First, the total length of the wires will affect the amount of resistance.
The longer the wire, the more resistance that there will be. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire.
More collisions mean more resistance. Second, the cross-sectional area of the wires will affect the amount of resistance. Wider wires have a greater cross-sectional area.
Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe. This can be attributed to the lower amount of resistance that is present in the wider pipe. In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires.
A third variable that is known to affect the resistance to charge flow is the material that a wire is made of. Not all materials are created equal in terms of their conductive ability. Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors but is never used in wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with suitable conducting ability to permit their use in wires of household circuits.
The conducting ability of a material is often indicated by its resistivity.
The resistivity of a material is dependent upon the material's electronic structure and its temperature. For most but not all materials, resistivity increases with increasing temperature. The table below lists resistivity values for various materials at temperatures of 20 degrees Celsius. As seen in the table, there is a broad range of resistivity values for various materials.
The colors reveal information about the resistance value. However, resistance and conductance are extensive rather than bulk properties , meaning that they also depend on the size and shape of an object. Put another way, the diameter of wire B is two times greater than the diameter of wire A. In fact, A must have one-fourth the cross-sectional area of B. This page was last edited on 25 November , at Thus, gauge wire has a wider cross section than gauge wire.
Those materials with lower resistivities offer less resistance to the flow of charge; they are better conductors. The materials shown in the last four rows of the above table have such high resistivity that they would not even be considered to be conductors. Resistance is a numerical quantity that can be measured and expressed mathematically. The standard metric unit for resistance is the ohm, represented by the Greek letter omega -. The equation representing the dependency of the resistance R of a cylindrically shaped conductor e.
Consistent with the discussion above, this equation shows that the resistance of a wire is directly proportional to the length of the wire and inversely proportional to the cross-sectional area of the wire. As shown by the equation, knowing the length, cross-sectional area and the material that a wire is made of and thus, its resistivity allows one to determine the resistance of the wire.
Resistors are one of the more common components in electrical circuits. Most resistors have stripes or bands of colors painted on them. The colors reveal information about the resistance value. Perhaps you're doing a lab and need to know the resistance of a resistor used in the lab. Use the widget below to determine the resistance value from the colored stripes. Household circuits are often wired with two different widths of wires: Thus, gauge wire has a wider cross section than gauge wire. Whether or not a material obeys Ohm's law, its resistance can be described in terms of its bulk resistivity.
The resistivity, and thus the resistance, is temperature dependent. Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance. The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as.
The factor in the resistance which takes into account the nature of the material is the resistivity. Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.
It should be noted that it is being presumed that the current is uniform across the cross-section of the wire, which is true only for Direct Current. For Alternating Current there is the phenomenon of " skin effect " in which the current density is maximum at the maximum radius of the wire and drops for smaller radii within the wire.
At radio frequencies, this becomes a major factor in design because the outer part of a wire or cable carries most of the current. The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more convenient.