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So, I know that when you're talking about something like a circuit that includes a battery, a circuit LOOKS like a loop, but that's merely because of the fact that the power supply is physically close to the output.

However, this doesn't have to be the case, right? Batteries are just self-contained because it's convenient, but you could drain a power source by connecting it to ANY positively charged endpoint, if I understand this right.

Are circuits therefore considered loops just because it's convenient to think of it that way? As a river cannot flow in a circle, neither can electricity because it would work out to a net zero potential thus meaning no movement!

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    \$\begingroup\$ "but you could drain a power source by connecting it to ANY positively charged endpoint" .... wrong! \$\endgroup\$ – Trevor_G Oct 12 '17 at 17:17
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    \$\begingroup\$ The river does flow in a cycle, it's just that part of that loop involves the ocean, clouds, and rain. \$\endgroup\$ – Andrew Morton Oct 12 '17 at 17:18
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    \$\begingroup\$ Your issue is trying to fit energy and electrons/current into the same model. Current is a chain, a chain that transfers energy from the power source to the load. Like all chains, in order to continue to work.. they need to loop back to the source. \$\endgroup\$ – Trevor_G Oct 12 '17 at 17:26
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    \$\begingroup\$ Ohms Law and everything in EE is a convenient abstraction of the real mess which is going down there. \$\endgroup\$ – Eugene Sh. Oct 12 '17 at 17:41
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    \$\begingroup\$ Not a technical answer, but from an etymological standpoint, "circuit" means "loop": etymonline.com/word/circuit \$\endgroup\$ – DukeZhou Oct 12 '17 at 20:38
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Most circuits are considered loops because charge in conductive materials tends to equalize electrostatic potential differences relatively quickly. Take a long wire/rod for instance. Let's say you can add electrons to one side of it. At first you start with 0 electrons. When you add the first electron, there's nothing else around so it can go basically wherever. When you add the second electron, it will push the first electron as far away as possible to try and create charge balance in the rod. This first electron moving is actually a tiny current and its movement could be used to extract work from it (because it took work to add the second electron to the system). Adding a third electron will push the second electron to the middle. The second electron's movement is half of the first so you could only extract half the amount of work out of it. The first electron is at the other end and hasn't moved at this point. If you keep adding electrons to the rod at one end, the movement of the other electrons will be less and less. Soon, you'll be at thousands of volts and not capable of extracting any work out of it because there's simply nowhere for the electrons to go.

Instead, what if we took electrons off of one side and added it to the other side? Now every electron you do this to will cause all the other electrons to move in response in one direction the same amount. Now you can extract a uniform amount of work out of the system for each electron you move. But what have you done? You've created a loop with your hand moving single electrons at a time. This is why most circuit utilize a loop. There's something that pushes electrons in one (or maybe both) direction. In your case, it's a battery, but generators and various other methods can be used to "pump" electrons to extract work from them at a different location.

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    \$\begingroup\$ This is a really good explanation, thank you. So basically a battery is a "pump" and also "catcher" to PUSH electrons into the system and then catch them as they fall out the other end? \$\endgroup\$ – dudewad Oct 12 '17 at 17:50
  • \$\begingroup\$ I want to clarify the point of my question, though: the "pump" side of the battery and the "catcher" side of the battery are not connected, except through the circuit itself, i.e. this is not a completely closed loop in the literal definiton. Am I right in saying that? \$\endgroup\$ – dudewad Oct 12 '17 at 17:52
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    \$\begingroup\$ @dudewad they're actually intimately connected. If you look into a battery, you'll see that the only reason it works at all is because it's allowing ions (charged atoms) to rebalance the charge difference that moving electrons causes. For every electron (-) that moves, a positive ion has to move internally to the other plate of the battery to balance this out. \$\endgroup\$ – horta Oct 12 '17 at 17:56
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    \$\begingroup\$ @dudewad If you were to take a battery apart and remove the electrolyte connection between the two (prevent ions from moving), then your battery would quickly stop working. \$\endgroup\$ – horta Oct 12 '17 at 17:57
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    \$\begingroup\$ @dudewad the battery has "magical chemical stuff" in it that moves the electrons from the positive side to the negative side. \$\endgroup\$ – immibis Oct 12 '17 at 20:14
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You can think of a battery as an electron pump, it chemically moves electrons from the positive terminal to the negative so as to maintain a certain 'electrical pressure' (That is what the early guys called potential difference in some old books, and it is not a bad model).

To actually make this thing do anything useful you need to provide a path for the electrons to flow that happens to use the moving electrons to do some kind of interesting task[1]. This might be heating a thin wire to make light, or powering some other electrochemical reaction to recharge another battery, or making a magnetic field in a motor or whatever. This path must clearly be a loop if you want the system to run more then very briefly (think nano seconds).

Note that at no point is there any mention of ground or such, all voltages are measured relative to some arbitrary point in your doings, and for that voltage to do anything useful there must be a loop for current to flow [2].

Ground is one of those really crap words that means at least 3 different things in a highly context dependent way, ignore for now.

