General > General Technical Chat
Veritasium "How Electricity Actually Works"
Sredni:
--- Quote from: Naej on May 02, 2022, 10:43:10 pm ---The relativistic answer (convention) is that a momentum density p of matter corresponds to an energy flux of pc².
If you want general relativity then the answer is, well, complicated. https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html
Classical physics is simple and says you have a potential energy of -GMm/r, you're not moving 0 energy, you're just converting: potential->kinetic->heat.
--- End quote ---
In the quasistatic idealization I had in mind, the mass takes an infinite time to travel from point A to point B. So, velocity is basically zero along the path and only changes during 'generation' (the machine raises the weight) and 'dissipation' (the weight does work against the gravitational field and it all becomes heat).
In my view the energy is in the gravitational field of the system planet + weight. It is being added to the system during 'generation' and it is extracted from the system during 'dissipation'. The system occupies all space, so does it even make sense to ask if the energy is traveling?
electrodacus:
--- Quote from: hamster_nz on May 04, 2022, 08:58:06 pm ---We already know the answer to that from the video - some energy will travel a shorter distance, via the EM fields, due to the changes in that fields. The bulk of the energy will start once the a stable field has built up around the whole length of the wire.
It doesn't answer if energy is transported inside or outside of the wires, just that energy can travel outside of wires (through the EM field), and that energy can travel along wires.
For me the interesting observation is that only the tiniest bit of EM energy travels the shortest distance, and after that the energy is then being supplied by changes in the EM field that are happening further and further away on the wires.
--- End quote ---
What video ? The one with the wrong info made by Derek ?
The energy is transported trough wires that is why there is no energy transfer from battery to load until electron wave has the time to travel from source to load along the wire and not trough air.
It is also important I think to talk about the electric and magnetic field separately in this context.
Before you close the switch all you have is a constant electric field so no magnetic field.
Only the exact moment when the two conductors of the switch get close enough so that an electron can jump to the other conductor that has a different distribution of electrons the energy starts to be transferred or do any work.
If you just move the switch but not close enough for any electrons to move you have some local small variation in the electric field but no magnetic field and no work done so no transfer of energy.
But if you think you understand better the magnetic field think about at permanent magnet that has fields extending quite some distance outside the magnet but they can not do any work or transfer any energy.
if you move a conductive loop in a magnetic field then the energy to the system is provided by the one that moves the loop and as electrons move they create an equal and opposite field. It is not the magnetic field that moves the electrons is the person that moves the loop trough a magnetic field and that opposite field is the result of electrons moving.
hamster_nz:
--- Quote from: electrodacus on May 04, 2022, 10:06:14 pm ---
--- Quote from: hamster_nz on May 04, 2022, 08:58:06 pm ---We already know the answer to that from the video - some energy will travel a shorter distance, via the EM fields, due to the changes in that fields. The bulk of the energy will start once the a stable field has built up around the whole length of the wire.
It doesn't answer if energy is transported inside or outside of the wires, just that energy can travel outside of wires (through the EM field), and that energy can travel along wires.
For me the interesting observation is that only the tiniest bit of EM energy travels the shortest distance, and after that the energy is then being supplied by changes in the EM field that are happening further and further away on the wires.
--- End quote ---
What video ? The one with the wrong info made by Derek ?
The energy is transported trough wires that is why there is no energy transfer from battery to load until electron wave has the time to travel from source to load along the wire and not trough air.
It is also important I think to talk about the electric and magnetic field separately in this context.
Before you close the switch all you have is a constant electric field so no magnetic field.
Only the exact moment when the two conductors of the switch get close enough so that an electron can jump to the other conductor that has a different distribution of electrons the energy starts to be transferred or do any work.
If you just move the switch but not close enough for any electrons to move you have some local small variation in the electric field but no magnetic field and no work done so no transfer of energy.
But if you think you understand better the magnetic field think about at permanent magnet that has fields extending quite some distance outside the magnet but they can not do any work or transfer any energy.
if you move a conductive loop in a magnetic field then the energy to the system is provided by the one that moves the loop and as electrons move they create an equal and opposite field. It is not the magnetic field that moves the electrons is the person that moves the loop trough a magnetic field and that opposite field is the result of electrons moving.
--- End quote ---
There will be a potential difference across the switch (otherwise no current will flow when the switch is closed). And as you point out it is a small capacitor - different charges separated by distance. There is then a force across the gap - and with simple geometries it can be calculated - https://physicstasks.eu/1535/force-acting-on-capacitor-plates. Force x distance = something or other... You can listen to this on electrostatic speakers if you want.
