Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You can compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: small diameter wire has high resistance (a lot of opposition to current), and large-diameter wire has low resistance (not much opposition to current). The type of metal makes a difference too. For example, steel wire has higher resistance for a given diameter than copper wire.
The standard unit of resistance is the ohm. This is sometimes symbolized by the uppercase Greek letter omega (Ω). You’ll sometimes hear about kilohms (symbolized k or kΩ), where 1 kΩ = 1000 Ω, or about megohms (symbolized M or MΩ), where 1 MΩ = 1000 kΩ = 1,000,000 Ω.
Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft or Ω/ft) or the ohm per meter (ohm/m or Ω/m). You might also come across the unit ohm per kilometer (ohm/km or Ω/km). Following Table shows the resistivity for various common sizes of solid copper wire at room temperature, as a function of the wire size as defined by a scheme known as the American Wire Gauge (AWG).
|Wire size, AWG #||Resistivity, ohms/km|
When 1 V is placed across 1 Ω of resistance, assuming that the power supply can deliver an unlimited number of charge carriers, there is a current of 1 A. If the resistance is doubled to 2 Ω, the current decreases to 0.5 A. If the resistance is cut by a factor of 5 to 0.2 Ω, the current increases by the same factor, to 5 A. The current flow, for a constant voltage, is said to be inversely proportional to the resistance. Following Figure is a graph that shows various currents, through various resistances, given a constant voltage of 1 V across the whole resistance.
Current as a function of resistance through an electric device for a constant voltage of 1 V.
Resistance has another property. If there is a current flowing through a resistive material, there is always a potential difference across the resistive component (called a resistor). This is shown in following Fig.
Whenever current passes through a component having resistance, a voltage exists across that component.
In general, this voltage is directly proportional to the current through the resistor. This behavior of resistors is useful in the design of electronic circuits. Electrical circuits always have some resistance. There is no such thing as a perfect conductor. When some metals are chilled to temperatures near absolute zero, they lose practically all of their resistance, but they never become absolutely perfect, resistance-free conductors. This phenomenon, about which you might have heard, is called superconductivity.
Just as there is no such thing as a perfectly resistance-free substance, there isn’t a truly infinite resistance, either. Even air conducts to some extent, although the effect is usually so small that it can be ignored. In some electronic applications, materials are selected on the basis of how “nearly infinite” their resistance is.
In electronics, the resistance of a component often varies, depending on the conditions under which it is operated. A transistor, for example, might have high resistance some of the time, and low resistance at other times. High/low resistance variations can be made to take place thousands, millions, or billions of times each second. In this way, oscillators, amplifiers, and digital devices function in radio receivers and transmitters, telephone networks, digital computers, and satellite links (to name just a few applications).