# Why is there alternating current in my wall socket?

Why is the standard of delivering electricity to our homes is via alternating current, and not direct? As far as I know almost every electronic device has an AC»DC converter because their internals use direct current.

• Because Edison got it wrong and Tesla got it right :-) – user207421 Nov 20 '12 at 5:18
• Related – Dean Nov 20 '12 at 16:44
• With alternating current, the power company gets to sell you something, then suck it back on the next half cycle ;-) What a business! – Olin Lathrop Jan 14 '13 at 15:08
• @OlinLathrop Reminds me of another, much older business with similar mechanics :p So I guess it's a fair kind. – user1306322 Jan 14 '13 at 21:56

From Wiki:

Transmission loss

The advantage of AC for distributing power over a distance is due to the ease of changing voltages using a transformer. Available electric power is the product of current × voltage at the load. For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have an almost fixed electrical resistance, some power will be wasted as heat in the wires. This power loss is given by Joule's first law and is proportional to the square of the current. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, high-current, low-voltage transmissions will suffer a much greater power loss than low-current, high-voltage ones. This holds whether DC or AC is used.

Converting DC power from one voltage to another requires a large spinning rotary converter or motor-generator set, which was difficult, expensive, inefficient, and required maintenance, whereas with AC the voltage can be changed with simple and efficient transformers that have no moving parts and require very little maintenance. This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts.

• AC prevents corrosion of the various metals too. Not sure if that was a welcome coincidence or a smart design requirement in the old days. – jippie Nov 20 '12 at 8:06
• @jippie: Well noted. Keeping a constant polarity can contribute to ionization, since ions from the surroundings will flock to whatever is charged in reverse. I'd say that's a nice bonus considering the huge advantages of using transformers. – Jonny B Good Nov 20 '12 at 17:42
• "Converting DC power from one voltage to another requires a large spinning rotary converter or motor-generator set" - but there are solid-state DC-to-DC converters. Are these simply unusable for very high amounts of current? – thomasrutter Nov 21 '12 at 6:52
• @thomasrutter Remember, back then you didn't even have a transistor and vacuum tubes were relatively new. DC to DC converters such as those that exist today weren't feasible back then. – AndrejaKo Nov 21 '12 at 7:01

Power loss in any resistive element is $$P = I^2 * R1$$

Power delivered to a load is $$P= I * R2$$

We can think of R1 as being our transmission wire and R2 as being the device being powered (OK, in reality most devices don't behave like resistors, but the story remains the same)

1: So, the loss (wasted power) increases with the square of the current, but the power delivered to the load does not. This means, to deliver the same power, it is better to use a low current in a transmission wire at a high voltage than using a low voltage at a high current.

2: It is very simple and efficient to use a transformer to convert AC from one voltage to another. Converting DC from one voltage to another is costly and complex.

Add all this together and it makes more sense to transmit power using AC than DC. Less power is wasted, wasted power means wasted money.

Also because the currents are less, the size of the wire is smaller and lighter, this means the cost of the infrastructure is lower.

The main advantage is that it is much easier to convert AC to different combinations of voltage and current. This was nearly impossible with DC back when the standard emerged. Also large machines like motors and the generators that power the grid inherently produce AC. This can be rectified with diodes or some types of communtation, but the result will still be rippling DC at best.

DC has advantage in power transmission since there is no capacitive and radiative loss, and the conductors don't suffer from skin effect. The fact that most transmission, even the high power main lines, are AC today is evidence of the difficulty in converting to DC and back to AC again at the other end. DC transmission is used in a few places today, limited to long distances and/or to transfer power between two power grids that are not phase locked. The greater efficiency over the long distance makes up for the cost of doing the conversion at each end.

One example of such a DC line is the hydro-Quebec feeder into the New England power grid. This runs for something like 1000 miles from large dams in northern Quebec down to as far as a power substation in Ayer Massachusetts not far from my house. The facility to receive the DC power and convert it for connection to the local grid is not trivial. Take a look at 42.5705N, 71.5242W if you want to see the scale. However, that is still apparently cheaper overall than paying for the power losses and more expensive cable over 1000 miles of transmission line.