An electric current is a flow of electrons through a conductor such as a copper wire. All materials are made up of atoms. An atom has a nucleus around which one or more electrons circle in different orbits. An electron has a negative electric charge and a nucleus has a positive one. Semiconductors are materials whose free electrons' ability to move falls between that of conductors and insulators. Electromotive forces are produced by power sources and there are three principal types of power source: chemical, magnetic and thermal. The electric potential at the terminals is caused by a chemical reaction between the metal plates and the electrolyte. The electrical pressure at any point in an electrical system is called the potential. The potential difference is the difference in electrical pressure between two points in an electrical system. A capacitor is a device that acts as a temporary store of electricity.