How exactly does a salt bridge close the circuit? I understand its role of neutralizing the solustions but I can't see how it forms a complete circuit. Do charges flow back to the anode through it? But why? Isn't the purpose of a battery to carry charge from high potential (anode) to low potential (cathode)? Isn't the external connecting wire enough?

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    $\begingroup$ Do you know the force, by which there would be attracted the opposite charges in 1m distance, caused by the current 1A by just 1s ? About 9 GN. Do you you know about any battery able to withstand comparable force ? Without a salt bridge or equivalent feature, the current in the wire would ceased after passing just very small charge. $\endgroup$ – Poutnik May 29 '19 at 4:44
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    $\begingroup$ SE sites focus on high quality answers to high quality, not easily answered, questions, to be a gain for all. Quick questions, where one can easily find answers in any relevant textbook or within few clicks on Wikipedia, do not count as high quality questions. They will be answered only if there is explicitly provided failed elaboration effort to raise their quality a little. This applies to literal homework, self-studies, puzzles, worked examples etc - forming the Homework class of questions. $\endgroup$ – Poutnik May 29 '19 at 4:45
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    $\begingroup$ I dunno, this question seems conceptual enough not to count as homework. If you think it's poorly researched, you're free to downvote, but I'm voting to leave it open. $\endgroup$ – M.A.R. May 29 '19 at 16:08
  • $\begingroup$ Well, it is rather a basic concept for physics SE about consequences of big charge displacement. Displace electrons by current 1 A for 1-2 minutes to the distance 10 km away and then observe the GC+ lightning 10 km long with the potential difference 1 billion volts. :-) $\endgroup$ – Poutnik May 30 '19 at 6:59

It is the same principle as why there is not going any current from a voltage source to a powered device, if there is currupted galvanic connection anywhere. The initial current very quickly balances potentials of power outlets with potentials on the wires and no current is flowing after charging the wire parasitic capacitance.

Let consider the classical Daniel cell $\ce{Zn|ZnSO4||CuSO4|Cu}$ with $||$ being a salt bridge ( or a diaphragm permeable for ions, but avoiding mixing of solutions).

$\ce{Zn}$ anode has more negative potential than the $\ce{Cu}$ one and provides excess of electrons by dissolving and hydrating $\ce{Zn^2+}$ ions.

The electrons flow via the wiring to the copper cathode with higher potential, where they reduce $\ce{Cu^2+}$ to the metallic copper.

In case of the absence of the bridge or the diaphragm, the accumulating negative charge at the cathode compartment very quickly creates solution potential difference that balances the electrode potential difference and the current vanishes.

In case of the presence of the salt bridge or the diaphragm, the potential difference between the solutions makes ions to migrate to the respective electrodes, according to their charge and direction of the potential gradient.

This keeps the compartment potential difference lower than electrode potential difference and the current is free to flow via the wire.

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