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I'm sorry if this question may seem wrong in many cases.

  1. What would happen if we had a wire with the length of 1 kilometer that connected the two terminals of the battery?

  2. Do electrons care if the other end of the wire is the positive terminal OR do they just flow inside the wire no matter what's at the other end of it until they are exhausted?

  3. What would happen if we connected a battery not to itself but to another terminal that drains it and never reconnect it to the positive terminal? does the battery deplete or does it stop working? Care to explain? Here's a related question that wasn't answered in detail.

Thanks

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Electrons will flow as long as there is an electric field to move the. When you first connect the wire to the negative terminal of the battery, the electric field generated by all of those electrons stuck on the negative terminal will cause them to move into the wire. They will basically move to distribute the electrons evenly along the wire. This happens fast. Really fast. The exact speed depends greatly on your particular wire, but we're talking microseconds, even over 1km of wire.

Now eventually the electrons reach the far end of the wire, where they get to interact with whatever you hooked it up to. If this is the positive terminal of the battery, then there's a bunch of positively charged molecules that they can combine with to become neutral. This is basically always a desirable thing, from an energy perspective, so they do it really fast. Once those molecules are neutralized, the chemical reaction in the battery is knocked out of equilibrium, and it starts generating more electrons at the negative side and more positive ions on the positive side. It does so by depleting chemical energy. This opens the door for more electrons to flow through the wire, and the expected result occurs: a short circuit.

Your third scenario basically never happens in reality to any meaningful degree (outside of exotic things like Van de Graaff generators... and scuffing your feet on the carpet). What ends up happening is an electrostatics problem. You keep plucking electrons off of the end of the wire, so the electrons keep re-distributing. As they do so, the entire structure (wire and batteries) becomes more positively charged.

Now how did you pluck the electrons off the wire in the first place? You had something that was more positively charged, so that the electrons wanted to go in that direction. Short of tiny tweezers, that's really the only way to pull the electrons off. But now your whole object is becoming more positively charged. This diminishes the electric field you were relying on to pull the charges off the wire. Eventually you reach an equilibrium where the charge on the wire is exactly right to have no electric field between the end of the wire and your device that's pulling electrons. The flow stops there. (and, incidentally, the positive side of the battery is ever so slightly higher in voltage. Whatever charge the negative side had to reach equilibrium, the positive side has that plus the EMF of the battery).

Now keep this going to an extreme, and weird things start to happen. If your object gets positively charged enough, you'll eventually reach the ionization potential of the air. When this happens, electrons will flow through the air onto your object, creating an arc. This is exactly what is happening when you charge up a Van de Graaff generator (except in reverse. Typically those generators create a highly negative object... but the same rules apply).

Now your battery and wire has become "battery, wire and walls" (and possibly you--did you remember to leave the room before engaging in high voltage activity?). This means you have more electrons to distribute. Eventually you will swamp whatever positive source is drawing all of those electrons away.

If you want to see what happens in this situation, check out the Dueling Tesla Coil Dudes. They have some generators configured so that they take a bunch of electrons from one guy and move them to the other. Most of the time they just interact with the air around them, neutralizing their charges. But when they get close enough, it's time to pay the piper!

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  • $\begingroup$ Thanks again for the detailed answer. This really opened my mind on how the electrons work at the terminal of a battery. $\endgroup$ – Cypher Jun 15 at 16:25
  • $\begingroup$ Cort, note that the electric field in the wire gets established immediately, but individual electrons move through the wire VERY slowly. Your first paragraph can be misinterpreted by a novice reader. $\endgroup$ – David White Jun 15 at 18:24
  • $\begingroup$ @DavidWhite your description isn't that accurate either. The E field propagation speed is about half the speed of light in vacuum (so you're right, "instantly") but the electrons that are responsible for the currents have speeds near the Fermi velocity, i.e. about 1% of the speed of light in vacuum. But they are very few of them, about 1 in ten billions of the free electrons are actually able to carry the current produced by the applied E field. But the point is that they move very fast, not slow (the so-called drift velocity). $\endgroup$ – thermomagnetic condensed boson Jun 15 at 20:07
  • $\begingroup$ @thermomagneticcondensedboson, I'm sure that the electrons move very fast when they are moving in between atoms. However, they quickly collide with atoms and momentarily stop when they do so. Implying that a few electrons move at the Fermi velocity all the way through the wire is incorrect. $\endgroup$ – David White Jun 15 at 20:10
  • $\begingroup$ @DavidWhite Most free electrons cannot collide with atoms (more precisely with impurities and phonons). The ones that do are the ones having a speed near v_F. When one applies an E field, these interactions with phonons change the direction of the electrons in the opposite direction (in average), but keeps the magnitude of the speed intact, i.e. still v_F. So electrons having -v_F get to v_F when the E field points towards the minus direction. $\endgroup$ – thermomagnetic condensed boson Jun 15 at 20:20
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Electrons are actually always moving, even when there's no potential difference (aka voltage). They just move in random directions.

