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"Veritasium" (YT) - "The Big Misconception About Electricity" ?
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SiliconWizard:
The fact it was a chicken-and-egg question was pointed out pages ago. =)
SiliconWizard:

--- Quote from: SilverSolder on January 18, 2022, 09:46:02 pm ---
--- Quote from: SiliconWizard on January 18, 2022, 06:48:51 pm ---
--- Quote from: SilverSolder on January 18, 2022, 04:26:28 pm ---Energy cannot be created or destroyed, it can only be transformed into another form!

--- End quote ---

I suppose you are assuming conservation of energy, which holds if we consider an isolated system.
Is the universe an isolated system? I'm not sure this has been fully answered yet. =) But this sure goes beyond the points made in this thread.

--- End quote ---

Can you think of an example where energy is created or destroyed,  rather than transformed?  -  I have always thought it a very basic law of the known universe...

--- End quote ---

You might watch this as a quick introduction to the matter:
rfeecs:
In the steady state, DC, does energy flow in the wires or in the fields?

Start with the fact that energy is contained in electromagnetic waves.  If you have used solar panels, you should believe this is true.

Consider a plane wave propagating through free space.  We know that energy is flowing with the plane wave.  We can measure it, and we can calculate it.  And the energy flux agrees with the Poynting vector.  We don't even need the Poynting vector to calculate it, but it does agree with the calculation:

https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Book%3A_University_Physics_II_-_Thermodynamics_Electricity_and_Magnetism_(OpenStax)/16%3A_Electromagnetic_Waves/16.04%3A_Energy_Carried_by_Electromagnetic_Waves

Now consider a transmission line.  It can be a twin lead transmission line as we have discussed.  Let's just feed it with a lumped source with say a sine wave.  Again we know energy is propagating.  We can measure it and we can calculate it.  We know the wave fronts are moving and we know that since there is energy in the field, the field is carrying the energy.  We can't say the energy flows in the wires or we would count twice the power that we measure.

Now lets say instead of a sine wave, we just have a battery and a switch.  At time t=0 we turn on the switch.  We have a rising edge, say 0V to 5V.  Now we know that this edge propagates down the line.  We can measure it.  We know that the energy is moving down the line in the fields.  We can see that if we slice up the space into thin slices of width dz, then as the wave front arrives at a point on the line, the energy of the slice goes up, and we know that that energy has to come from the slice behind it.  So one by one slices are filling with energy and energy is flowing all along the line in the fields.

Now if we look at a point where the wavefront has already passed, the fields are constant.  The voltage is constant and the current is constant DC.  But we know that energy is flowing through this point to fill the slices up to the wave front.  So here everything is static, except power is flowing through the fields.

Now we terminate the line with a resistor that matches the impedance of the line.  When the wavefront reaches the resistor it is totally and continually absorbed in the resistor.  But nothing has changed at the previous point on the line we looked at.  Energy is still flowing there in the fields even though the whole system is now in a steady state and DC only.

That's one way of looking at it.
SilverSolder:

--- Quote from: rfeecs on January 22, 2022, 12:27:15 am ---In the steady state, DC, does energy flow in the wires or in the fields?

Start with the fact that energy is contained in electromagnetic waves.  If you have used solar panels, you should believe this is true.

Consider a plane wave propagating through free space.  We know that energy is flowing with the plane wave.  We can measure it, and we can calculate it.  And the energy flux agrees with the Poynting vector.  We don't even need the Poynting vector to calculate it, but it does agree with the calculation:

https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Book%3A_University_Physics_II_-_Thermodynamics_Electricity_and_Magnetism_(OpenStax)/16%3A_Electromagnetic_Waves/16.04%3A_Energy_Carried_by_Electromagnetic_Waves

Now consider a transmission line.  It can be a twin lead transmission line as we have discussed.  Let's just feed it with a lumped source with say a sine wave.  Again we know energy is propagating.  We can measure it and we can calculate it.  We know the wave fronts are moving and we know that since there is energy in the field, the field is carrying the energy.  We can't say the energy flows in the wires or we would count twice the power that we measure.

Now lets say instead of a sine wave, we just have a battery and a switch.  At time t=0 we turn on the switch.  We have a rising edge, say 0V to 5V.  Now we know that this edge propagates down the line.  We can measure it.  We know that the energy is moving down the line in the fields.  We can see that if we slice up the space into thin slices of width dz, then as the wave front arrives at a point on the line, the energy of the slice goes up, and we know that that energy has to come from the slice behind it.  So one by one slices are filling with energy and energy is flowing all along the line in the fields.

Now if we look at a point where the wavefront has already passed, the fields are constant.  The voltage is constant and the current is constant DC.  But we know that energy is flowing through this point to fill the slices up to the wave front.  So here everything is static, except power is flowing through the fields.

Now we terminate the line with a resistor that matches the impedance of the line.  When the wavefront reaches the resistor it is totally and continually absorbed in the resistor.  But nothing has changed at the previous point on the line we looked at.  Energy is still flowing there in the fields even though the whole system is now in a steady state and DC only.

That's one way of looking at it.

--- End quote ---

I totally get that with AC, we can transfer energy through the field.  But DC...   hmmm.

There's another Veritasium video that is quite instructive:  how a Slinky spring falls...   Check this out:




First, the fall starts as a wave motion.  Then, once that initial transient is over, the slinky continues as a coherent block of moving particles - a current!

That's another way of looking at DC in a transmission line - a fast wave, followed by a coherent movement of the particles - a current!
adx:

--- Quote from: SilverSolder on January 21, 2022, 08:32:54 pm ---I have a quick question, maybe someone can help me understand:

What happens if we put the bulb inside a steel Faraday cage?  That would mean an outside electrical field cannot reach it, and any magnetic field would not reach the bulb either (it would by-pass the bulb via the steel box).

If the Veritasium video is right and the energy is carried by the magnetic/electric field and not the wire itself...   wouldn't that mean the bulb would not be able to light up inside the Faraday / magnetic shield cage?  (Similarly, we could put the battery inside a steel box, or both...)



--- End quote ---

If RF land (not my main skill);

* the highest frequency waves will simply reflect off the box and away.
* Lower frequencies will couple in the wires that remain unshielded. Making the holes (gaps) too small and long will create capacitive feedthroughs which will short out the medium frequencies with a low impedance, reducing the current which flows through to the bulb, and taking it around the Faraday cage, but it will still flow.
* Low and DC frequencies will flow 'through' the cable like normal, by that stage the fields which would otherwise rob the transient of power (to the bulb) are set up and no longer taking energy.You can still say the energy flows in the fields. Or the cables. If you didn't have holes in the cage and loop, there would be no voltage differences. If you didn't have a hole in space (for the wire etc) there'd be no current.
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