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I have a traditional circuit (a conductor wire, and voltage source and a resistor). The voltage source may be a battery in this case.

My questions(s) are:

  • From where do the electrons that flow in the circuit come from? Do they come from the wire or from the voltage source? I assume they come from the wire itself since the conductor has electrons in its last level that it can easily get rid of?
  • Looking at voltage in primitive terms: Is it a kind of repelling power? For example, will a negative voltage source repel electrons in the conductor, thus causing a current? Is that correct?
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    \$\begingroup\$ You may have difficulty with the 'cause' word in your second question. When you get down to the basic physics, it's a bit difficult to remain coherent about which came first, fields, potential, forces mediated by photon exchange, the role of energy etc. It's best to stay away from electrons and just worry about volts and amps \$\endgroup\$
    – Neil_UK
    Jun 27 '16 at 8:47
  • \$\begingroup\$ Variants on this question get asked a lot. Search for "electron" on the site. See e.g. electronics.stackexchange.com/questions/72875/… \$\endgroup\$
    – pjc50
    Jun 27 '16 at 13:29
  • \$\begingroup\$ Basic physics will sure help in understanding theses concepts if you want to dive in deeper, electrons are present every where they are major constituents of the atom, Voltage is the source which causes movement of electrons considering duality it can be thought as the changes in temperature which causes the flow of air, if you want to understand the concept of negative potential, try understanding battery and how they work. \$\endgroup\$
    – MaMba
    Jun 27 '16 at 15:15
  • \$\begingroup\$ +1 for "stay away from electrons". It will make you get all sorts of wrong ideas about electronics. Somewhat unintuitive, I think that if you are interested in electrons, you are not doing electronics - you are doing physics. \$\endgroup\$
    – pipe
    Jul 10 '16 at 3:29
  • \$\begingroup\$ Related: electronics.stackexchange.com/questions/233851/… \$\endgroup\$ Jul 10 '16 at 9:29
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Electrons are present in every atom.

enter image description here

In materials such as metals (good conductors) the outermost electrons are very loosely bound to the nucleus and tend to wander around randomly in the material matrix leaving 'holes' in the normally neutral atom's electron structure.

These 'free' electrons take their energy from thermal energy. Any individual electron (negative charge) wandering by a positively charged 'hole' will be attracted and may fill the gap. It may then sit there for a while and move off. Its a very random/chaotic process.

The result is that the material stays electrically neutral even though billions of free electrons are randomly wandering about in all directions at any given time. There is no net drift in a particular direction.

In a battery we produce a potential difference or voltage by accumulating and maintaining a surplus or decifit of electrons at the terminals. Note that the charge is produced in pairs so there always equal amounts of electrons and holes.

One terminal is positive (electron deficit, hole surplus) and the other is negative (electron surplus, hole deficit). This accumulation and maintenance of excess charge requires energy provided by a chemical reaction inside the battery.

enter image description here

When a circuit is made current flows almost instantly. This depends upon the wire but is somewhere between 0.5 and 0.9 times the speed of light.

This cannot be the physical (drift) movement of electrons due to the applied electric field as the drift speed of electrons is much (much) lower (see https://en.wikipedia.org/wiki/Drift_velocity). It must be an electro-magnetic wave.

A thought experiment: Imagine at the negative terminal end of the battery one (excess) electron moves from the terminal into the wire . This minute pulse of current (change in electric field) is transmitted through the wire at near the speed of light and pushes one electron out of the wire into the positive terminal of the battery to retain the overally electrical neutrality (balance) of the circuit

Now imagine this happening to billions of electrons. The result is what we term current.

The force that moves the electrons is an electric field produced by the voltage difference between the terminals and the basic rule is 'like charges repel, unlike charges attract'.

Electrons will be repelled by the negative voltage and attracted by the positive. Note that the electron does not need to travel through the wire, it only needs to move a small distance.

In terms of current direction we still tend to think that current flows from positive to negative (because that's what we teach in schools), this was due to an historical mistake made by Benjamin Franklin who simply got it wrong.In reality it doesn't matter as long as you are consistant in any calculations.

This loss and accumulation of electrons has the effect of reducing voltage at the battery terminals. It unbalances the chemical reaction inside the battery. The chemical reaction then tries to rebalance this by producing extra pairs of charge (electron + positive ion) to maintain the voltage at the terminals.

The more current that is taken by the circuit, the faster the reaction has to take place in the battery and the quicker the battery will be depleted. Measuring the voltage at the terminals you will see this drop as more current is taken.

For calculation purposes we think of a battery having an 'internal resistance' It doesn't have a real resistor inside, it just has a physical limit as to how quickly it can replace the neutralised charge pairs.

Rechargeable batteries can have their chemical reaction reversed by sending a current into the battery. The energy provided is stored in chemical form.

Fuel cells can be be provided with external chemicals to provide power. e.g hydrogen fuel cell - air (oxygen + nitrogen etc.) + hydrogen = electricity + water (plus nitrogen etc.)

