All conducting materials offer a resistance to the flow of current. This resistance is determined by the structure of the material’s atoms. For example, copper atoms offer negligable resistance to an electrical current because a larger number of their electrons are free to move from atom to atom. Glass atoms, on the other hand, offer considerable resistance to electrical current because a larger number of their electrons are not free to move from atom to atom. The resistance of a material varies according to its:
- Length – resistance increases with length because when the electrons travel further, they suffer a greater number of collisions with atoms in the material, whereas electrons moving through a short wire only feel resistance for a short time.
- Cross-sectional area – a narrower wire would have fewer paths available for the electrons to move through, while a larger wire has many more potential paths. This makes conduction easier, and resistance less.
The relationship between current, voltage and resistance
The relationship between current, voltage and resistance is expressed in Ohm’s Law, named after the physicist who discovered it. In the year 1826, George Simon Ohm conducted some experiments regarding current in conductors. As a result of these experiments he arrived at the conclusion that the current flowing through a metal wire is proportional to the potential difference across it (providing the temperature remains constant).
Ohm’s law simply translates to the equation:
V = I x R
or
Voltage = Current x Resistance.
Ohm’s law applies to metal conductors as well as certain other materials, and is obeyed provided that, not only the temperature, but all physical conditions remain constant. For example, the resistance of certain conductors will vary if they are bent or placed near a strong magnetic field. Certain conducting materials disobey ohm’s law entirely. These are mainly semiconductors and gases.
Conductors which follow ohm’s law are called Ohmic conductors, while those that disobey ohm’s law are known as Non-ohmic conductors.
Ohmic conductors
Ohmic conductors are most easily identified by a graph plotted for the change in voltage against the change in current. These “V-I graphs” for ohmic conductors are seen as a straight line passing through the origin. This indicates that the increase in voltage is proportional to the increase in current, and thus indicates that ohm’s law is obeyed. The steeper the line, the lower the resistance. Copper wire and all other metals give this shape of graph under constant temperature:
Non-ohmic conductors
- Filament lamp – as more current flows through the metal filament, its temperature increases. The increase in temperature causes an excess amount of vibrations between the atoms, and this constant vibration obstructs the flow of electrons, thus decreasing the current, and increasing the resistance. Therefore when the voltage increases, the current decreases, and ohm’s law is not obeyed. The V-I graph of a filament lamp can be seen below.
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Thermistor – a thermistor is used in electronics and is made of a semiconductor substance. As more current flows through a thermistor, the kinetic energy of the electrons increases and they flow through the material with a greater speed. As this happens, the current increases and the resistance, therefor, decreases. The V-I graph of a thermistor can be seen below.
Conductors and semiconductors
The electron theory states that all matter is comprised of molecules, which in turn are comprised of atoms, which are made up of protons, neutrons and electrons. The electrons in an atom play the most important role with reference to the conduction of electricity. All metals are conductors of electricity as they contain many free electrons, which can travel from atom to atom, creating a charge and transferring energy. The conductivity of a metal varies according to the type of metal. For example, silver and copper are the best metal conductors, whereas tungsten is a poorer conductor. The malleability and ductility of certain metals such as copper makes them easy to use in electrical circuits. The copper is draw into wires of different thickness according to the level of conductivity required in a circuit. As mentioned before, the resistance of a wire alters with its length, cross-sectional area, material and temperature. These conditions apply to all metallic conductors. Therefore, a thick, short piece of copper will offer negligible resistance when compared with a long copper wire. This is because it is much easier for the electrons in the copper to move through a greater area and the smoother the flow of electrons, the larger the current. Also, if electricity is passed through two equally sized copper wires, each of varying temperatures, the heated wire will offer much more resistance when compared with the cooler one. This is because the heat in the wire causes its particles to obtain kinetic energy and vibrate vigorously. Their vibrations obstruct the flow of electrons so the current is less and the resistance is greater.
The energy band theory
The key difference between semiconductors and conductors is that a conductor’s conductivity decreases with an increase in temperature, whereas a semiconductor’s conductivity increases with an increase in temperature. At any temperature above absolute zero, there is a possibility that an electron in the lattice will be knocked loose from its position, leaving behind a deficiency called a “hole”. If a voltage is applied, then both, the electron and the hole can contribute to a small current flow. As the thermal energy of the electrons increases, they breach the “hole” present in the semiconductor into what is called a conduction band. Thus, unlike with metals, in semiconductors, the resistance decreases with an increase in temperature. The conductivity of a semiconductor can be modeled in terms of the energy band theory. The theory suggests that at ordinary temperatures there is a possibility that electrons can reach the conduction band and contribute to electrical conduction.
Intrinsic and extrinsic semiconductors
The term intrinsic distinguishes between pure semiconductors, and extrinsic (doped) semiconductors.
The conductivity of semiconductors such as Silicon (Si) can be increased by adding small, controlled amounts of impurities that have roughly the same atomic size that the semimetal itself. This process of adding impurities to increase conductivity is known as doping.
