General Question

ninja_man's avatar

What is the difference between alternating current and direct current?

Asked by ninja_man (1133 points ) August 29th, 2012

What distinguishes the two? Are there different uses for them?

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13 Answers

Afos22's avatar

Direct current is a constant flow of electrons in one direction. Alternating current means that the current moves back and forth and transfers the electricity that way. DC is typically used with battery power and AC is used for long distance electricity transportation.

Nullo's avatar

In addition to @Afos22‘s post, AC is typically found in wall sockets. DC is the domain of the battery. A device built for one will require an adapter to use the other, which is why your cell phone charger has the clunky box on it.
Video here.

bostonbeliever's avatar

Thomas Edison championed Direct Current.
Nikola Tesla championed Alternating Current.
Alternating Current won, because it’s much more effective and because Tesla was far more of a boss than Edison.
Support the Tesla museum! http://www.indiegogo.com/teslamuseum

Nullo's avatar

IIRC a power inverter will make DC into AC, and a rectifier will make AC into DC. Also IIRC, AC gets better distance over copper wire.

Tesla pioneered DC, Edison (a contemporary and friend competitor) championed AC. Eddie rather infamously used DC electricity to fry an elephant, to “prove” that it was somehow more dangerous than his AC.

Afos22's avatar

Nullo. You have it backwards. Tesla championed AC, and Edison Dc.

dabbler's avatar

All our digital electronics run on DC.

filmfann's avatar

If it uses a circuit board, it’s DC.

Nullo's avatar

@Afos22 I stand corrected. Edison was still a jerk, though.

Paradox25's avatar

I’m an electrician, but I’ll try to keep this simple. Let’s just pretend that your typical 9 volt battery generated AC instead of DC. This battery has your typical two terminals in which one is the positive and one is the negative, so with the chemical reactions within the battery cells that generate DC the polarity of the positive and negative terminals stay the same.

Now, getting back to the hypothetical battery which will generate AC, the polarity of each of those positive and negative terminals would constantly reverse. Depending upon the frequency of the AC waves (which could be sine, square, sawtooth, etc) the polarity of each of those terminals will always change at a certain time. The typical American house circuit has a 120/240 volt single phase system which operates at 60 hz. 60 hertz would mean that the polarity of those two battery terminals would switch at a rate of 60 times per second.

As far as uses for each go is pretty variable. AC is better suited for power transmission because it can be transformed easier and thus the voltage can be stepped up. The higher the voltage the easier it is for it to travel longer distances due to voltage drops by the resistance within the conductor used itself. AC transforms better than DC because the polarity of any inducted current reverses itself, and because DC doesn’t change polarities it can only be inducted once, unless you pulse it. Electric fences and ignition coils are examples of using transformers to induct and transform voltage using pulsated DC.

DC is good for many electronics uses. DC is also good for loads which depend upon strong magnetic fields to function properly such as electric magnets, coils, certain motors, etc. Well I won’t drown you here but I hope that helped. I’m not a great teacher.

Nullo's avatar

If you’re interested, Radio Shack sells a book that covers the basics – definitions, concepts, components, etc. Runs about $20, though. Wikipedia’s got the information as well, just not as handily formatted.

ninja_man's avatar

@Paradox25 No worries, that was great!

dabbler's avatar

AC is actually not as good as DC for very-long-distance transmission, but it is can be transformed easily as @Paradox25 mentions, which is why we use it for power systems up to regional size. With its natural cycles it’s easy to convert voltages high-to-low or vice-versa.
Due to the interactions of its electrical field and magnetic field AC power travels on the surface of the cable and high-voltage transmission lines have to have a big cross-section, making them heavy, or they are hollow/woven tubes. The interaction of the electric and magnetic fields also encounters more impedance than DC would resulting in power losses.

In cases where electrical power is sent a long way without the need for voltage change DC is used. E.g. Pacific DC Intertie that brings hydro power from Washington state down to California. That uses huge semi-conductor switches to convert AC to DC on sending end and back to AC on the receiving end.

jerv's avatar

Think of it as the difference between being shaken and being pushed.

One other note; in general, getting shocked with AC will make you twitch while DC will make your muscle clench up. You can often let go of an AC item that is shocking you, but DC… not so much. As for how I know, Navy electricians like to live dangerously ;)

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