Tanya Pawsey, of Ilam School, asks :-

Jemimah Walker, of Ilam School, asked:- Eloise Bacon, of Henley Primary School, asked:-

How do you get electricity into batteries?

Peter Harland, a chemist at the University of Canterbury, responded.

The energy in a non-rechargable battery is put in when the chemicals it contains are manufactured.

In a chemical reaction, one set of substances, called the reactants, is converted into a new set of substances, called the products, through the breaking of some chemical bonds in the reactants and the formation of new bonds to form the products. If the products are more stable than the reactants (stronger bonds) then the reaction proceeds with the release of energy, usually as heat, light or sound or a combination of these forms. For example, an open fire in which coal (mainly carbon) reacts with oxygen from the air to form the more stable carbon dioxide releases energy in the forms heat, light and sound.

A special class of chemical reactions, called redox reactions (standing for reduction-oxidation), form products of higher stability which differ from the reactants only by the number of electrons. Such reactions convert chemical energy directly into electrical energy. This is achieved by transferring electrons between the reactants to form products, instead of breaking chemical bonds. Since the transfer of electrons from one reactant to another only requires that the electrons released by the donor are available to the acceptor, it is possible to physically separate the reactants providing there is an electrical conductor between them to transport the electrons and a return path for the current.

Inside a battery, suitable redox reactants are physically separated and no chemical reaction can take place until a conductor is connected between the battery terminals. Once a connection is made, electrons can pass through the wires and whatever else is in the circuit, for example: torch bulb; cassette player; smoke alarm. If a switch in the circuit is opened the electron flow stops and the chemical reaction stops. Eventually, all of the reactants will be converted to products and the battery is dead.

In rechargeable batteries the redox materials are chosen so that the passage of a reverse electrical current through the battery converts the products back to the original reactants again. The electrical energy from the house supply drives the chemical reaction backwards, like pushing a bicycle up hill, to be released as electrical energy once again.