For a constant magnetic field what you say is true. Likewise a transformer with DC over the primary windings has 0V over the secondary winding.
But if the magnetic field is changing, then you also have an electric field. How else would transformers work - the wires are not moving, yet energy is transferred?
As captured in the Faraday-Maxwell equation:
--- Quote --- The Maxwell–Faraday equation states that a time-varying magnetic field always accompanies a spatially varying (also possibly time-varying), non-conservative electric field, and vice versa.
--- End quote ---
Connecting the battery causes a time-varying magnetic and electric field, and this transfers (a limited amount of) power across the 1m gap between wires.
electrodacus:
--- Quote from: hamster_nz on May 04, 2022, 11:00:53 pm ---
There will be a potential difference across the switch (otherwise no current will flow when the switch is closed). And as you point out it is a small capacitor - different charges separated by distance. There is then a force across the gap - and with simple geometries it can be calculated - https://physicstasks.eu/1535/force-acting-on-capacitor-plates. Force x distance = something or other... You can listen to this on electrostatic speakers if you want.
For a constant magnetic field what you say is true. Likewise a transformer with DC over the primary windings has 0V over the secondary winding.
But if the magnetic field is changing, then you also have an electric field. How else would transformers work - the wires are not moving, yet energy is transferred?
As captured in the Faraday-Maxwell equation:
--- Quote --- The Maxwell–Faraday equation states that a time-varying magnetic field always accompanies a spatially varying (also possibly time-varying), non-conservative electric field, and vice versa.
--- End quote ---
Connecting the battery causes a time-varying magnetic and electric field, and this transfers (a limited amount of) power across the 1m gap between wires.
--- End quote ---
The potential difference is due to imbalance of charges and the electric field is the effect of that.
You mentioned electrostatic speakers but those same as electrostatic microphones/condenser microphones require a DC bias voltage / phantom voltage in order to work unlike dynamic microphones / speakers that generate energy by moving the coil installed on the membrane in a constant magnetic field.
(piezoelectric effect is a different story and plays no role here).
So you can not produce energy by moving the plates of a capacitor like example condenser microphone. You push the energy stored in the capacitor in and out in that DC bias (usually you measure the voltage drop on a resistor).
So you charge or discharge the capacitor and that phantom voltage / DC bias is there to cover any losses and keep the capacitor charged.
But even if there was a piezoelectric effect you will have energy generated just when there is motion so say air was piezoelectric (not the case) and when you moved the switch you produced some energy then as soon as you close the switch (relevant moment) no more energy is generated.
That is why I mentioned a few times that nothing moves in this circuit.
The transformer will not work with DC not that energy is transferred trough the field it is still transferred trough wires.
If there are no electrons to move in primary then you do not have any electron movement in secondary.
If you apply DC to a transformer you are using that coil as a heater. The transient at connection is not DC.
Yes the transient is due to electron wave that starts to form electrons just moving in random direction canceling out to starting to have a defined direction.
You are charging the line witch is a capacitor thus you have a current flow due to creating an electric field by moving electrons on the capacitor plate that results in an electron vacating the opposite plate and that happens at light speed so depends on the gap between capacitor plates.
hamster_nz:
--- Quote from: electrodacus on May 05, 2022, 12:22:01 am ---The transformer will not work with DC not that energy is transferred trough the field it is still transferred trough wires.
If there are no electrons to move in primary then you do not have any electron movement in secondary.
--- End quote ---
You might have missed a word in there somewhere.
Are you saying if I have 1A DC in the primary I will have current flowing in the secondary?
Or just that with no current in the primary windings there will be no current in the secondary?
--- Quote ---The transient at connection [of a DC source to a transformer] is not DC.
--- End quote ---
Agreed - during the transient there is a changing magnetic field, and that changing field will causes a voltage across the transformer's secondary windings during the transient.
Once the magnetic field in the primary becomes stable then no more energy will be transferred to the secondary windings (apart from heat). That is until the DC source is disconnected. Then more as second pulse of energy will be transferred into the secondary (even though the DC source is disconnected!)
A resistor connected across the transformer's secondary windings will get (slightly) warmer during each of these transients - energy has been transferred from DC source to the resistor without them being electrically connected. That energy has definitely flowed between the two insulated wires that make up the transformer's winding (yes, through the insulation!).
The original Veritasium experiment has very long wires and so has a very long transient, during which a small amount of energy is transferred into the load through the space between the wires. But where along the wires this energy transfer is happening continuously changes depending on where the transient has got to. Only a tiniest initial amount is will be transferred directly over the 1m gap.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version