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Im going back to electricity 101 to understand several things Im missing. One of them is exactly how does current flow through a DC source.

Heres my understanding: A voltage source is one which has an excess of electrons in one side of its terminals, and a lack of electrons in the other, when connected together via a resistance or cable, electrons will flow from the side with an excess of electrons, to the side with a lack of electrons.

Inside the source itself theres no current flow, only "around the supply", meaning if I have say a battery, the only way that electrons will go from one side to the other, is by connecting a cable from its + to - side, the electrons will flow through the cable all the way to the other side. When both sides have reached an equilibrium in which both have the same amount of electrons, theres no voltage potential (dead battery), thus no current flow.

Very often a textbook will substitue components for DC sources, for example if we have a voltage source connected to a diode in series with a 2 resistances, the equivalent circuit will substitute a 0.7V DC source for the diode, and add the 2 resistance in series with it such as this:

Lets take this circuit:

Diode circuit 1

The equivalent circuit is the following, where V2 equals the diode:

Duide equivalent

So the way I understand it, in the circuit above, replacing the diode with say a 0.7V battery or 0.7V DC power supply would be the same as having the diode, yet, how is it possible that current will flow from the positive terminal of V1, through the resistor, THROUGH the 0.7V battery (or dc supply), through the second resistor into the negative terminal? I thought that no current could flow through a battery or dc source.

I used a diode to make an example but there are several places in which Ive seen multiple voltage sources in a circuit, yet I dont understand how can current go through them.

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  • \$\begingroup\$ Inside the source, current flows from + Positive to - Negative. \$\endgroup\$ – Optionparty Nov 19 '12 at 19:24
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    \$\begingroup\$ @Optionparty, the direction of the electric current through a voltage source depends on whether the source is supplying or receiving power. The electric current through the voltage source is out of the positive terminal if it's supplying power and into the positive terminal if it's receiving power. \$\endgroup\$ – Alfred Centauri Nov 19 '12 at 20:19
  • \$\begingroup\$ I understand that, but I thought that electrons went from positive to negative (conventional current), and stayed in the negative side, say like a glass of water being emptied to another glass. But what you are saying is basically that one glass of water is being emptied to a 2nd glass of water which is returning the water to the first glass, thus creating a loop? Then if all electrons return to its original side, why its not an endless loop, why batteries ran out and why is there power consumption? \$\endgroup\$ – S.s. Nov 19 '12 at 21:13
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    \$\begingroup\$ @JoeM, in a battery, the electrolyte reacts with two different metals causing excess electrons to accumulate on one while electrons are removed from the other. This chemical reaction is stopped by the electric field between the terminals, due to the accumulated charge, if the battery isn't connected to anything. However, when the battery is connected to a circuit, electrons flow from one terminal, through the circuit and to the other terminal and this allows the chemical reaction to proceed until the reactants are exhausted. \$\endgroup\$ – Alfred Centauri Nov 19 '12 at 22:52
  • \$\begingroup\$ Thank you that helps, the thing is this, I understand that the electrons will flow though the circuit from the + terminal to the - negative terminal, my question is, an electron comes out of the + terminal, goes throught the circuit and reaches the - terminal, what happens to the electron when it reaches the - terminal, it jumps back to the positive terminal throught the electrolyte? \$\endgroup\$ – S.s. Nov 20 '12 at 1:28
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My favorite educator, Bill Beaty, often rants at the many misconceptions that all too many people have been infected with. One of the many common misconceptions involves batteries.

"Frequently-Asked Electricity Questions": "THE LIQUID BETWEEN A BATTERY'S PLATES IS A GOOD CONDUCTOR. SO WHY DOESN'T IT SHORT OUT THE BATTERY?"

"Why is electricity so hard to understand?" "...mistaken belief that no charge flows through batteries. ... This leads to the traditional incorrect flashlight-current explanation (current comes out of battery, flows...etc.) It also leads to the misconception that batteries SUPPLY CHARGE, and have a storage place for "used" charge. This might make sense if we believe that there's no path for charge through the battery. But it's wrong, because there is a path, a path provided by flowing charged atoms. Charge must flow around and around a circuit, passing THROUGH the battery over and over."

"But how SHOULD we teach kids about 'electricity'?" "A battery is a chemically-fueled charge pump. Like any other pump, a battery takes charges in through one connection and spits them out through the other. A battery is not a source of the "stuff" being pumped. When a battery runs down, it's because its chemical fuel is exhausted, not because any charges have been lost. ... When you "recharge" a battery, you are pumping charges through it backwards, which reverses the chemical reactions and converts the waste products back again into chemical fuel."

'Which way does the "electricity" really flow?' "When you connect a lightbulb to a battery, you form a complete circuit, and the path of the flowing charge is through the inside of the battery, as well as through the light bulb filament. Battery electrolyte is very conductive."

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  • \$\begingroup\$ Thank you very much, that was a perfect answer, and if I may add those links are a great source of information, it really explains straightforward a lof of misconceptions. \$\endgroup\$ – S.s. Nov 20 '12 at 4:22
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I thought that no current could flow through a battery or dc source.

An electric circuit is a closed path through which there is an electric current. Think of a simple series circuit - a battery and a resistor. Since the circuit elements are in series, the current through each element is identical.

An ideal voltage source will support any current through an external circuit.

Physical voltage sources, like a cell or battery, have a maximum, short circuit current.

Inside a physical battery, there is an internal electric current through the electrolyte. Usually, this current is due to the flow of ions rather than the flow of electrons.

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I thought that no current could flow through a battery or dc source.

It can and does. Even in your first diagram, you have current flowing through the 12V source.

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  • \$\begingroup\$ So, in my first diagram, current (from a conventional approach) is going from the + side through the resistor, through the diode, through the second resistor to the - side, and from the - side of the battery internally through to the + side? \$\endgroup\$ – S.s. Nov 19 '12 at 21:10
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    \$\begingroup\$ Yep, that's it. \$\endgroup\$ – Jim Paris Nov 19 '12 at 21:41
  • \$\begingroup\$ So what is actually moving the electrons? \$\endgroup\$ – S.s. Nov 19 '12 at 22:03
  • \$\begingroup\$ The battery, of course. I'm not sure I understand your question. If you're asking how a battery does that, see this. \$\endgroup\$ – Jim Paris Nov 19 '12 at 23:01
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    \$\begingroup\$ Yes, the voltage source pushes on the pre-existing charged particles. \$\endgroup\$ – davidcary Nov 20 '12 at 3:19

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