In this question :
What is the difference between a battery and a charged capacitor?

It is said that the battery does not have a charge separation on its electrodes and only the chemical reaction generates voltage. But in the comment, it is said again that there's charge separation on the electrodes thus producing the electric field and voltage.

so at the end there's a small charge separation in the battery so how a battery differs from a capacitor? can we say it is due to chemical reaction there's charge separation? When the circuit is closed, there's drop in electric field, why the chemical reaction does not maintain the charge separation as in the open circuit?

  • $\begingroup$ In very simple terms, the battery creates the charge and hence the potential gradient via a chemical reaction. A capacitor on the other hand just discharges the charge that it stores on charging. Two very different mechanisms. $\endgroup$ – sbp Nov 8 '15 at 9:27

The difference is that batteries chemically "pump" electrons from one side to the other. There is a small amount of charge separation in a battery even when it is not connected to a circuit. This charge creates an electric field that opposed the chemical action of the battery to prevent further charge separation. This makes the battery act somewhat like a constant voltage source at low current draw. This is a bit like a capacitor but capacitors store much more charge than batteries and have no charge separation mechanism of their own.

  • $\begingroup$ during the discharge of the battery (when it is connected to a circuit), why the electric field decreases? and why the chemical reaction does not maintain the charge separation? $\endgroup$ – Tonylb1 Nov 8 '15 at 10:21
  • $\begingroup$ When the circuit is complete, there is a path for electrons to flow so they never build up at the ends of the battery. $\endgroup$ – Robert Stiffler Nov 8 '15 at 10:23
  • $\begingroup$ but there's a potential difference in the battery during discharge thus charge separation $\endgroup$ – Tonylb1 Nov 8 '15 at 10:28
  • $\begingroup$ There is a potential difference, yes, but the electrons have a circuit to flow through meaning the chemical pump in the battery has to be constantly working to maintain the potential difference. $\endgroup$ – Robert Stiffler Nov 8 '15 at 12:50
  • $\begingroup$ An open circuit is a bit like having a capacitor across the terminals of the battery. Once the capacitor is charged up to equal the potential difference of the battery, current stops flowing because the potential on the capacitor exactly equals that of the battery. In the real world, though, the actual amount of charge that gets separated in a battery is very small. $\endgroup$ – Robert Stiffler Nov 8 '15 at 12:52

The chemical reaction in a battery can only happen when there's a complete circuit; the chemicals at one electrode can then dump electrons into the circuit, while at the other electrode they take electrons away. The rest of the time, the chemicals are electrically neutral.

So while the whole principle of a capacitor is to keep charges separate, batteries merely use chemicals with the potential to produce charges.


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