[quote author=Fu-Kwun Hwang link=topic=941.msg3485#msg3485 date=1234092088]
One of the key concerns in the transmission of electricity is the power loss in transmission lines, dissipated as heat due to the resistance of the conductors.

High-voltage transmission lines are used to transmit electric power over long distances. Normally, high voltage (HV) transmission power lines are made of high voltage (between 138 and 765 kilovolts)  conducting lines of copper and/or aluminum.

Assume the power to be transmitted is P, and the resistance of the transmission line is r.
If the power is transmitted with voltage V, then the current flow through the transmission line is I=P/V.

The power loss P[sub]loss[/sub]=I[sup]2[/sup]*r=(P/V)[sup]2[/sup]*r
Since P and r are fixed conditions, less power will be lost if  high voltages V are used.

Some students will raise questions like: From Ohm's law. if the voltage is increased, the current will increase ,too. Why is the current smaller when high voltage is used to transmit the power.

Textbooks forgot to tell students that the transmission line needs a transformer to step down the voltage.
And the transformer does not have a fixed impedance. If higher voltage is used to transmit the power, the ratio of the transformer will also change which will change the impedance of the transformer.
The following applet was developed to help you understand the high power transmission line.

You can change the Power/Voltage V and resistance r in the transmission line with sliders.
I will show current flow through the transmission line.
Z is the total impedance of the transmission line, Zt is the impedance of the transformer.
N:n shows the ratio of the high voltage transformer (Assume user voltage is 100V).
Efficiency of the power line is also shown at the right side.

I hope this applet will help you understand more about the power transmission line.