12

We know that at point A and B there is same voltage.Then why electron moves from one point to the another one?enter image description here

Abdullah Al Zami
  • 315
  • 1
  • 7

6 Answers6

28

For the same reason an object can theoretically slide at constant velocity on a surface having zero friction without the application of a force, electrons can theoretically flow between points A and B without a voltage between A and B if there is zero resistance between the points. The electrical resistance is analogous to the mechanical friction.

However, in the real world there is no such thing as zero friction and no such thing as zero resistance (an exception being low temperature super conductors). In electrical circuit diagrams the wires connecting electrical components are assumed to have resistance so much less than the components that it can be ignored so that there are no voltage differences along the wires.

Then velocity of electron will become less after passing through a resistor?

No, and that is a common misconception.

The current in a resistor (rate of charge transport across any cross sectional area of the resistor) is the same throughout the resistor. The electrons do not slow down on average as they move through the resistor. If less electrons per unit time exit the resistor than enter it, electrons would pile up in the resistor, which doesn't happen. However, for a given voltage between two points, the greater the resistance between the points, lower the current (the slower all the electrons will move through the resistor).

The reason why the electrons don't slow down in the resistor is that the kinetic energy they lose in collisions with the atoms and molecules of the resistor as heat is continuously replenished by the work done by the force of the electric field applied to the resistor, so that the average kinetic energy of the electrons remains constant (the average drift velocity of the electrons remains constant). A mechanical analogy is pushing an object at constant velocity on a surface with friction.

Hope this helps.

Bob D
  • 71,527
  • Then velocity of elecrton will become less after passing through a resistor? – Abdullah Al Zami Nov 14 '20 at 19:59
  • @AbdullahAlZami I've updated my answer in response to your follow up question. – Bob D Nov 14 '20 at 20:36

  • The current in a resistor (rate of charge transport across any cross-sectional area of the resistor) is the same throughout the resistor.

    Do you think a continuity agreement (similar to how we do in incompressible fluid flow) be correct to apply here? I.e: current - in = current-out

     – tryst with freedom Nov 15 '20 at 05:39
  • 1
    @Burian in a way yes. For steady flow of an incompressible fluid the continuity equation is rooted in the law of conservation of mass. For current it’s the law of conservation of charge – Bob D Nov 15 '20 at 05:48
  • There is an extra 'a' in the name, I didn't get the ping because of that. Anyway, it was a good answer! – tryst with freedom Nov 15 '20 at 06:24
  • 2
    @Buraian sorry about that. Used my mobile phone. For some reason names don’t auto fill making it more error prone – Bob D Nov 15 '20 at 06:33
  • @BobD Note that electrons moving slower doesn't necessarily imply a pile-up; they could also get closer together. (Though they actually can't, because they're electrons, not cars – but all I'm saying is that the analogy doesn't hold.) – wizzwizz4 Nov 15 '20 at 13:53
  • Really nice answer. – Árpád Szendrei Nov 15 '20 at 16:36
22

"We know that [at] point A and B there is same voltage." There isn't. There will be enough of a voltage difference between A and B to drive a current through the wire. The resistance of the wire between A and B is, we assume, very low, so only a very small voltage difference will be needed.

Philip Wood
  • 35,641
  • 4
    While I don't disagree with your answer given the last sentence, I think you know that, theoretically, it is not necessary to have a voltage difference between A and B in order for current to flow between A and B. – Bob D Nov 14 '20 at 22:45
  • 4
    @Bob D I assumed that Abdullah al Zami was thinking about an ordinary copper connecting wire, as suggested by his circuit diagram. – Philip Wood Nov 14 '20 at 22:55
  • 2
    I don't see anything in his circuit diagram suggesting an "ordinary copper connecting wire". I only want to know if you agree that if there were theoretically zero resistance between points A and B then points A and B would be at the same potential but current could still flow between A and B. Do you agree with that, or not. – Bob D Nov 14 '20 at 23:06
  • 2
    @Bob D. The fact that he's used ordinary circuit symbols suggests to me that he's thinking about an ordinary circuit. But I do agree with your last sentence. – Philip Wood Nov 14 '20 at 23:27
  • OK.Then I think we are on the same page. i – Bob D Nov 15 '20 at 00:40
  • @BobD I don’t see anything in the circuit diagram suggesting that A and B are at the same electric potential. In addition to what Philip Wood said (ordinary circuit symbols), the question overall reads like a question from a student just starting to learn about electric circuits. Such students are sometimes taught that wires have zero resistance. This is the obvious explanation for the claim that the points have the same potential. – Brian Drake Nov 15 '20 at 09:29
  • @Brian Drake of course that’s what he was thinking and that’s the basis of my answer too. But it’s also true that technically you don’t need a voltage difference between A and B for current to flow. Superconductors can maintain a current with no applied voltage whatsoever. So I’m not sure what’s purpose of your comment. It seems like a criticism but I don’t know why – Bob D Nov 15 '20 at 10:10
  • @BobD Now I’m not sure myself why I posted that comment. Probably because of this line: “I don't see anything in his circuit diagram suggesting an ‘ordinary copper connecting wire’.” The line seems to suggest that Philip Wood’s assumption is unreasonable. I’m not trying to criticise the actual physics here – in fact I upvoted your answer. – Brian Drake Nov 15 '20 at 10:49
  • @Brian Drake OK. I never meant to imply Philip’s assumption was unreasonable either. – Bob D Nov 15 '20 at 10:54
6

