Electric circuits

A battery (cell)

Crudely, we can think of a battery as a container with two metal ends and two mixtures of substances inside, one mixture in each half.

Click on 'Play', below, to see what happens when the mixtures are added to make a battery:

• A chemical change involving the substances of mixture 1 releases electrons. These electrons join the delocalised electrons in metal end 1. This gives a higher number of delocalised electrons in the metal end than normal - a higher concentration. There is a limit as to how high this concentration of electrons can go - there is an equilibrium concentration. The reaction stops when it gets to this limit even though there is still plenty of the mixture left to react. This is the negative end of the battery.

• A chemical change involving the substances of mixture 2 captures some of the delocalised electrons from metal end 2. This gives a lower number of delocalised electrons in the metal end than normal - a lower concentration. There is a limit as to how low this concentration of electrons can go - there is an equilibrium concentration. The reaction stops when it gets to this limit even though there is still plenty of the mixture to react. This is the positive end of the battery.

• As an overall condition, mixture 1 can only react to release electrons if at the same time, the same number of electrons are captured by the reaction of mixture 2.

• Were it to happen, as soon as electrons move out of the ‘negative end’ (i.e. the concentration goes down), further chemical change can take place (as long as some of the reactants are still left) to restore the concentration of electrons to the equilibrium value at this ‘end’. Similarly, as soon as more electrons move into the 'positive end', further chemical change can take place (as long as some of the reactants are still left) to reduce the concentration back to the equilibrium value at this ‘end’.

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