What happens when there is a potential difference, i.e. when the wire is connected to two terminals of the battery, is that although the electrons continue to move in random directions, there's now a slight bias towards one direction. That's what constitutes the current. The speed due to current is actually very slow, usually a few millimeters per second. This is in contrast to their random speed which is much faster. See the Wikipedia article on drift velocity.

If only one end of the wire is hooked up to the battery, then there is no potential difference, and no current. You can't discharge a battery by connecting only one terminal to "something" either. That something will just become charged too.

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  • $\begingroup$ When I think about electrons in water terminology the battery seems like a barrel full of water, if we have a barrel full of water and let lose the water, then we can use the flow to move something, but in case of a battery and electrons, you're telling me that letting lose the electrons doesn't do anything (like lighting up a bulb), why is that? shouldn't the electrons just flow from the battery and light the bulb and then get to the ground (the wire is connected to the ground and not the battery)? why can't a one way connection from battery to the ground like this work? $\endgroup$ – Cypher Jun 15 at 9:47
  • $\begingroup$ Did you read the link? The second sentence makes it clear that what you're asking doesn't make sense. $\endgroup$ – Allure Jun 15 at 10:07
  • $\begingroup$ There are two of the free electrons that aren't moving at all (zero speed), though they're delocalized over the whole sample. The drift velocity concept is misleading to explain what really happens in a metallic conductor. $\endgroup$ – thermomagnetic condensed boson Jun 15 at 20:16
  • $\begingroup$ If you hook one end of a wire up to a battery, there's a little bit of a potential difference: the wire is effectively one plate of a capacitor, and a few electrons flow in to charge it up. $\endgroup$ – Mark Jun 16 at 5:00
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1-Current is flow of electrons . Electrons will only move only and only if there is a potential difference between the ends of a wire . This case also applies to battery as the positive end is maintained at a higher potential than that of negative terminal . Also if you connect an end of a 1km long wire to battery and ground the other end . You will notice a current flowing provided you have maintained the battery potential at a very high potential as compared to earth ( 0 volts ) and your ammeter is sensitive enough to detect the current .

2- The electrons will move from lower to higher potential . This can be noticed in an electric field . There is random motion of electrons inside a wire because electrons in a conductor continously adjust themselves in such an orientation such that net electric field is zero and hence electric field inside a conductor is zero .

3- I am not clearly able to understand your question .The problem suggested in the link shows that that bulb does not glow . This shows that bulb requires continous current to glow spontaneously . The bulb in that condition will glow for a very short period of time and then go off which might be hard to notice . This also follows from fact in these conditions you require a closed circuit for it to glow .

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  • $\begingroup$ Thanks. about answers #1 & #2, how do electrons know that at the end of the 1 Km wire the potential difference is enough for them to start moving towards it? do they automatically flow inside the wire until they're exhausted (like water does inside a pipe until the pressure can't push it any further) or do they somehow get a feel of the potential difference between their current position and the destination (the ground the wire is attached to 1 Km ahead) ? $\endgroup$ – Cypher Jun 15 at 9:42
  • $\begingroup$ @Cypher they don't "know" anything. If an electron has other electrons more bunched together on one side of if than on the other side, the difference in electrostatic force will move it to even up the spacing. Note, some of the answers may give the wrong idea that some electrons move the whole length of the wire. That is completely wrong - the disturbance to their positions moves fast along the wire, but each individual electron will only move a small fraction of a millimeter, if we are thinking about a typical low voltage battery. $\endgroup$ – alephzero Jun 15 at 18:13
  • $\begingroup$ Warning, this answer is wrong. The E field inside a conductor is not necessarily zero. In fact, if it were zero then the charges, in average, wouldn't have a net displacement and there would be no current. So when one applies a voltage (or electric field), one is creating a non vanishing E field inside the conductor. Maybe Cheesykid296 had the electrostatics case in mind, but it is hard to tell. As is, this answer is wrong. $\endgroup$ – thermomagnetic condensed boson Jun 15 at 20:12
  • $\begingroup$ Not to mention that "current is flow of electrons" is not really true either. How do you call a flow of protons? Or positrons? A current :) $\endgroup$ – thermomagnetic condensed boson Jun 15 at 20:13
  • $\begingroup$ I am talking from the conventional point of view . Electric current is then flow of charge carriers . Here electrons are then the charge carriers .Also that's true that I had electrostatic case in mind . But I would still say that E field inside a conductor will be zero if you are not applying a potential difference.You maybe missing a point that electrons continously orient themselves to cancel out the natural electric field which is weak . Like a metal rod kept on table . When a strong E field is applied which cannot be cancelled, electrons then drift rapidly which causes a current . $\endgroup$ – Cheesykid296 Jun 16 at 5:15

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