How does a battery work?

Every atom consists of a positively charged nucleus called a proton surrounded by a number of smaller, negatively charged particles called electrons. If atoms lose or gain electrons, they are left with a positive or negative charge and they are then called ions. In an electrically conducting solid such as a wire, some of the negatively charged electrons are able to move around fairly freely around the fixed array of positively charged protons. Although these 'free' electrons can move, there is a force which keeps the electrons — and so the charge — evenly spread along the length of the wire.

If all the electrons are concentrated at one end of the wire they would want to rush back to cancel out the charge imbalance. In doing so they would create an electric current for a very short time.

A conductor in its natural state — negative electrons (blue) and positive ions (red) evenly distributed along its length.

The same conductor in a state of imbalance where all the negative particles (blue) have been forced to one end.

An electron "pump" 

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