the light bulb. (also correct).

So, why bother with a second (return) wire at all? Think of all the savings in wiring costs, complexity, weight. Seems hard to believe.

[cleeds]

Originally Posted by clpetersen

I need the emoji for beating a dead horse ... why bother with a second (return) wire at all? ...

The answer to your question is, of course: because the electricity wants to flow back to the source. I'm not sure why that's relevant to this thread or why you think you're beating a dead horse.

[Macuser]

Makes for an interesting read no doubt. If I'm understanding this theory correctly there shouldn't be differences in cabling since most of the activity happens in the magnetic field or am I overlooking something?

[clpetersen]

First, don't forget it is Mothers Day! Heading to Fenway with the family as my wife is a baseball fan. It is cold outside and the Red Sox are dead last, but always fun.

Less fun:
I rewatched the video just now - nothing wrong, but also not a 'big misconception' - at least since they laid that first undersea cable 150 years ago or so.

Here goes:
1. Circuit open. The wire from the positive terminal has an electron deficit, or a net positive charge, and negative wire has an electron accumulation. There is no overall movement of charges - i.e current = 0.

There is a radial electric field around the wire due to these charges, but no net magnetic field as nothing is moving.

2. Instant switch is closed. The potential drop across the circuit starting at the switch causes a longitudinal electric field along the wire, positive to negative. This starts things moving and creates the circular magnetic field, per Maxwells equations (actually something called the Biot-Savart law - Maxwell's genius was to take these various laws and unify them)

The now-electromagnetic field is where the energy is (yes, immediately outside the wire), and Poyntings vector, shown nicely in the video, shows that the energy is moving towards the lightbulb. This is the energy due to the radial electric field from the charges and magnetic field from the current. The initial field moves very fast, and so there is net current everywhere and the light turns on more or less instantly.

3. The weaker electric field along the wire (this is the voltage drop of the circuit) and the magnetic field created by the current creates another Poynting vector, this one pointing into the wire - energy is deposited into the wire. These are your resistive losses.

4. At the lamp, the longitudinal electric field is much stronger (biggest voltage drop over a short distance), the vector points into the filament and the light heats up and starts working.

So, the power to the bulb is the product of the longitudinal electric field (voltage drop) and the magnetic field due to the current. Power = V*I

5. What about the return current? This is bit trickier. Thinking of the current flow back to the battery, you would expect the Poynting vector to deliver energy to the battery. However, this current is caused by electrons being pushed away from the negative terminal, causing a net negative charge on this wire. This reverses the electric field vector and since the current is away, not towards, the magnetic field direction around the wire also reverses (clockwise to ccw). The net is that the Poynting vector still points to the light bulb, per the video.


So, the video is not wrong, but sort of minimizes the need for current flow and gets caught up instead in the movement of an individual electron.

Cool and nicely done, but a not quite a breakthrough.

(and to an earlier post - the fields are very localized around the wire, so shielding works, but you do need to pay attention to spacing - in EE terms, this is the capacitance of the your shielded cable. )