The Big Misconception About Electricity

Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
So what happens if you puy the lamp inside a mu-metal shield?
Draw a diagram or picture of what you mean. I would assume you mean something other than a completely sealed Faraday cage with a disconnected lamp inside. The circuit wires still must be connected to the lamp from the outside so the field disturbances from charge displacement (separation) that carry energy outside the shield follow (surround) the wire through the holes in the shields as energy carrying fields inside the shield where the wiring is connected to the lamp.

It's useful to see that the EM fields already exist in the space surrounding all charged particles, charge displacement causes a disturbance in the existing fields and the energy of the displacement(s) will propagate into space as electrical energy with the wiring as the guide to channel that energy. They work as a system.
It's not the moving electrons in the connecting wiring that lights the bulb, it's the electrical energy surrounding the connecting wiring that causes the local effect of electrons inside the lamp filament to move (accelerate) causing the lamp to light. The effect of energy pushing electrons is thermal KE due to collisions. We use low resistance conductors to reduce the 'speed' of electrons in wires, by lowering resistance we also lower the thermal KE of electrons while increasing the ratio of EM energy surrounding the conductors.
 
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atferrari

Joined Jan 6, 2004
4,470
Where were you, Mr Poynting, when I met Oersted and Faraday?

I don't recall reading of this before. Why?

My reality.png

/Edit to add: y sin entender nada aún.
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Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
Where were you, Mr Poynting, when I met Oersted and Faraday?

I don't recall reading of this before. Why?

View attachment 253352

/Edit to add: y sin entender nada aún.
Edit/
https://www.physicsforums.com/insights/circuit-analysis-assumptions/
A recurring problem on PF is raised by students who may have learned about components and CA and wish to go a “little bit” deeper to understand electrical conduction. To make it worse, many are unwilling to go further with serious study and unwilling to deal with math beyond what they already know. Sometimes, the tip-off is when the student mentions electrons.

Fields are simplified in the very practical and useful circuit theory with no geometrical information to lumped circuit components with zero sized interconnects. No need for Mr Poynting in a traditional electronics education until maybe RF or high speed digital. For a traditional physics education where force and energy are basics Mr Poynting is talked about early and often.

 
I can't believe this simple, yet major, mistake is made in the Post#1 video.

there is a potential between + and - terminals. The battery as a whole has potential energy but one pole of the battery cannot be compared to the other as having "more" energy. "Energy" has a very specific definition so the video is just plain sloppy. There are some other issues with the video - but this one really irks me.

195FE5C0-7660-4E66-9C08-E89D250E96A7.jpeg
 

Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
An Australian response…

Great video for the most part but I would say there are many times where the fact the energy is outside the wire matters (even at DC). Any time where the rate of change (wavelength) is a sizable fraction of the circuit dimensions we need to account for the dielectric media (vacuum, air, PCB substrate) the energy passes through as a propagation delay, phase shift and radiation. We can use lump-sum transmission line solutions but IMO that don't give the level of intuition you get by knowing and visualizing the energy flow paths across space. Today we use EM solvers in daily engineering to see the fields so we do care because the tools are much easier to use today with high speed computers on every desk.
https://en.wikipedia.org/wiki/Electromagnetic_field_solver

For the DC case, energy outside the wire is what makes DC motors move and CRT plates deflect charged particles like electrons. The DC circuit transferring energy is not static, it's in a non-equilibrium state.

Circuit theory describes how much energy is transferred, but it never makes any claim about where energy is located within a circuit element nor where energy crosses a lumped element's boundary. There is no conflict here (with energy outside the wire) because circuit theory makes no claim on the question.

https://forum.allaboutcircuits.com/...ic-field-in-a-ground-wire.129568/post-1063006

My missing link from that thread: https://www.tu-braunschweig.de/inde...oken=90bbd237cd4ef87cf2941a99eb3c3d610de3d0d3
 
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shortbus

Joined Sep 30, 2009
8,955
Circuit theory describes how much energy is transferred, but it never makes any claim about where energy is located within a circuit element nor where energy crosses a lumped element's boundary. There is no conflict here (with energy outside the wire) because circuit theory makes no claim on the question.
But that may be where the "Energy Harvesting" fringe people get some of their ideas from.
 

Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
A EM solver solution.

What this also demonstrates is the efficiency problem with room-scale 'wireless' energy systems. Yes, the energy flows outside the wire directly to the load but as you can see above, the wires guiding the energy makes for a much more efficient energy transfer system than simply moving electrical energy across more than very short distances. Like chain links on a bicycle chain the electrons don't possess the circuit energy but they do provide the mechanism for electrical energy transport from point A to B via tension (potentials).
 

Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
So how does the field carrying the energy work with a DC current and how does it change with a change in the DC voltage?
A simple example is the DC motor (magnetic field) or energy storage in a DC filter capacitor(electric field)? The DC circuit is not static because there is a energy flow from the combination of electric and magnetic field components from a electrical energy source via the wires to physical mechanical load in a DC motor. The magnetic field (component of the EM force) energy is what pushes or pulls the motor round, not the electrons in the wires that stay neatly insulated from each other wire and the motor. Like I said, the electrons are like chain links in a power chain, they don't physical change, or increase then decrease in energy during the transfer of power from sprocket to sprocket but they do move round and round (like current/movement of charge). The analogy to fields is the bike chain tension that changes with the load and level of effort applied to the pedals (energy). There is nothing in circuit theory that says how the energy actually moves or is transferred, Circuit theory provides the solutions to quantities of energy, not solutions to the physical implementation of energy transfer. There is no conflict with the EM field explanation of energy transfer.

 
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Thread Starter

nsaspook

Joined Aug 27, 2009
8,898
Holes do exist in conductors but they are not persistent in most metals the way they are in semiconductors as they only exist near the Fermi energy (electron 'hops' between atoms at the Fermi velocity) in good conductors. In most metals electron flow is the primary means of conduction.
https://en.wikipedia.org/wiki/Drift_velocity
https://en.wikipedia.org/wiki/Fermi_energy
As a consequence, even if we have extracted all possible energy from a Fermi gas by cooling it to near absolute zero temperature, the fermions are still moving around at a high speed. The fastest ones are moving at a velocity corresponding to a kinetic energy equal to the Fermi energy. This speed is known as the Fermi velocity. Only when the temperature exceeds the related Fermi temperature, do the electrons begin to move significantly faster than at absolute zero.

The electromagnetic field is a single entity so the magnetic effect we see is a viewpoint transformation of existing energy not a new field.
https://en.wikipedia.org/wiki/Length_contraction#Magnetic_forces
In 1820, André-Marie Ampère showed that parallel wires having currents in the same direction attract one another. To the electrons, the wire contracts slightly, causing the protons of the opposite wire to be locally denser. As the electrons in the opposite wire are moving as well, they do not contract (as much). This results in an apparent local imbalance between electrons and protons; the moving electrons in one wire are attracted to the extra protons in the other. The reverse can also be considered. To the static proton's frame of reference, the electrons are moving and contracted, resulting in the same imbalance. The electron drift velocity is relatively very slow, on the order of a meter an hour but the force between an electron and proton is so enormous that even at this very slow speed the relativistic contraction causes significant effects.
This enormous coulomb force is why fusion requires enormous energies to crack the barrier.

For the details on the relativistic effects read: Ed Purcell in chapter 5 of Electricity and Magnetism
https://ocw.mit.edu/courses/physics/8-022-physics-ii-electricity-and-magnetism-fall-2006/readings/


https://books.google.com/books?id=A2rS5vlSFq0C&pg=PA281&lpg=PA281&dq=Ed+Purcell+in+chapter+5+of+Electricity+and+Magnetism&source=bl&ots=kuSVnNCf15&sig=ACfU3U0i5KUxdLM3mLJn3zu2ccY9A7v9Bw&hl=en&sa=X&ved=2ahUKEwis2q254Lv0AhXBGTQIHVyuDlQQ6AF6BAgPEAM#v=onepage&q=Ed Purcell in chapter 5 of Electricity and Magnetism&f=false
 
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