This opposition to current flow is known as RESISTANCE (R), and the unit of measure is the OHM.
It is known that the directed movement of electrons constitutes a current flow. It is also known that the electrons do not move freely through a conductor’s crystalline structure. Some materials offer little opposition to current flow, while others greatly oppose current flow.
The standard of measure for one ohm is the opposition provided at zero degrees Celsius by a column of mercury having a cross-sectional area of one square millimeter and a length of 106.3 centimeters.
A conductor has a unit of one ohm of opposition when an applied potential of one volt produces a current of one ampere. The symbol used to represent the ohm is the Greek letter Omega. (Shown here)
Symbol for ohms.
Resistance, although an electrical property, is determined by the physical structure of a material. This opposing property of a material is governed by many of the same factors that control current flow. Therefore, in a subsequent discussion, the factors that affect current flow will be used to assist in the explanation of the factors affecting the aforementioned opposing property.
The magnitude of opposition is determined in part by the "number of free electrons" available within the material. Since a decrease in the number of free electrons will decrease the current flow, it can be said that the opposition to current flow is greater in a material with fewer free electrons. Thus, the opposition of a material is determined by the number of free electrons available in a material.
A knowledge of the conditions that limit current flow and, therefore, affect this opposition of force can now be used to consider how the type of material, physical dimensions, and temperature will affect the resistance of a conductor.
Depending upon their atomic structure, different materials will have different quantities of free electrons. Therefore, the various conductors used in electrical applications have different values of resistance.
Consider a simple metallic substance. Most metals are crystalline in structure and consist of atoms that are tightly bound in the lattice network. The atoms of such elements are so close together that the electrons in the outer shell of the atom are associated with one atom as much as with its neighbor. As you can see in view (A) of the illustration below.
As a result, the force of attachment of an outer electron with an individual atom is practically zero. Depending on the metal, at least one electron, sometimes two, and in a few cases, threeelectrons per atom exist in this state.
In such a case, a relatively small amount of additional electronenergy would free the outer electrons from the attraction of the nucleus. At normal room temperature materials of this type have many free electrons and are good conductors. Good conductors will have a low resistance.
Atomic spacing in atoms.
If the atoms of a material are farther apart, as illustrated in view (B), the electrons in the outer shells will not be equally attached to several atoms as they orbit the nucleus. They will be attracted by the nucleus of the parent atom only. Therefore, a greater amount of energy is required to free any of these electrons.
Materials of this type are poor conductors and therefore have a high opposition to current flow. Silver, gold, and aluminum are good conductors. Therefore, materials composed of their atoms would be low.
element copper is the conductor most widely used throughout electrical
applications. Silver has a lower opposition to the flow of current than
copper but its cost limits usage to circuits where a high conductivity
is demanded. Aluminum, which is considerably lighter than copper, is
used as a conductor when weight is a major factor.
Cross-sectional area greatly affects the magnitude of resistance
Definition of resistors
Composition of Resistors