# Why our homes are supplied with an AC not DC???

#### Hurdy

Joined Feb 27, 2006
137
I have a question also.

Why do they say to transmit power by increasing the voltage the current goes down?

According to ohms law (V/I*R) they're proportional. I can't get my head around it.

Say for the sake of this a cable has a resistance of 2ohms and the voltages is 2000V

then I=2000/2 = 1000A

but if we increase the voltage to say 4000V

then I=4000/2 = 2000A

hence proportional.

Thank you.

#### SIcam

Joined Aug 9, 2008
61

Car batteries can store energy and it is DC. AC is very difficult to store.

This prevents people from having a basement full of batteries to store power before a hurricane hits and the prices are raised. Isnt it great.

Look at the lines at the pumps when gas prices are about to raise. Some even buy extra an store it in gas cans.

Actually - just jokin.
Simplified version.

With DC the electrons actually have to flow the full distance from the power station to the usage point. (Mechanical thinking - with DC the current is water and to move the water wheel at the end of the tranport line the water must leave the pipe and turn the water wheel.)

With AC no electrons do not truely flow they just vibrate back and forth a small distance. This vibratory pumping action is translated in to motion at the end.
(Mechanical thinking - Take the pipe with water and plug the end of the pipe with a piston connected with connecting rod to a crank shaft. The water is then given positive pressure for 50% of the time then the water retreats (negative pressure) and this happens at 60 cycles/sec. Through the pipe there is frictional loss of the water to the pipe walls and itsself but as the pressure increase this is less noticeable. Now the water does not come out the end of the pipe but oscillates a small distance back and forth turning a crankshaft as the end.

Both AC and DC can do the work but AC is more efficient at doing it.

Also transformers are cool and DC doesn't work with transformers easily.

#### TanTJ

Joined Mar 6, 2008
21
*edit* This is in response to what Hurdy said.

Ohms law is not used for what you're looking for.

The amount of power used at different voltages is what you're looking for. Since power = volts times amps as the voltage goes up the required amperage goes down for the same amount of power. For example:

Let's say a you need 500,000 watts of power to a business. At 220 volts you will be drawing 2272.22 amps through the circuit. That's quite a lot of amps!

Now let's say that the power transmission line going to the transformer for the business is at 50,000 volts then you're only running 10 amps through that circuit to supply the required 500,000 watts to the business.

On another note regarding what the OP was talking about here's an experiment you can try to explain why AC is preferred over DC when it comes to power transmission over long distances.

Go to your local automotive store and find a spotlight that operates on 12vDC. Find something that is rated for at least 100 watts. When you're there pick up 2 spools of 12 gauge wire that is at least 20-30 feet long each.

Now go to your car and hook it up directly to the battery with about 12 gauge wire that is less than a couple feet in length. The light is pretty bright right?

Now take the 2 spools of wire and completely unroll them. Hook up the light to the long lengths of wire and see what happens to the light. It got pretty dim didn't it?

The voltage drop across the wire when using DC voltage is pretty big. To counter this you need to run really large gauge wire over longer distances which is not cost effective.

Now have you noticed that you can hook up your weed whacker to a 50 foot extension cord and it still works just as well as if it was plugged directly into the wall? This shows you that the voltage drop across the wire when using AC voltage is not near as much as when using DC.

Unfortunately I wouldn't be the person to describe in really technical terms why exactly this is, but as they say, the proof is in the pudding.

#### SIcam

Joined Aug 9, 2008
61
Hurdy to answer your question you need to think in Energy transfer kWh (kilowatt hour) as the power station sells you.

So simplify get rid of the h ( hour and worry about kW).

You said 2000 Volts with 1000 Amps usage or 2000 kW.

Now if we doubled the voltage how many amps need to move to get the same kW of 2000?

2,000,000 Watts / 4000 volts = 500 Amps.

So if power control is protected by amperage and the wire diameter must be large enough to allow that much current to move at once then 1000 Amps is more costly to move than 500 Amps.

The only problems with the higher voltage is insulation (Plastic must be thicker and wires must be hung further away from things ). Copper is costly so they run extremely high voltage which allows for smaller diameter wire for current to flow.

I hope this helps.

#### Hurdy

Joined Feb 27, 2006
137
ah brilliant I understand!! Thank you both.

#### blocco a spirale

Joined Jun 18, 2008
1,546
This thread contains some of the weirdest theories I've ever heard

#### SIcam

Joined Aug 9, 2008
61
This thread contains some of the weirdest theories I've ever heard

Go ahead, make up your own theory, post it and lets see how it fairs against ours.