We are able to compare the values of resistance and hose diameter (conductance vs. practical diameter). Regarding metal wires, this analogy works well. Small-diameter wires have a higher resistance than large-diameter wires made of the same metal.
The standard unit of resistance is the ohm, sometimes expressed as the capital Greece letter Omega (Q). You will also hear about kilowatts (denoted as k or kQ) with 1 k = 1000 ohms during which or about megaohms (denoted as M or MQ) with 1 M = 1,000,000 ohms = 1000 k. In this book, we will never use the omega symbol. Instead, we'll always write "ohm" or "ohms" intact.
In some cases, the resistivity of wires is evaluated. The standard unit used for this purpose is ohms per foot (ohm/ft) or ohms per meter (ohm/rn). You may also encounter units of ohms per kilometer (ohm/km). Table 2-1 shows the resistivity values of various common sizes of solid copper wire at room temperature, which is a function of wire diameter as a function of the wire diameter of the scheme boundary called "United States Wire Gauge (AWG)".
When we apply a potential difference of 1V to a component with a resistance equal to 1 ohm, the current is 1A, assuming that the power supply is capable of providing an infinite number of charge carriers. If the resistance is doubled to 2 ohms, the current will be reduced to 0.5A. If the resistance is reduced by a factor of 5 to only 0.2 ohms, the current will increase from 1 A to 5 A by the same multiple. The current with respect to the stable voltage, which changes inversely with respect to the resistance, shows the current passing through the various resistive components with a stable potential difference of 1 V.
Circuits always have a certain resistance. There is no such thing as an aspirational conductor (an object that is mathematically zero resistance) in the practice international. When scientists cool certain metals to temperatures close to absolute zero, these substances literally lose all resistance, so that electric current can flow for a long time. This phenomenon is called superconductivity. However, nothing can be absolutely perfect for command. Such an object would in practice constitute a perpetual motion machine.
Just as there can be no material that is completely free of resistance in the practice world, we will never encounter absolutely infinite resistance. Even dry air conducts an electric current to some extent, although its effect is usually so small that scientists and engineers are able to overlook it. In some electronic uses, engineers choose materials based on the "simply infinite" appearance of resistance. But when they say that, they use analogies. What they really mean is that the resistance is so great that we can see it as "infinite" for all practical uses.
In electronic devices, the resistance of a component usually changes depending on the conditions under which the component operates. For example, transistors may sometimes have high resistance and sometimes low resistance. High/low resistance changes can occur thousands, millions, or billions of times per second. In this way, oscillators, amplifiers, and digital devices work in radio receivers and transmitters, telephone networks, digital computers, and satellite links, to name a few.
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