The point of alternating currents?

  • #1
Femme_physics
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Main Question or Discussion Point

I'm not sure I understand the point of alternating currents. From what I understand, it's electrons spinning in circles. So, is alternating currents essentially the way we store electrons? That way we keep them spinning around themselves?
 

Answers and Replies

  • #2
dlgoff
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I'm in a little bit of a hurry this morning so for now I'll give you an animation that might help.

"[URL [Broken] motors and generators
[/URL]
 
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  • #3
Femme_physics
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I'm not sure if this animation helps me understand. I get that AC is a sinosoidal wave... I see 4 magnets causing AC motion, whereas 2 magnets cause DC motion... I'm not sure I see how it's related-- it's just more element adding to my confusion!
 
  • #4
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One reason is that DC has higher losses than AC when it comes to transmitting the power to where it is required.
 
  • #5
sophiecentaur
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Steady on chaps. A few misconceptions here.
Electrons don't spin round in circles. Even with high power AC, the electrons only move at a few mm per second and move back and forth by a small distance with no net movement at all. There are SOOO many electrons in a wire that this seemingly small amount of electron movement corresponds to a high current.

Actually, DC has LOWER losses than AC, for a given voltage and current. However, because you can transform AC to any desired voltage, you can choose to transmit your power at high voltage and then transform it back down to more usable levels. High voltage transmission means lower current which, in turn, means less resistive loss.
It may well be that, as technology produces suitable switching devices, it may be possible to convert DC voltages efficiently and transmission could end up being at DC.

btw, all commonly used generation is basically AC, (because a coil rotates in a magnetic field) followed by rectification if required. The old fashioned 'dynamo' needed a nasty 'commutator' which switched its output constantly in order to produce unidirectional current flow. They were dropped for almost every application as soon as silicon diodes became robust enough - and cheap.
 
  • #6
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One point (advantage) of alternating currents is that transformers don't work on dc currents. This is one of the points of contention between Edison and Westinghouse. See

http://en.wikipedia.org/wiki/War_of_Currents

Bob S
 
  • #7
Femme_physics
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It's as though you guys are speaking Chinese!... I just started out on electronics. I know nothing! But I'll keep looking at sources to see if I can clear my confusion about that issue...thanks
 
  • #8
dlgoff
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This may or may not be more than you've bargained for at this time but poke around a bit and see.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/accircon.html" [Broken]
 
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  • #9
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AC current is sort of like a tidal current. the tide rises and current comes rushing in as electrons. then the tide falls and electrons go rushing out. and just like water, all that current has energy. that energy can be transformed or converted. often it is converted to heat. to transfer energy in the form of magnetic fields, the current has to be constantly changing. and that is why AC is used in power transmission. magnetic coupling allows you to increase voltages and decrease current. a decrease in current basically amounts to less energy lost to "friction", or resistance in the wires.

anyhoo, don't worry too much. it took a couple of years before many things started to click with me. and if you haven't had any calculus yet, you may need to take some things on more faith than you would otherwise need. but over time, as it becomes more familiar, things will make more sense.
 
  • #10
Drakkith
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Basically, AC has certain advantages that DC doesn't have. It is cheaper to produce in a generator and is much easier to transform into different voltages/currents. Overall it is much cheaper and easier to use AC for power than it is for DC.
 
  • #11
davenn
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I personally know of only one DC transmission line. Thats in New Zealand, running from the Benmore Hydro in the central South Island to the Wellington, at the bottom end of the North Island. Its a distance of some 500km, and uses 600V.

Wonder if there are any other significant others around the world. ?

Dave
 
  • #12
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I'm not sure I understand the point of alternating currents. From what I understand, it's electrons spinning in circles. So, is alternating currents essentially the way we store electrons? That way we keep them spinning around themselves?
Hello!

We need to clear up some misconceptions you have. First, it is true that electrons have a property called "spin" however it doesn't really have anything to do with what we typically call current.

Electrons have a quality we call charge in addition to quality we call spin. When we talk about current we are referring to charge, not spin.

In a circuit we get a current by getting electrons to move. Specifically we define current as the number of electrons that move past a certain point in a given time.

AC current is essentially a change in direction in which the electrons move.

One of the reasons that we use AC current for power lines is that it is a lot easier to change the size of an AC voltage vs. a DC voltage. Transmission lines typically have huge voltages. To get a feel the voltages in your house (if you're in the US) is 120V from an outlet. The voltages in a transmission line are 100-1000 larger than 120V. So we need a way to reduce the voltage. AC transmission lines allow us to use transformers which have no moving parts, low maintenance. DC voltages would require mechanical devices which would need servicing, etc.

So essentially it's a matter of choice and we generally tend to think that AC is better than DC for transmission by looking at the pro's and con's of each.

However we have transistors which act like voltage switches and can control the size of a DC voltage. The problem in the past with using transistors for transmission lines is because in the past transistors could only regulate relatively small voltages however now we can regulate kilovolts using transistors so there is a push to start using DC for power lines.
 
  • #13
dlgoff
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I personally know of only one DC transmission line. Thats in New Zealand, running from the Benmore Hydro in the central South Island to the Wellington, at the bottom end of the North Island. Its a distance of some 500km, and uses 600V.

Wonder if there are any other significant others around the world. ?

Dave
Dang. Take a look at this http://en.wikipedia.org/wiki/List_of_HVDC_projects#Europe".
 
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  • #14
davenn
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WOW,
thanks Don,

they are going nutz over them in Europe !!

Down in the references on that wiki page is a link to the NZ one, looks like its undergoing a total overhaul

Dave
 
  • #15
Femme_physics
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I think I understand it all better now, thanks!
 

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