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  • \$\begingroup\$ Wow, that's half a semester of physics in one post. Well done! \$\endgroup\$
    – uint128_t
    Jun 27 '16 at 15:59
  • \$\begingroup\$ @uint128_t - Thank you, that's very kind. I know its a bit of a long post to what appears a 'simple question' but I hope it will help others to understand the basic processes involved. \$\endgroup\$ Jun 27 '16 at 17:44
  • \$\begingroup\$ All good except battery: chemical reactions are on the surfaces, not between plates as shown. The electrolyte itself is a passive conductor which connects the battery-plate surfaces to each other. The two reaction sites are called "half-cells." So, all batteries behave as two separate cells connected in series. Important: they are connected by a conductive electrolyte which is a non-electron conductor. For example, in acidic electrolytes, the current is a flow of protons, not electrons. The arrows in your diagram are wrong inside the battery. (Many textbooks make this same error) \$\endgroup\$
    – wbeaty
    Jul 10 '16 at 7:25
  • \$\begingroup\$ @wbeaty You are totally correct. That's because I simply took a 'representive diagram' from the net which as you point out has failings in clarity as to exactly where and what type of reaction takes place. The question wasn't really about how a particular battery works (in this case a leclanche cell) but how current flows in a circuit with a voltage source that may (or may not be) a battery. It was a very long answer which could have ended up much longer. To improve clarity I shall edit the diagram. \$\endgroup\$ Jul 10 '16 at 10:38
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Electrons are everywhere in the conducting wire, or we can say there are millions of electrons in conducting wire.

Now, what is the current? Current is just flow of electrons.

Universally, we have assumed that the direction of current is equal to the flow of positive charges. Since, electrons are -vely charged. Thus, current flow in opposite direction to that of electrons.

Now, term "voltage" only symbolises, of how much electromotive force is on electrons. Here, the term " electromotive force" tells with which force, electrons are being attracted or repelled.

Example: Now, let suppose we have 9V battery. For instance imagine electrons inside 9V battery, how they will move. Now assume positive terminal of battery as +9 and negative as 0. Thus, electrons are being attracted to +9 side from 0V side, this is what voltage is.

Now, you can easily answer your own Question.

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>

Looking voltage in primitive term : Is a kind of repelling power ?

Yes, that's essentially correct (but, voltage also an attracting power, where a positive pole attracts negative charges.)

Circuit-physics involves magnetic fields and electric fields, the b-fields and the e-fields, corresponding to amperes and volts.

What is voltage? It's a measure of the electrostatic fields which produce electric forces and move charges around in circuits. Voltage is a way to measure E-fields, just as gravitational potential is a way to measure gravity fields, and magnetic potential is a way to measure magnetic fields.

If you magnetize a steel bar, you can pick up bits of iron, and that's a demonstration of magnetic fields. If you electrify a plastic rod, you can pick up bits of paper, and that's a demonstration of voltage; of e-fields.

One thing that gets missed by many intro textbooks: voltage is "static electricity," and electric circuits are based on electrostatics.

Rub a balloon on your arm-hairs, and you're producing extreme voltage but at zero current. Scuffing on rugs and zapping the doorknob, that's a matter of 5,000V or so. A battery is a charge-pump, and it pumps charges by producing a static surface-charge and a voltage in a thin layer between a metal and a conductive solution. (The chemical reactions on a battery plate; they're much like millions of tiny balloons rubbing on millions of wool sweaters! But only producing a few volts, not tens of thousands.)

Given a closed circuit, a voltage can pump charges along, as when connecting a light bulb to a battery. Voltage causes current, and voltage acts much like "electric pressure," but it's not a pressure exactly, since pressures only act upon surfaces, while e-fields can act throughout the entire volume of a collection of charges. Also, voltage can exist in vacuum, while physical pressure certainly cannot! :)

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Just to add to the above, the electron flow is actually quite slow, usually a few mm/s. That does confuse many people. What actually drives the circuit is the electric field induced when you close the circuit. That does move at the speed of light, C.

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    \$\begingroup\$ Just a small correction, its actually slightly slower than C, depending on the material it's travelling through (about 0.95C in copper 12-gauge wire) \$\endgroup\$
    – BeB00
    Jun 27 '16 at 9:26
  • \$\begingroup\$ Ok, true. I was just trying to explain the difference between the speed of the electrons through the circuit and the force that actually drives them! :) \$\endgroup\$
    – F. Bloggs
    Jun 27 '16 at 10:02
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  1. The electrons from the voltage source (- connector), travel trough the wire and load, and finally arrive back at the + connector of the source. That's why it is called an electrical circuit, because the electrons arrive back at the source. When you connect your source to the circuit, the electrons of the source enter the wire, while the free electrons in the wire also move in the same direction. So from the beginning, you have moving electrons everywhere along the wire.

  2. Its repelling forces at the negative supply and attractive forces to the positive supply, caused by the difference in potential and a corresponding electric field. It's the electric field that makes all the electrons move (or drift) at at the same time everywhere in the wire.

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  • \$\begingroup\$ > The electrons come from the voltage source... wrong. The electrons populate the entire metal circuit, all the time, even when the wires are not connected. Physicists call it "electron sea" or the "ocean of charge." All metals are conductive because they're jam-packed with mobile electrons: either one or two free electrons per metal atom. To attack the misconception, we could say that, first, electrons are drawn into the voltage source + terminal. Also, the electrons don't start out anywhere, any more that the rubber of a rotating tire "starts out" at one point. A battery is a charge-pump. \$\endgroup\$
    – wbeaty
    Jul 10 '16 at 7:30

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