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Why does Ohm's Law not apply in this example?
A power station supplies electrical energy at a rate of 2 MW to an industrial park from a local power station, whose output voltage is 10 kV. The total length of the cables connecting the industrial park to the power station is 15 km. The cables have a diameter of 10 mm and are made from a material of resistivity 5 X 10^-8 ohm meters,
1.Calculate the Total Resistance of the Cables?
I worked this part out to be 9.55 ohms
2. Calculate the Current flowing in the cables?
I get 2 answers for this:
Using ohm's law:
I = V/R
I = 10000/9.55 = 1047 Amps
Using Power = Voltage * Current
2000000 = 10000 I
I = 200 Amps
The correct answer is 200 amps but how come the ohm's law method doesn't work?
1. I think this is wrongl
R = 15000 * 5 X 10^-8 = 0.00075 Ω
You might have to double this (or triple if it is 3 phase)
2. Your problem is that there is not a 10kV voltage drop on the wires. If there were there would be nothing left for the industrial park.
So you have to use the power equation. Now from that you can calcutate the voltage drop on the wires by:
V = IR
V = 200( 0.00075 ) volts.
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