I'm afraid this is like asking "if the river is level, why does the water move?" Because the water is under pressure.

Voltage is akin to pressure. The point of confusion arises because you're ignoring the fact that the battery is creating "pressure" (a potential difference). And what you're asking is, "why does the water in the river have the same pressure at two points along its length?"

Perhaps what you've forgotten is that you can't actually measure voltage at one point. It must be measured against a reference point. You've forgotten that the potential difference between two arbitrary points on a wire and "ground" is the same. From a very simplistic perspective, it's the "ground" reference (or a bit more accurately, the voltage potential between the reference and the test point) that's making the electron move.

JBH
  • 158
  • 2
  • 13
2

If you pump water through a frictionless pipe, why does the water move? It moves because there is water pushing it from the pump. This force is transferred from one molecule to the next. The force between water molecules "wants" to average out, therefore the whole body of water must move. Electrons repel each other. If you force electrons into one end of a wire and there is somewhere for them to go, then, by keeping the mean distance between each other constant (by electrostatic repulsion), they must move to the current sink. Imagine a series of compression springs attached end-to-end. Push at one end and they all move even if the measured pressure is the same between any two.

In fact, this is why a solid moves if you push at one end. You are compressing zillions of tiny springs and, in the absence of friction, this pressure will even-out along the length of the solid.

chasly - supports Monica
  • 3,006
0

in an ideal wire, $R_{ab} $ is $0$, so the electrons will have zero resistance, and their direction moving won't change.

dissolve
  • 109
-2

Because of Newton's first law.

WillO
  • 15,072
  • 1
    Electrons in wires are not free. See the mean free path in wires. It is not clear whether the Newton's laws apply, in the way I think you are suggesting – Cryo Nov 14 '20 at 20:48
  • @Cryo Electrons in metals are relatively free, that is, they are highly mobile. – Bob D Nov 14 '20 at 23:17
  • 1
    @BobD https://homepages.rpi.edu/~galld/publications/PDF-files/Gall-116.pdf Mean free path is tens of nm. So no, not free. You can describe electrons as being free by defining conduction electrons, which are quasi-particles. But it is not at all obvious that Newtons laws will apply then, since you are swapping continuous translational symmetry of the free space for discrete translational symmetry (hence why you get the crystal momentum). So I stand by comment Newton's laws, in solid state, do not apply in the simplistic way implied above. Picture changes a bit in plasmonics – Cryo Nov 15 '20 at 11:05
  • Electrons in metals are considered to be mobile charge carriers. – Bob D Nov 15 '20 at 11:13
  • @BobD. They may be mobile, but they are not moving in empty space, they keep colliding with each other, lattice defects, phonons etc. So transport of electrons is diffusive. If you give electron a momentum within tens of nanometers it will start colliding with other stuff and that momentum will be passed to the wire as a whole, whilst energy will become heat. The end result is that if you push an electron, but will not keep up the pressure, the electron will 'stop' very soon – Cryo Nov 15 '20 at 18:13
  • 'stop' in quotes is because the Fermi velocity of electrons in metals is 10e5 m/s, transport speed is (if I remember correctly) mm/s. So motion of electrons due to applied voltage is a VERY small extra bias on top of extremly high speed due to thermal motion and Fermi statistics of electrons. – Cryo Nov 15 '20 at 18:15
  • @Cryo Yes yes of course they don't move in empty space. I never said or implied that . When I say they are mobile I mean it in the sense of the following quote from the link below "instead of orbiting their respective metal atoms, they form a “sea” of electrons that surrounds the positively charged atomic nuclei of the interacting metal ions. The electrons then move freely throughout the space between the atomic nuclei". Here is the link https://courses.lumenlearning.com/introchem/chapter/bonding-in-metals-the-electron-sea-model/ So let's stop going back and forth on this. – Bob D Nov 15 '20 at 18:27