When any material flows through a pipe, it meets opposition. The opposition comes from friction between the material and the inner surface of the pipe. The friction resists the flow of the material, and it produces heat. The same is true of electricity flowing in a conductor. The moving electrons meet opposition, and the opposition produces heat. This opposition is called the resistance of the conductor.
Factors Resistance Depend on
Resistance is a property of the material, its size, and its temperature. Some materials have a greater resistance to the flow of electricity than other materials of the same cross-sectional area and at the same temperature. For example, metals generally have lower resistance than nonmetals. For most conductors, increasing the cross-sectional area decreases the resistance. Raising the temperature generally increases the resistance.
Various materials have different atomic structures. The number of free electrons differs in each material. The freer electrons a material has, the easier it is for electric current to flow through it. The fewer free electrons, the more difficult it is for electricity to flow. The opposition to electric current is called resistance. Resistance is a physical quantity, a property of a material. Materials that conduct electric current have definite values of resistance ranging from zero to very high values. The resistance also depends on the length, the cross-sectional area, and the temperature of the material.
Certain materials lose their electrical resistance completely at temperatures near absolute zero. These materials are called superconductors. If an electric current is started in a loop of wire made of superconducting material, the current will continue to flow even after the power source is removed.
An ideal conductor, used for carrying electricity from a power source to a load, should have no electrical resistance. However, the ideal conductor does not exist at ordinary temperatures. Therefore, electrical wires are made of materials having the lowest possible resistance at a reasonable cost.
Copper is the material used most often for electrical conductors. Where it is important to save weight, aluminum is used instead. Aluminum wiring is used in aircraft wiring and in long-distance power transmission lines that are suspended from towers.
In some applications, even the slight resistance of the wires connecting parts of a sensitive electronic circuit can affect the performance of the circuit. In such applications, silver wires keep the circuit resistance to a minimum. Silver has the lowest resistance of all metals at ordinary temperatures
High resistance is needed for electric heating elements and for the filaments of light bulbs. These elements must have high resistance to control their temperature during use.
The heating element in an electric heater is usually made of high-resistance nichrome (nickel-chromium) wire. The cable supplying power to the heater is made of low-resistance copper wire.
The high-resistance wire becomes hot when an electric current passes through it because of its high resistance. The copper wire remains cool because its resistance is much lower than that of the nichrome element.
Another purpose of high resistance is to limit the current in a circuit. All electrical equipment has some resistance, but sometimes resistance must be added to control the current. Most de motors require extra resistance in the circuit during starting to prevent too much current from flowing through the motor windings. The most effective method of controlling the current during starting is to add several units of resistance in series with the motor windings. These units are then gradually disconnected during the starting period until all the extra resistance has been removed.
Resistance Unit and Symbol
The resistance unit is OHM.