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In galvanic cell, salt bridge maintains charge balance between solutions. If there's no salt bridge, charge neutrality will be broken as solution(anode) becomes positive and that of cathode becomes negative. Many textbooks just say galvanic cells thus stop working due to broken charge neutrality.

But I want to know why. Why broken charge neutrality stops electrons flow? does it have to do with any potential energy?

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Let's see what happens in a Daniel cell (Zn/Cu). In the beginning, a Zinc atom looses 2 electrons and becomes a zinc ion $\ce{Zn^{2+}}$. The electrons are attracted by the copper ions $\ce{Cu^{2+}}$ in the other compartment. And the $\ce{Zn^{2+}}$ enters the solution. But look ! After some time there will be 2, 3 or more $\ce{Zn^{2+}}$ ions in this solution. This creates a electrostatic positive charge which is diluted in the whole solution around the Zinc electrode. This positive charge repells new positive charges that could be produced on the Zinc plate. So the creation of electrons at this electrode stops after some microsecond.

The same thing happens on the Copper electrode. On this electrode the electrons coming from the zinc electrode through the outer connection react with copper ions $\ce{Cu^{2+}}$ and this produces a metallic copper deposit on this electrode. But this will not last long, because if the electrons destroy positive ions, they are progressively disappearing. As a consequence negative ions (probably $\ce{SO_4^{2-}}$) become more and more numerous. After some time, the solution gets negatively charged and will prevent new electrons from arriving to the copper electrode.

So after a short while, the cell stops working. If you want it to continue working, you must find a way of getting rid of the sulfate ion around the copper plate, and to send them to the zinc compartment. That is the goal of the salt bridge. The excess of $\ce{SO_4^{2-}}$ ions around the copper plate are attracted by the excess $\ce{Zn^{2+}}$ ions from the anode. They move to them through the salt bridge. And the cell can continue working.

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  • $\begingroup$ I'll expand this just a bit. (1) As Maurice indicated there is no continuous current flow, only a brief pulse which polarizes the electrodes. (2) An electron can travel through a foot of wire in about 1 nanosecond. So the current pulse that will polarize the electrodes will be very small and the pulse with travel between the electrodes very very quickly. $\endgroup$ – MaxW Apr 28 '20 at 8:20
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    $\begingroup$ @MaxW You may confuse speed of the field propagation and speed of electron flow. The latter is very slow, some mm/s at the best. $\endgroup$ – Poutnik Apr 28 '20 at 8:35
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    $\begingroup$ @Poutnik - Thanks for keeping me honest. A particular electron doesn't travel through a wire in a nanosecond. So the situation is sort of people on a moving sidewalk. Suppose that there is is a constant flow of a person getting off the moving sidewalk every 5 seconds. Well then there must be a constant flow of people getting on the moving sidewalk every 5 seconds on the other end. But the sidewalk travels at 1.5 feet per second so the hundred yard ride takes a particular person 200 seconds. $\endgroup$ – MaxW Apr 28 '20 at 8:51

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