# Write the differences between alternating current (AC) and direct current (DC).

There are no doubt many differences that could be mentioned, but I think the most important ones are adequately covered below:

Alternating current periodically changes the polarity of voltage and consequently the direction of flow also periodically changes. A full set of changes from zero (volts or amps) through all of the positive values, zero, then all the negative values and back to zero again is called “one cycle”. The number of cycles occurring every second is called its “frequency” and the unit of measurement is “Hertz” (Hz) with one cycle per second being equivalent to one Hertz. AC power is commonly supplied at a substantially constant value of 50Hz or 60Hz dependent upon the National specification. AC power is most commonly generated by an electro-mechanical machine usually called an “alternator”, which is usually designed to deliver either a “single phase” or a “three phase” output with approximately “sinusoidal” waveform.

Direct current is delivered from a supply whose polarity of output voltage is constant, thus it causes current to flow in a single direction. The value of the dc output voltage is substantially constant, and in such circumstances, it is meaningless to talk about its “frequency”, though it is sometimes said to be zero Hertz. Note that if a dc supply is periodically “interrupted” (turned on and off or “chopped”) it will become a unipolar square wave and effectively acquire “a.c. components” whose frequencies can be specified. DC supplies are commonly obtained from electrochemical “cells” or “batteries”; photoelectric cells; and thermocouples; as well as electromechanical devices sometimes called “dynamos”.

Current flow in a pure dc circuit is determined by the value of the voltage, and the resistance(s) of the component(s) in the circuit, according to “Ohm’s Law” which may be expressed by the formula

`` I = V/R ``

where I is the current in Amps; V is the voltage in Volts and R is the resistance in Ohms.

In an ac circuit the current is determined by the voltage (which is usually expressed as an r.m.s. value or less often as an “amplitude” a.k.a. “peak value”) and the so called “impedance” (Z) where Z is determined by both Resistance (which is largely independent of frequency) and the frequency dependant “Reactance” (X) , the latter having two forms referred to as “Capacitive Reactance” and “Inductive Reactance”.

Reactance has no real meaning in a pure d.c. circuit, but is of fundamental importance in a.c. circuits.

There are no doubt many differences that could be mentioned, but I think the most important ones are adequately covered below:

Doh! - Please ignore the last two lines - they duplicate the first paragraph, and should have been deleted before posting.

Thank you so much Estragon for detial explain

Adding to Estragon’s excellent info there is a lot to the AC/DC story. There was a huge battle between Thomas Edison who favored electrifying America with direct current (DC) and Nicola Tesla who wanted to electrify America with alternating current (AC). Edison held that AC was too dangerous, especially high voltage AC and Tesla countered that DC was impractical since it couldn’t efficiently travel long distances. George Westinghouse backed Tesla while J. P. Morgan formed General Electric and backed Edison. Tesla wisely panteted his AC technology but Westinghouse convinced Tesla to sign his patents over to Westinghouse Electric Company to attract shareholders and better compete with General Electric. In the end J. P. MOrgan had much deeper pockets and threatening ruinous litigation Morgan convinced Westinghouse to sign Tesla’s AC patents over to GE. Tesla probably underestimated the value of his patents but Tesla already had far more ideas to develop. Tesla started experimenting with higher frequencies including radio frequencies and even envisioned producing AC in the frequency of light (visibile light ranges in frequency from 420 to 789 THz or trillions of cycles/sec). We all think of Marconi as the inventor of radio but Tesla had a pretty good claim to be radio’s inventor. The courts ruled in favor of Marconi since in turn Marconi would allow the US government to use radio without paying royalties. While many of Tesla’s ideas proved wildly impractical he also invented things we use every day like florescent lights, neon signs, remote control vehicles and X-rays. Tesla was one of the greatest scientific minds that ever lived but he died broke, alone, and largely unrecognized for his contributions.

50-60 Hz was determined by compromise. Lower frequencies favor transmission efficiency since a circuit’s impedence is closer to its resistance but higher frequencies allow smaller and more efficient generators, transformers, and power supplies. Military and aircraft generators usually run at 400 Hz. On an aircraft lighter gear is more important than transmission efficiency. Lots of things work fine with 400 Hz current. Light bulbs and resistive loads are fine. Some well or over- designed power supplies designed for 60 Hz work fine on 400 Hz. I know some people get away with plugging cell phone or laptop rechargers into 400 Hz outlets but most electric motors will have problems. The vaccums used to clean up airplanes between flights are specially designed for 400 Hz AC and would burn out if plugged into 60 Hz mains.

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