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# Characteristics of Ohmic and Non-Ohmic Conductors.

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Introduction

Characteristics of Ohmic and Non-Ohmic Conductors.

INTRODUCTION

## Conduction

Conduction takes place in solid matter of certain materials where energy can be transferred from atom to atom. A conductor is a material that allows an energy transfer of this nature to take place. All metals are good conductors of electricity, which is why all circuits contain mostly metal wires and components. When energy is supplied to one part of a conducting wire, the electrons in the wire move from one atom to the next, producing an electrical charge.  Semi-conductors are elements such as Silicon and Germanium, which do not conduct at low temperatures, and whose conductivity increases with the temperature.

## Current, Voltage & Resistance

Current electricity is the flow of charged particles through a circuit. In all dry conductors, the charged particles are electrons, therefore the charge is negative. The electrons in a circuit always flow from the negative terminal of a cell to the positive terminal. the same current flows through any given point in a series circuit, but in a parallel circuit, the current divides in a manner that the current in each of the separate branches sums up to the current in the main circuit. The size of the current in a circuit is measured in Amperes, or  “amps” using an ammeter.

Middle

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

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.

Conclusion

## 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.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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