There are electrons and protons in the atoms of all materials , but to do useful work the charges must be separated to produce a potential difference that can make current flow . Some of the more common methods and sources of providing electrical effects are listed below.
Static electricity by friction:
It is the first source of electricity, in this method, electrons in an insulator can be separated by the work of rubbing to produce opposite charges that remain in the dielectric.
Conversion of chemical energy:
It is second source of electricity, in this method wet or dry cells and batteries are the application. Here a chemical reaction produces opposite charges on two dissimilar metals, which serve as the negative and positive terminals .
It is third source of electricity, , in this method electricity and magnetism are closely related. Any moving charge has an associated magnetic field; also, any changing magnetic field can produce current. A motor is an example of how current can react with a magnetic field to produce motion; a generator produces voltage by means of a conductor rotating in a magnetic field.
It is fourth source of electricity. Some materials are photoelectric, meaning they can emit electrons when light strikes the surface. The element cesium is often used as source of photo-electrons. Also, photo-voltaic cells or solar cells use silicon to generate output voltage from the light input. In another effect, the resistance of the element selenium changes with light. When this is combined with a fixed voltage source, wide variations between dark current and light current cab be produced. Such characteristics are the basis of many photoelectric devices, including television camera tubes, photoelectric cells, and photo-transistors.
It is last source of electricity, Some materials when heated can “boil off” electrons from the surface. Then these emitted electrons can be controlled to provide useful applications of electric current. The emitting electrode is called a cathode, while an anode is used to collect the emitted electrons.