September 20, 2008


Electromotive Force

In a piece of wire, the electrons move in random without direction. To produce a current flow, all electrons must move in one direction. To produce motion in one direction, energy must be provided to the electrons in the conductor or wire.

The electrical force that forces the electrons to move in uniform direction is called as voltage. Voltage can also be called as potential difference or electromotive force (EMF). Capital letter “E” is used to symbolize voltage in a circuit, which is taken from the term electromotive force.

The unit used to express voltage or electromotive force is referred to as volt. Volt is named after Count Alessandro Volta, the scientist who invented the first voltaic cell. Subunits used to express voltage are given below.


There are several methods of producing electricity. It is important to consider that energy is not created; rather, there is simply a transfer of energy from one form to another. The source supplying the voltage is not simply a source of electrical energy. Instead, it is the means of converting some other form of energy into electrical energy. The six most common sources are friction, magnetism, chemicals, light, heat and pressure.

Friction is the oldest known method of producing electricity. A piece of glass can become charge when rubbed with a piece of fur or silk. A Van de Graaf generator is a device that operates using the same principles as the glass rod and is capable of producing millions of volts.

Magnetism is the most common method of producing electrical energy. If a wire or conductor is passed through a magnetic field, voltage is produced, as long as there is motion between the magnetic field and the conductor. A generator uses this principle. A generator can produce either alternating current or direct current depending on how it is wired. In direct current (DC) electrons flow in only one direction. When electrons flow in one direction then in the opposite direction, the current is called as alternating current (AC). A generator may be powered by steam coming from nuclear power or coal, wind water, gasoline or diesel engines.

Chemical cell is the second most common method of producing electrical energy. The cell is consists of two dissimilar metals, copper and zinc, immersed in a salt, alkaline or acid solution. The metals copper and zinc are the electrodes. The electrodes established contact with the electrolyte 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 negative charge. Several of these cells can be connected together to form a battery.

In a photovoltaic cell, light energy can be converted directly to electrical energy by light striking a photosensitive substance. A solar cell consists of photosensitive materials mounted between metal contacts. When the surface of the photosensitive material is exposed to light, it removes electrons from their orbits around the surface atoms of the material. This happens because light has energy. A single cell can generate a small voltage.

Thermocouple is a device that converts heat energy to electrical energy. A thermocouple device is consists of two dissimilar metal wires, twisted together. One wire is copper and the other wire is zinc or iron. When heat is applied to the twisted part, the copper wire gives up free electrons that are transferred to the other wire. Therefore the copper wire develops a positive charge and the other wire develops a negative charge and a small voltage occurs. The output voltage is directly proportional to the amount of heat applied.

When pressure is applied to certain crystalline materials such as quartz, tourmaline, and barium titanate, a small voltage is produced. This method of creating electricity is called as piezoelectric effect. At first, positive and negative charges are distributed randomly throughout a piece of crystalline material, and no charge can be measured. When pressure is applied, electrons leave one side of the material and accumulate on the other side. As long as the pressure remains, charge is produced. When pressure is removed, the charge is again distributed randomly, and no charge is produced. Uses of piezoelectric effect include crystal microphone, phonograph pickups and precision oscillators.


manoj singh said...

Thanks for providing information on emf which is responsible for flow of electric current in the conductor.
Electromotive Force