power loss in cable

Yes, you could also use P=V^2/R to calculate the power loss in a transmission cable, but be aware that the [V] refers to the voltage across the cable, NOT the voltage of the secondary coil of the step-up transformer.
By conservation of energy, the power input at the primary coil of a transformer equals the power output at the secondary coil (assume 100% efficiency).
Thus we have, V1I1 = V2I2
where V1, I1 and V2,I2 are the voltages and currents at the primary coil and secondary coil respectively.
For a step-up trnasformer, V2>V1, hence I2<I1
If we apply Ohm's law to the transmission cable with resistance R,
voltage across cable if transformer is not used Vc1= I1.R
voltage across cable if step-up transformer is used Vc2= I2.R
hence, Vc2<Vc1
i.e. although the transformer steps up the voltage, the voltage across the cable is actually reduced because of the decrease in secondary current (this is a result of the conservation of energy).
That is to say, if you apply the equation P=V^2/R to calculate the power loss (where V is the voltage across the cable) , the power P is reduced at the secondary.

因為電流大小唔係由條cable話事, 所以同一電壓, 電流亦有大有細, 咁個power loss就會唔同.
除非你測到條電線頭到尾既potential difference, 咁就可以用.
(但係對電線黎講一般都太接近測唔到)

2007-04-16 01:08:53 補充：
電流大細取決於成個circuts既電阻.但係計電纜時因為冇睇到個電阻所以冇數據你計,得用電既電流資料,咪唯有用P=I^2R囉