[1] Electrons in a copper conductor at any sort of current you want to play with move on average really slowly, think less then a mm per second, but a wire is like a tube full of ball bearings, you push one in at one end, one pops out the other far faster then any ball actually moves down the tube.

[2] Yea, I know, flash memory gates, electrostatic lenses, laser printers, all sorts of slight exceptions, but roll with it for now.

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  • \$\begingroup\$ Its comforting that you say that about ground.... because yes I am confused by it as it seems to be a moving definition. It's amazing that my house hasn't burned down by now. Thanks! \$\endgroup\$ – dudewad Oct 12 '17 at 18:18
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Sorry, no. A battery or any power supply must, in the long run, remain electrically neutral. The restoring forces on separated charge are very large, and permanent separation of charge at a circuit scale simply isn't going to happen. This means that if current flows out of one terminal it must flow in through another. About the closest you'll get to an exception is an electret microphone, which contains permanently separated charges - just not a lot.

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  • \$\begingroup\$ But, could the negatively charged particles be stored, say, on one end of a hallway, and the receiving medium be placed on the other end? This would constitute not a loop but rather just some sort of an "equalization" so to speak. Does that make sense? I guess I'm trying to clarify whether "loops" is to be taken literally or not. \$\endgroup\$ – dudewad Oct 12 '17 at 17:25
  • \$\begingroup\$ @dudewad, the loops are taken VERY literally. All circuit analysis is based on the loop. People love to point out exceptions that happen in very extreme situations. But you will literally find circuit analysis impossible if you convince yourself that charges can flow into a circuit element and stay there. The macroscopic physical world matches this behavior well enough that you probably don't have to worry about any exceptions. \$\endgroup\$ – mkeith Oct 14 '17 at 2:26
  • \$\begingroup\$ Not only in the long run, but instantaneously also. I won't quibble over a few extra electrons fired from an electron gun or something, but from the perspective of circuit analysis, every circuit element must remain electrically neutral instantaneously as well as over the long run. \$\endgroup\$ – mkeith Oct 14 '17 at 2:39
  • \$\begingroup\$ @mkeith - I added the phrase specifically to account for electron guns and similar ion sources. And since electron guns used to be part of electronic circuits (although not configured to produce non-neutral components) I don't see how you can dismiss them. \$\endgroup\$ – WhatRoughBeast Oct 14 '17 at 3:17
  • \$\begingroup\$ @dudewad If you are just beginning to study circuits, take the idea of "loops" absolutely literally. But bear in mind that "loops" and only tell part of the story. If you want to explain what happens at the atomic level when a current flows through a wire, or what happens at high frequencies, or when electromagnetic radiation (e.g. radio transmission) is involved, or how any type of semiconductor device (even the most basic) works, you need much more theory than just "loops". But you have to start with simple ideas, and build on them ... \$\endgroup\$ – alephzero Oct 14 '17 at 3:23
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A circuit MUST be a loop. When the loop is closed, current flows through the load. When the loop is open, the circuit is turned-off.

Maybe you can assume that the voltage is the "force" which push the charges to flow in the loop. The current is that flow by unit of time.

Without loop, there isn't current.

... but you could drain a power source by connecting it to ANY positively charged endpoint, if I understand this right.

No. Look at the Energy Conservation and Charge Conservation. If we drain charges from a positive potential point, the charges MUST return through a reference point (negative terminal of baterry, GND, etc.), i.e. flow in a closed-loop.

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  • \$\begingroup\$ okay, that is helpful... does that mean there is a relationship between the source and the load that has anything to do with proximity? \$\endgroup\$ – dudewad Oct 12 '17 at 17:32
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    \$\begingroup\$ ...I say that because if you can have a 'ground', where literally the electricity flows out of the loop to the earth, then .... the earth becomes part of the "loop" somehow? That's the bit that's a little mysterious to me. \$\endgroup\$ – dudewad Oct 12 '17 at 17:33
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    \$\begingroup\$ This isn't accurate. Charge can and does flow from higher potential to lower potential. A loop isn't necessary. \$\endgroup\$ – horta Oct 12 '17 at 17:34
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    \$\begingroup\$ Look at "the earth" as the "return path" \$\endgroup\$ – Martin Petrei Oct 12 '17 at 17:34
  • \$\begingroup\$ Okay so my question isn't totally insane? I literally am just wanting to clarify that this is more like equalization of charge and not a true loop where effectively electrons somehow magically move in a circle (because they don't). \$\endgroup\$ – dudewad Oct 12 '17 at 17:36
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I believe there is a more generic answer and we should think of current not voltage.

All circuits have to follow Kirchoff's Law - founded on Maxwell's fundamental equations that describe how current flows in any media.

If a node is unconnected (via any media at any frequency) then it can't form part of the circuits. Conversely any node that is connected in any way forms part of the circuit.

Kirchoff's Law can be simply phrased as "The sum of currents in any circuit is zero" i.e. For every current leaving (in this case we'll call this a positive current) from a node an identical and opposite (negative) combinatorial sum of currents must enter the same node.

If you take this to a logical conclusion all nodes must be connected in one or more loops for the sum in the circuit to be zero. All the negatives and positives must cancel exactly.

