The Problem of Transmitting Power:

DC power was mainly in used in the 1880's and it was hard to transmit over distance because:

-To transmit over long distance you need high voltage on a skinny wire or low voltage on a wide wire. High voltage on DC is very dangerous, and with low voltage the wire would have to be so thick that it would not be practical. Also with high voltage you couldn't not step down the voltage so it could be used with home light bulbs.

Using the water analogy: imagine that a small wire with high voltage is like a garden hose with high pressured water moving fast inside. Imagine that this hose fills 2 gallon jugs of water in one minute. Now think of a 6" wide drain pipe filled with water. You can deliver the same amount of water to the destination in the same time period without needing so much pressure.

With AC power you also use high voltage to move the electricity down a long wire. AC becomes more practical because once you send the power to the destination, you can use a transformer to change the voltage down to a manageable level. The power is stepped down several times by the time it reaches you home. The power line coming into your home is at 240 volts, from your breaker box it is split into lines of 120 volts for most of your home sockets and 240 for appliance sockets. In Europe and other regions 220 V is the standard home socket.

Transformer - a device that transfers electrical energy from one circuit to another circuit using inductively coupled conductors. In other words by putting two coils of wire close together while not touching, the magnetic field from the first coil called the primary winding effects the other coil (called the secondary coil). This effect is called "inductance". Inductance was discovered by Joseph Henry and Michael Faraday in 1831.

Now if you would like to change the voltage on a powerline, you could do this by changing current going into the primary coil (voltage stays high). The current level affects the induced voltage on the secondary coil. A changing magnetic field induces a changing electromagnetic force (EMF) or "voltage". To put it simply: by changing the current you can obtain the desired voltage on the other side.