[Physics] Are the plates of a battery really charged

electric-circuitselectric-fieldselectricityelectromagnetismelectrons

In a zinc/copper Daniell cell correct me if I am wrong :

  1. Zinc has 2 valence electrons. So it wants to get rid of them. To do so it sends them to the copper which needs 2 to complete its valence shell.

  2. There needs to be a wire between the zinc and the copper for this reaction to happen.

  3. So technically the plates are not charged. It's just the charges flowing out that create the electric field.

    TLDR : Are the plates of a battery more like a capacitor with excess charges on the plates? Or do they simply throw in and out electrons near their terminals and the individual plates of zinc and copper are neutral?

My confusion is this : I understand that the zinc wants to get rid of electrons, and the copper wants more electrons, but : The zinc and copper atom are "neutral". it's only the defecit of electrons on the conductor near the positive terminal and the excess of electrons near the negative terminal, That for me would make an electric field .

Or Maybe it's the "wanting to get rid" and the "wanting to get more" electrons that create an electric field, if it's indeed that please confirm !

Thanks !

Best Answer

I think that most confusion about batteries comes from ignoring the electrolyte. For example:

Zinc has 2 valence electrons. So it wants to get rid of them. To do so it sends them to the copper which needs 2 to complete its valence shell.

The actual reaction is between the metallic zinc and the dissolved zinc. Zinc wants to get rid of two electrons and it does so by becoming an ion and going into solution.

This reaction is energetically favorable and can occur even if there is a small electric field opposing it at the surface of the electrode. However, the reaction products near the electrode surface, zinc ions and electrons, are highly charged and quickly produce a strongly opposing field which overcomes the energetic favorability and halts the reaction. For the reaction to proceed the reaction products must be removed from the region near the electrode surface.

The electrons can be removed from the electrode surface by transport through the wire, and the ions can be removed from the surface by transport through the fluid. The transport of electrons requires the complementary reaction at the copper electrode, and the transport of the ions requires a complementary transport of the solute ion in the electrolyte. Understanding the electrolyte is essential for understanding batteries, and is the usual neglected piece.

There needs to be a wire between the zinc and the copper for this reaction to happen.

The purpose of the wire is not to make the reaction happen. The reaction is energetically favorable, so it briefly happens regardless. The purpose of the wire is to remove the reaction products so it can continue to happen.

Note that in this process not all of the excess electrons on the plate are normally removed. As soon as a few are removed the reaction proceeds and replenishes those few. The reaction thus proceeds at the rate that the products are removed from the immediate vicinity of the electrode surface. In abnormal situations, like a short circuit, a substantial fraction of the excess charges at the surface can be depleted and the current is limited by the reaction kinetics. This manifests as an “internal resistance” for the cell.

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