How can stepping up and down the voltage save energy in power transmission?

[Angie K.]

Homework Statement

If 39.4 kW of power is to be transmitted from a generating station over two 0.141-Ω lines, estimate how much power is saved if the voltage is stepped up from 120 V(rms) to 1200 V(rms) and then down again, rather than simply transmitting at 120 V. Assume the transformers are each 95 percent efficient.

Homework Equations

Power dissipated = I^2*R

The Attempt at a Solution

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Would I find the current using Ohm's Law, of the 120 V and then of 1200 V ? Using .142 as the resistance?

The compare it to the given power?

Then I'm not sure what to do or even if that is the right approach.

 

[Orodruin]

Would I find the current using Ohm's Law, of the 120 V and then of 1200 V ? Using .142 as the resistance?

No. The power loss depends on the potential drop over the transmission lines, which is not 120 V and 1200 V. You need to compare the potential drop over the load and compare it to that over the transmission.

 

[CWatters]

In both cases 39.4 kW of power (at 120V) has to arrive at the load. I would make two diagrams (yes I know they are simple) and then starting at the load, mark them up with Voltages and current as appropriate based on what you know and can calculate. Work back to the source.