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  • \$\begingroup\$ This is over simplified. For example, a radio transmitter definitely has a "current" flowing to the antenna - you can measure it with an ordinary meter - but the antenna itself is not a "closed circuit." Physically, it's just a collection of bits of conducting material constructed in a particular geometrical configuration. If you measure the DC resistance between the "termials" of the antenna, it is an open circuit! \$\endgroup\$ – alephzero Oct 14 '17 at 3:52
  • \$\begingroup\$ An antenna IS a closed circuit. A conventional antenna has two 'plates' with an alternating collapsing charge moving between them. Because of the ground plane effect even a simplified antenna has one real conductor and an imaginary one, see en.wikipedia.org/wiki/Monopole_antenna. \$\endgroup\$ – Jason Morgan Oct 15 '17 at 10:13
  • \$\begingroup\$ Simple is good. The Kirchoff/Maxwell simplification stands true. Perhaps you were considering the emitted RF as part of the circuit from a current perspective? This is no more true than the heat emitted in a resistor being part. Remember Maxwell's equations are general and don't just describe free waves in space. \$\endgroup\$ – Jason Morgan Oct 15 '17 at 10:21
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You are partly right, in that electrons will move from a potential to a higher potential. So you may think that if I have a voltage at a point in space A with a wire running to a different point in space B that electrons will flow through the wire.

schematic

simulate this circuit – Schematic created using CircuitLab

However, voltage on its own means nothing. A voltage is a difference in potential between two points.

schematic

simulate this circuit

That is, you need a common reference point in order to actually assert the voltages in the first place. You therefore end up with a loop, with some resistance, whether you like it or not.

However, that also brings up another point. In the circuit above, even without the reference wire, there is still some resistance between the negative sides of the voltage sources, albeit huge. As such, you have a loop and a tiny current will flow in the wire, though it may be too small for you to measure it.

It is also important to separate the notion of current from the notion of electron movement. Current is an abstract representation whereas electron movement is a physical phenomenon. We say current loops through the battery or capacitor, but in actuality, the electrons do not. Rather, an equal number of electrons exit them as do enter the other side. That difference is subtle, but important.

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  • \$\begingroup\$ I see. Yes do understand that current != electron movement, that much is clear to me. Thanks for dumbing it down for me :) \$\endgroup\$ – dudewad Oct 12 '17 at 18:21
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    \$\begingroup\$ @dudewad ha.. I thought I was dumbing it down for myself :) \$\endgroup\$ – Trevor_G Oct 12 '17 at 18:22
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Current is a flow of electrical charge carriers, usually electrons or electron-deficient atoms.

if we take a battery: the current flow in closed loop that is an electric current

enter image description here

but some one tell me OK, but what happen when you touch a positive of the source whey we are chocked,

enter image description here

well the answer is very simple : the person makes a closed loop with the earth to make the electrons flows within his body, but the bird are not shocked because it does not make a close loop

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    \$\begingroup\$ The battery diagram is wrong. It doesn't show the current through the center of the battery. If there is one ampere in the wire, then there is one ampere through the electrolyte. Electric circuits are closed loops. (A battery is a charge-pump, and the path for current is through the battery and back out again, same as with any component.) \$\endgroup\$ – wbeaty Oct 14 '17 at 1:34
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a circuit that includes a battery, ...LOOKS like a loop

It IS a loop. The electrolyte of a battery is a good conductor, a short cicuit.

Perhaps you have the misconception that batteries "supply charge," or that the current through the electrolyte is zero amperes? Nope, doesn't work like that. Batteries behave like short circuits, very low internal resistance, and a simple circuit with a battery is a closed loop.

Batteries don't supply any charge to circuits. The flowing charge comes from the copper itself, from the electron-sea of the metal.

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  • \$\begingroup\$ You mean good conductor of ions, right? If it was a good conductor of electrons, the battery would short circuit and catch on fire. chemistry.stackexchange.com/questions/2644/… \$\endgroup\$ – horta Oct 13 '17 at 17:37
  • \$\begingroup\$ > conductor of ions A good conductor, period (conducts high amperes.) The electrolyte is a "shorting jumper" between the plates. That's how all batteries actually work \$\endgroup\$ – wbeaty Oct 13 '17 at 19:12
  • \$\begingroup\$ True. It's an ion short rather than a metal wire short, but you're correct that an ion conductor is still a conductor. I feel like it makes it confusing if you don't make that distinction. \$\endgroup\$ – horta Oct 13 '17 at 19:26
  • \$\begingroup\$ Less confusing: just don't mention electrons at all. Don't mention ions either. The electric current takes the form of a closed loop, with no beginning or end. The path for current leads through the middle of the battery, just as with light bulbs the path for current leads through the filament. Or with dynamos, the path for current leads through the coil. Common misconception: one plate of the battery is a source of charge, while the other plate collects "used charge." That's wrong, since charge flows through the electrolyte, while the battery, as a whole, functions as a charge-pump. \$\endgroup\$ – wbeaty Oct 14 '17 at 1:41

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