about electricity

make sure you know the meaning of the symbols in each equation

for series citcuit:
P=(I^2)*R
I - current throught the circuit
R - resistance of the resistor on which you want to find out the power dissipated


for parellel circuit:
P=(V^2)/R
V - voltage across the resistor
R - resistance of the resistor on which you want to find out the power dissipated


If you use P=(V^2)/R V is the potential difference(voltage difference) at the two end points of the transmission cable.
V is not the absolute voltage of the cable with respect to ground(0 V).
R is still the resistance of the whole cable=]



2009-02-24 00:05:47 補充：
咁即係可以睇成條transmission cable係一個resistor 咁...就可以用個條式啦??
SURE!! the two eqts are equivalent but make sure you get the right substitution

如果我用P=V^2/R呢條式黎計, 出黎個數又好大wor

You probably has used the equation wrongly. The voltage "V" in the equation is the voltage across the cable, NOT the voltage output of the transformer. Because you are asked to find the power loss on the transmission cable, the I and V therefore refer to the current on and voltage across the cable. They are related by Ohm's Law, V=I.R where R is the resistance of the cable.
Hence, from the equation V = I.R, you could easily see that by reducing the current I on the cable using high voltage transmission would, at the same time, reduce the voltage V across the cable. Thus the power loss on the cable, irrespective of using either P=I^.R or P = V^2/R, would also be reduced.
Therefore, both of the equation, P=I^2.R and P=V^2/R would certainly give the same answer because they are actually derivable from each other using Ohm's Law. They both equally apply to series and parallel circuits.