Just When I Thought I Was Understanding Electrics To A Small Degree...

[OnlQQker]

I go and watch something like this...

 

[nicebutdim]

Watched this shortly after its release on Youtube and my first thoughts were that I'd like to hear the opinions of certain forum members.

 

[Petej999]

Watched this shortly after its release on Youtube and my first thoughts were that I'd like to hear the opinions of certain forum members.

Which ones?

 

[nicebutdim]

Which ones?

Quite a few and didn't want to start a list as I'd invariably forget some names. I'd certainly be interested in your thoughts, Pete, if you have time to watch the video.

 

[pc1966]

As with most things in life, there is the simple version that works for situation A and then the complicated version that works for A & B.

The basic idea that electricity is the movement of electrons is right, but that is not quite as it seems as electrons move really slowly. The "speed" of electricity is more like a pressure wave in a water pipe travelling at the speed of sound in water of ~1.5km/s (much faster than in air) even if your actual water is only moving at a couple of meters per second at most.

But it gets more bizarre as well when you do a more detailed analysis and conclude that in most cases (non magnetic conductors) the speed is determined by the insulator - hence the view that the conductors are really guiding the wave through the medium (i.e. insulator) they sit in.

At very high frequencies you end up with a waveguide - basically very expensive precision plumbing - that is used to route RF with a single conductor as the guide. Or beyond that to the likes of fibre optic where it is the change in dielectric (glass-air or plastic-air in simple case, glass-glass in fancy single-mode fibres) that acts as the guide without any conductor!

 

[Lucien Nunes]

It's a pretty good explanation, although by jumping directly to the physics he bypasses the opportunity to correct the 'lies' using conventional electrical concepts which are also valid.

If you work with RF a lot, you get used to the idea that electrical power flows through the dielectric of a cable and through free space and it starts to seem peculiar to think of the electrons themselves carrying it. They and their conductors provide equipotentials to set up the E-field and movable charges to set up the H-field, but the fields do the work.

But wait. A field isn't really a thing. It's an analytical concept that shows the disposition of certain effects within 3-dimensional space. There is potential difference and you can use the idea of an E-field to show how that potential difference is distributed through space. But the field is nothing in and of itself.

So don't get too attached to that idea either.

 

[marconi]

'A field is not a real thing'. I think many physicists would dispute this statement. One can describe and explain a field in terms of the force interactions it has - between electric charges for an electric field for example. These forces have a strength and direction associated with position in the field and magnitude of the two charges which are an interaction. (For electric fields and charges the force interaction, a non-touching force from a distance, is mediated by photons - a type of boson particle). An electric field is a vector field requiring three number to define it - position, force strength and direction.

So, one would say an electric field exists outside a charged hollow metal sphere but does not exist inside the sphere. The thing is the region of space where force interactions between charges or between the field and charges happen. For the example of a charged sphere, there is the unbounded region outside the sphere and bounded region inside the sphere. In the former the field is of infinite extent albeit it decays with increasing distance.

I think what Lucien may have meant is that a field is not really made up of field lines as is commonly drawn to represent a vector field. A field is continuous.

When Lucien writes 'The field is nothing in and of itself' I think he is saying that a field is not self-generating ie a charge must be present for an electric field and mass for a gravitational field. This follows from a field being a region where interaction takes place between charges or masses. Without them no field exists.

Here is a nice short piece by Scientific Amercian on fields:

 

[littlespark]

“To a small degree”

Well, if the degrees is 120, I guess you got 3 phase there…

It’s magic. Plain and simple.

We can give little Harry Potter a run for his money!

 

[Lucien Nunes]

When Lucien writes 'The field is nothing in and of itself'

It was a slightly exaggerated way of poynting out (ha) that many of the ways we 'see' some effect or other are analytical constructs that don't correspond to a physical entity. E.g. what we call an electric field is equivalent to the spatial representation of potential gradient. You can measure so many V/m on this vector at this point here, and so many V/m on that vector at that point there, and plot your findings in 3d space and the result is the same as the field.

We (OnlQQer) already knew there was a current along the conductor that produced a magnetic field around it, and a potential difference between the conductors. Although omitting to consider their arrangement in space, that is a step on the way to already knowing that there are E and H fields. The fields are not extra quantities being added to the mix, but stronger and better analytical representations of the same ones.

 

[marconi]

E.g. what we call an electric field is equivalent to the spatial representation of potential gradient. You can measure so many V/m on this vector at this point here, and so many V/m on that vector at that point there, and plot your findings in 3d space and the result is the same as the field.

I, and I reckon many eminent physicists, would say that there is an electric field produced by electric charge or charges. This electric field is to be understood by what I write to mean a region where other electric charges will experience a force of interaction mediated by the photon with the aforesaid charge or charges to produce action at a distance. Charges are fundamental entities and perforce they have an associated electric field.

Once can then consider the work required to move charges around in an electric field. In physics, work is a what happens when a force moves through a distance and involves energy transfers. Thus, if a charge experience a force in an electric field and it moves along some path, along the path work will be done. If the charge is moved 'against the force of interaction on it, it will gain in potential energy as work is done against the field. If it moves in the direction of the force of interaction on it. it will gain in kinetic energy as the field does work on the charge which changes the potential energy it had into motion. The long and the short of it is that one can derive(or formally define) a quantity called electric potential which is so much energy per unit of charge. It is the energy required to move a unit charge from infinitely far away to a point in the electric field. Then one can use this derived field to calculate the potential energy changes of a charge between points in the electric field just using the potential difference alone between these points and not the path along which they move.

Summing up, force fields exist; potential fields are derived fields which do not exist in objective reality.

Is this what you meant? Please understand I do not want to start a pointless lexicological argument.

 

[marconi]

I go and watch something like this...

OnlQQker and readers: One might like to consider whether the experiment in the film, which was only described, but if done in reality is identical to the supply of electrical energy from say a battery with a lamp next to it through a loop of parallel conductors 1 m apart 300km long and shorted at their end, and a battery connected to the same loop with a lamp at its very end 300km away. The conductors assumed to be resistance free.

Then consider whether the experiment in the film (second attachment) is physically representative of the end to end power supply system our man mentions.

 

[pc1966]

OnlQQker and readers: One might like to consider whether the experiment in the film, which was only described, but if done in reality is identical to the supply of electrical energy from say a battery with a lamp next to it through a loop of parallel conductors 1 m apart 300km long and shorted at their end, and a battery connected to the same loop with a lamp at its very end 300km away. The conductors assumed to be resistance free.

The usual 'thought experiment' I used to give students was a battery, switch, and an infinitely long pair of wires.

The questions were "What happens when the switch is closed? Will any current flow, and if so, how much? Would it matter what is at the end of the line given it is infinite distance?"

That was the introduction to the (no doubt tedious!) subject of transmission lines and the various aspects such as characteristic impedance, reflection coefficients, cable attenuation and the Neper/decibel, etc, etc.

 

[Vortigern]

Energy doesn't flow in wires....yeah tell your heart that when you contact one! Electrontology sounds like, thats the electrical branch of scientology. I'm sure Edison said "..that we need all these big words to hide the fact that actually, we haven't a clue what electricity is..."

 

[Lucien Nunes]

potential fields are derived fields which do not exist in objective reality.
Is this what you meant?

Sort-of. It was as I say a slightly tongue-in-cheek observation, as indicated by the informal language, that even the 'correct' understanding of how electrical energy is transmitted by a pair of conductors, relies on (as you say) the derivation of the concept of a field that is not fundamental. It was not intended to stand up to rigorous scrutiny.