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# Physics Electricity question help watch

1. The question I am working on:

"The power stations providing electricity for the National grid generate electricity at 400kV. The typical demand on an average winter's day is 6000MW. The current in the transmission lines is 15kA. Each pair of 400kV supergrid conductors has a resistance of 0.034Ω per km. How much power is lost per kilometer heating the conductors."

Upon looking at the answer, it was achieved using ; P=I^2R so that P=15kA^2 x 0.034 = 7650000
However I was using the other equation for power which is V^2/R and I ended up using 400kV^2/0.034 which gave the wrong answer. Why is the method I used wrong in this situation, and how do I know when to apply the correct power equation?
2. (Original post by scientific222)
The question I am working on:

"The power stations providing electricity for the National grid generate electricity at 400kV. The typical demand on an average winter's day is 6000MW. The current in the transmission lines is 15kA. Each pair of 400kV supergrid conductors has a resistance of 0.034Ω per km. How much power is lost per kilometer heating the conductors."

Upon looking at the answer, it was achieved using ; P=I^2R so that P=15kA^2 x 0.034 = 7650000
However I was using the other equation for power which is V^2/R and I ended up using 400kV^2/0.034 which gave the wrong answer. Why is the method I used wrong in this situation, and how do I know when to apply the correct power equation?
400kV is not the voltage to use in the V2/R formula
In that formula you use the voltage dropped across the length of the cables as that is the pd across the resistance in the cables.
400kV is the voltage at the generator end. At the consumer end it is somewhat less.
The voltage drop per km is V=IR = 15kA x 0.034 = 510V

The power loss per km is then this V2/R = 5102/0.034
3. (Original post by Stonebridge)
400kV is not the voltage to use in the V2/R formula
In that formula you use the voltage dropped across the length of the cables as that is the pd across the resistance in the cables.
400kV is the voltage at the generator end. At the consumer end it is somewhat less.
The voltage drop per km is V=IR = 15kA x 0.034 = 510V

The power loss per km is then this V2/R = 5102/0.034
I've been baffled for hours until now, thank you so much!

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