Rate of energy dissipated by a power transmission line

[hidemi]

Homework Statement

A power transmission line carries 400A of current at a voltage of 765 kV. If the line has a resistance of 29 μΩ/m, what is the rate at which energy is being dissipated in 800 km of line?

a.) 0 W
b.) 3.7 kW
c.) 310 kW
d.) 3.7 MW
e.) 310 MW

The answer is D.

Relevant Equations

Power = V^2/R = I^2*R

VR = IR
= 400 [ 29*10^-6 * 800 * 10^3 ]
= 9280

P = IV
= 400*9280
= 3.7 MW

I was able to calculate the correct answer from the above, but why can't I use the equation P= V^2 / R?

 

[haruspex]

VR = IR

I was able to calculate the correct answer from the above, but why can't I use the equation P= V^2 / R?

You mean V=IR.

What voltage did you use in V^2/R? If 765kV, what two points is that the voltage difference for?

 

[berkeman]

Homework Statement:: A power transmission line carries 400A of current at a voltage of 765 kV. If the line has a resistance of 29 μΩ/m, what is the rate at which energy is being dissipated in 800 km of line?

a.) 0 W
b.) 3.7 kW
c.) 310 kW
d.) 3.7 MW
e.) 310 MW

The answer is D.
Relevant Equations:: Power = V^2/R = I^2*R

I was able to calculate the correct answer from the above, but why can't I use the equation P= V^2 / R?

Because you weren't given the voltage drop along the wires. You were given the current I and the resistance -- that is enough to calculate the power loss along the wire.

 

[hidemi]

Thank you all.