An electric current flows when electrons [ electron : An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ] move through a conductor, such as a metal wire. The moving electrons can collide with the ions [ ion : Positively- or negatively-charged particles - eg positively charged hydrogen, sodium and potassium atoms. Ion charge helps determine a substance's acidity or alkalinity ] in the metal. This makes it more difficult for the current to flow, and causes resistance.