Characteristics of Ohmic and non-Ohmic Conductors.

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Characteristics of Ohmic and non-Ohmic Conductors

Some atoms contain loosely attached electrons. These electrons can be made to move easily from one atom to another. When these electrons move freely along a path, a current of electricity is created. Electrons can only flow freely in materials that conduct electricity. A Conductor of electricity is a substance that allows electricity to flow through it. Substances that do not conduct electricity are knows as Insulators. All metals are good conductors because inside them they have a large number of free electrons that can move easily from atom to atom. Insulators do not have free electrons inside them. Current is the flow of free electrons through a conductor in one particular direction. Current is also calculated by the amount of charges transferred per second. For current to flow, we need a cell to exert a force and provide energy. The Current flowing in a circuit depends on the cells ability to give energy. Voltage is the energy given by the cell per charge. Another factor that affects current is Resistance. As electrons pass through the conductor they encounter collisions with atoms or ions. Electrons lose their energy and the atoms gain. The atoms vibrate with more amplitude for a while, till it emits the extra energy as heat. The 1862, George Ohm discovered that the current flowing through a metal wire is proportional to the potential difference across it (provided the temperature remains constant). However this law does not apply to all conductors, due to resistance of a wire. Thus, conductors can be divided into Ohmic and Non-Ohmic conductors. In non-ohmic conductors current is not always proportional to voltage. Sometimes, when voltage increases current decreases. Also we can say that Semi conductors are non-ohmic conductors, because as a current increases in a semi conductor, it gets hotter and so the energy is used to liberate more free electrons. As the number of free electrons increase, resistance decreases.

Ohmic Conductors

Metallic Conductors

A metal contains a large number of almost ‘free’ electrons. This property makes metals extremely good conductors of electricity. But as the temperature of the metal increases the ions in the metal vibrate vigorously, colliding with the free flowing electrons, obstructing the flow of electricity, thus as temperature increases, resistance increases as well. Metallic conductors are Ohmic Conductors. In ohmic conductors, Current is always proportional to the voltage. As voltage increases, current increases too. This is because as the voltage increases, more electrons have the energy to flow through the conductor, causing the current to increase simultaneously, (provided the temperature of the conductor remains constant).

Voltage (V)







                                                             Current (I)

The graph of Current against Voltage against Current, for an ohmic conductor will usually give a straight line through the origin. The gradient of the wire will give us the resistance of the wire.  The greater the resistance the more voltage is needed to push a current through the wire. Different ohmic conductors will have different resistance, the graphs of all these conductors will pass through the origin, but their gradient will vary.


 Voltage (V)

             Good conductor


                   Poor conductor

Current (I)


  The steeper the graph, the lower the resistance. The flatter the graph, the higher the resistance.  We can explain this difference in conduction, by using Energy Bands. 


                                                                     Conduction Band


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When the electrons in the metal get energy they jump in the Conduction Band. The highest Band containing electrons is only partly filled. So the electrons in this band can easily move to unfilled levels rather than up the band. These electrons have broken free from individual atoms, and are able to move through the material. So this band is called the Conduction Band. The Valence Band is just is immediately below the conduction band. Electrons in the Valence Band are firmly attached to individual atoms.  In Good Conductors, the valence Band and ...

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