It is widely recognized that high voltages pose greater hazards than low voltages. In short, high voltages are more economical for transmitting power, and AC voltage is much easier to raise and lower, so that AC is used in most large-scale power distribution systems. But, as we shall see in a later chapter, there is a limit to current in superconductors, too. If superconducting lines could be economically produced, there would be no loss in the transmission lines at all. Of course, lower-resistance lines can be built, but this requires larger and more expensive wires. The lower the voltage, the more current is needed, and the greater the power loss in the fixed-resistance transmission lines. This would result in a power loss in the lines of 16.0 MW, or 16.0% rather than 0.250%.
Note that if 100 MW of power had been transmitted at 25 kV, then a current of 4000 A would have been needed. One-fourth of a percent is an acceptable loss. The AC voltages and frequencies commonly used in homes and businesses vary around the world. Figure 20.14 shows graphs of voltage and current versus time for typical DC and AC power. Examples include the commercial and residential power that serves so many of our needs. If the source varies periodically, particularly sinusoidally, the circuit is known as an alternating current circuit. Alternating current (AC) is the flow of electric charge that periodically reverses direction. Most well-known applications, however, use a time-varying voltage source.
It is the steady state of a constant-voltage circuit. Direct current (DC) is the flow of electric charge in only one direction. Once the current is established, it is thus also a constant. Most of the examples dealt with so far, and particularly those utilizing batteries, have constant voltage sources. Explain why AC current is used for power transmission.Calculate rms voltage, current, and average power.Explain the differences and similarities between AC and DC current.By the end of this section, you will be able to:
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