Quote from: TinselKoala on July 18, 2012, 11:05:18 PM
But B is in series with one 12 v battery and bulb C (removing the top of the circuit), and it's also in series with the other battery and bulb A (removing the bottom). Why should it not be lit at normal 12 volt brightness?

Well if you separate the bottom and top, Batt/A/B  or Batt/C/B , each will have an opposite flow of current through B. So when combined, those 2 different current flows will cancel out. No B flow.

The first circuit is like 2 weak 12v power supplies powering the B bulb. Take one of those weak supplies( Batt/A or Batt/C ) away and there will be less current available to B. Or, take away B and no current flows in the circuit.

The second one, we could look at B's connection to the batteries as 0v common with 12v+ and 12v-.  If that point between the batteries is our common, using a meter we will see 0v on the other end of B, due to the equal voltage division between A and C.
So B is connected across neutral points in the circuit thus 0v across B.

MaGs

Ah. So the flow of current between two points is determined by the voltage difference, or potential, between those points. If there is no potential, then there is no current flow.
And there is only one kind of current flow, there isn't a "positive current" and a "negative current", there is only the flow of (negative) charge from regions of relatively high potential to regions of relatively lower potential (aka voltage). (Thanks, Ben, for guessing wrong and screwing everyone up for ever after).
And the idea of a positive voltage on one side of the bulb "cancelling" or opposing the positive voltage on the other side is simply a matter of the charge pressure being equal and so there is no reason for it to "want" to go in either direction... because the pressure is equal or, to put it differently, the potential difference-- the voltage-- applied across the bulb is zero.

Quote from: TinselKoala on July 18, 2012, 11:57:24 PM
Ah. So the flow of current between two points is determined by the voltage difference, or potential, between those points. If there is no potential, then there is no current flow.
And there is only one kind of current flow, there isn't a "positive current" and a "negative current", there is only the flow of (negative) charge from regions of relatively high potential to regions of relatively lower potential (aka voltage). (Thanks, Ben, for guessing wrong and screwing everyone up for ever after).
And the idea of a positive voltage on one side of the bulb "cancelling" or opposing the positive voltage on the other side is simply a matter of the charge pressure being equal and so there is no reason for it to "want" to go in either direction... because the pressure is equal or, to put it differently, the potential difference-- the voltage-- applied across the bulb is zero.

Are you asking? ;]

Sometimes Ive use the analogy of water pipes and valves to explain electricity to people that dont know how it works. But I think air pressure is a better way to look at it, being water will not compress or decompress. But water pressure and valves and pipes are things most people are familiar with, so it is easier for them to relate.



MaGs

I prefer to use water because its very incompressibility means it can transfer energy "instantly" (really at the speed of sound) in spite of flowing slowly, just like electrons somehow can. Let's save the springy compressibility effects for components, like specifically inductors and capacitors.

OK, thanks for coming and I'm sorry I'm late. I was hoping to get to the Hydraulic Analogy's description of capacitors and inductors last night but other things, as you can tell, happened in between.

The Hydraulic Analogy (HA) to the Inductor.

Well, OK, to make up something that acts like an inductor we have to know how an inductor acts, and to know that we have to make up something that acts like an inductor, and to know that..... well, let's just start and see where we end up

Charge is fundamental, charge is conserved, charge is quantized, comes in two flavors, like repels like, the electron is inseparable from the unit negative charge; away from the center of atoms and cyclotrons, positive charge is the absence of electrons where they "should" be, or a region where electrons are depleted. And it takes a _heck_ of a lot of negative Unit Charges to make a Coulomb, which is an amp-second of charge.

And in the HA we imperfectly represent charge by water, in the aggregate, current by water flow, conductors by pipes, resistors by constrictions, switches and rheostats by valves, voltage by pressure, and power sources by pumps and elevated reservoirs.

But what about mysterious components like the inductor... how shall we represent it?

I like to think of an inductor as a section of pipe with a valve on the far end (low pressure end), closed, and springy elastic walls, and a valve on the near end (high pressure end), open.  When the flow of water (current) comes in and enters the near end, the pressure of the water makes the springy walls of the tube expand and accept more and more water, but also pushes back on the water so the more full, the harder to push more in. The far end valve is connected to the walls though and as the tube swells up the far end valve starts to open, and by the time the walls are maximally swelled out the valve is fully open and there is no  more constriction and the flow is now just what it would have been if there was only a smooth pipe there -- only now there is a reservoir of energy stored in the springy, expanded walls of the inductor. So the full flow is delayed for a time while the springy walls are stretching out and the valve is opening.  Then... when the upstream supply of water is cut off, the upstream (HV end) valve closes, and the springy walls then can squeeze out all that stored water thru the lowvoltage end valve--- so the current actually continues for a time.

The springy walls are of course the magnetic field produced by moving charges, arranged to be concentrated by various means like coiling wires and/or wrapping them around materials that encourage magnetic fields to penetrate them.

So an inductor is a component that stores the energy of an incoming current in a concentrated magnetic field like a springy reservoir, and as long as the current is constant, it might as well be just a pipe. Only when the current is increasing (delayed, swelling, storing energy) and decreasing (also delayed, shrinking, releasing stored energy) does the inductor make itself felt to the flow of current. In the steady state, the magnetic field of the inductor takes no extra energy to maintain, just as a permanent magnet doesn't take any energy from the outside to maintain its field once it's established.

This is why contacts arc when an inductive load is switched off.... the magnetic field collapses and tries to maintain the current flow in the same direction it was going, and will make an arc as the switch contacts separate, maintaining the circuit as the stored magnetic energy -- as voltage--- continues to push charge through the circuit until the energy is depleted.

OK, digest that for a little while. Thanks for your attention...

Next: Capacitors and the HA.

 Hogen, a Chinese Zen teacher, lived alone in a small temple in the country. One day four travelling monks appeared and asked if they might make a fire in his yard to warm themselves.
While they were building the fire, Hogen heard them arguing about subjectivity and objectivity. He joined them and said: "There is a big stone. Do you consider it to be inside or outside your mind?"
One of the monks replied: "From the Buddhist viewpoint everything is an objectification of mind, so I would say that the stone is inside my mind."
"Your head must feel very heavy," observed Hogen, "if you are carrying around a stone like that in your mind."