In a piece of copper wire, the electrons are in random motion with no direction. To produce a current flow, the electrons must all move in the same direction. To produce motion in a given direction, energy must be imparted to the electrons in the copper wire. This energy comes from a source connected to the wire. The force that causes the electrons to move in a common direction is referred to as difference of potential, or voltage
A current is produced when an electron is forced from its orbit around an atom. Any form of energy that dislodges electrons from atoms can be used to produce current. It is important to note that energy is not created; rather, there is simply a transfer of energy from one form to another. It is the means of converting some other form of energy into electrical energy.
The six most common voltage sources are friction, magnetism, chemicals, light, heat, and pressure.
Friction is the oldest known method of producing electricity. A glass rod can become charged when rubbed with a piece of fur or silk. This is similar to the charge you can generate by scuffing your feet across a carpet in a dry room.
Magnetism is the most common method of pro ducing electrical energy today. If a wire is passed through a magnetic field, voltage is produced as long as there is motion between the magnetic field and the conductor. A device based on this principle is called a generator. A generator may be powered by steam from nuclear power or coal, water, wind, or gasoline or diesel engines.
The second most common method of producing electrical energy today is by the use of a chemical cell. The cells establish contact with the electrolyte (the salt, acid, or alkaline solution) and the circuit. The electrolyte pulls the free electrons from the copper electrode, leaving it with a positive charge. The zinc electrode attracts free electrons in the electrolyte and thus acquires a nega tive charge. Several of these cells can be connected together to form a battery.
Light energy can be converted directly to elec trical energy by light striking a photosensitive (light sensitive) substance in a solar cell. A solar cell consists of photosensitive materials mounted between metal contacts. When the surface of the photosensitive material is exposed to light, it dislodges electrons from their orbits around the surface atoms of the material. This occurs because light has energy. A single cell can produce a small voltage.