Alternating currents, what's the use?

[johnnyies]

today i was introduced to AC currents in my electricty/magnetism class. Perhaps we're not to this point yet, but I'm not sure what's the use of an alternating current, compared to DC currents. dc currents can only go in one direction, but what's the practical use for alternating one?

sorry for stupid question.

 

[Perfection]

You can't use a transformer on DC. http://en.wikipedia.org/wiki/Transformer

Transformers are essential for modern power distribution.

 

[Born2bwire]

From a practical standpoint they are easy to generate and transmit efficiently over long distances. From the consumer side though, few things are able to use AC current directly. Two exceptions being the standard three phase induction motor and incandescent bulb.

 

[syhprum]

As Mr Edison demonstrated they are very useful for electrocuting elephants

 

[Jobrag]

If you accidently grap a live AC carrying cable you will tend to be thrown off, if you do the same to a DC cable you will grab the cable harder.
Heaters use AC directly as well.

 

[ruko]

As Mr Edison demonstrated they are very useful for electrocuting elephants

The reason Edison publicly electrocuted an elephant was to supposedly prove that AC was more dangerous than DC. Edison had invested a fortune in DC power distribution even though the hand writing was on the wall, so to speak, for the inevitable world use of AC. He also did it to embarrass Tesla, a former employee of Edison, who was really the founder of alternating current generation and distribution. See Wikipedia, War Of Currents.

Extremely high voltage DC power distribution is being used in some areas for very long distance power lines because there are less losses due to hysteresis and capacitive/inductive reactance. Also very efficient methods are available now for converting DC to AC for local distribution.

 

[pixel01]

The way you generate electricity, it's easier to make it AC. It is also much more effective to transmit long distances. A lot of motors are with AC, no sparks, simpler, more durable...

 

[ruko]

. It is also much more effective to transmit long distances.QUOTE]

No, it is not. Read the many articles on the net about very long transmission lines using high voltage DC. The longer the line the better DC is to transmit because of reasons I stated previously. There are DC lines in use right now that are over 1100 miles long. In my state there is a new DC line 465 miles long. DC is being used here for a reason and the reason is higher efficiency.

http://www.jcmiras.net/jcm/item/86/ ,
http://en.wikipedia.org/wiki/High-voltage_direct_current

 

[mgb_phys]

The way you generate electricity, it's easier to make it AC. It is also much more effective to transmit long distances.

Only indirectly true.
It's more efficient to transmit DC long distances for several reasons (peak voltage, radiative losses, skin effect) and practical reasons (no need to keep the distant grids in phase).
The reason we use AC grids is that it's easier to convert into high voltage and back - which is presumably what you meant.

And although AC is an electric shock risk at low currents, DC is very dangerous at high powers. It's very difficult to safely switch or disconnect a high current DC line.

 

[ruko]

Only indirectly true.
It's more efficient to transmit DC long distances for several reasons (peak voltage, radiative losses, skin effect) and practical reasons (no need to keep the distant grids in phase).
.

I wonder about skin affect at 60 Hz. Most AC losses are from reactance especially when dealing with under sea or underground cables. Peak voltage is a good reason. A 500 Kv AC line is really carrying 707 Kv peak. At 500 Kv AC the insulators have to have about a 50% longer path compared to 500 Kv DC.