# Alternating currents

## Homework Statement

Electrical energy supplied by a high voltage power has a total resistance of 4 ohms. At the input to the line the rms voltage is 400kV and the input power is 500MW

Calculate : A) the min voltage that the insulators which support the line must withstand without breakdwon

B) The value of rms current in the power line

C) the power loss in the power line

## Homework Equations

P=I^2R
P=0.5IV (average power)
Peak I = 0.707 Irms

## The Attempt at a Solution

Is it -400 kV? because I can't understand any other way it can be "minimum" without breakdown

B) I did P= IV and got 1250 Amps and if I use V=IR I get 100,000 Amps, since both current are ridiculously high, I doubt they are right.

C) P= I^2R?

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gneill
Mentor
The input voltage is specified as an rms value. What does that tell you about the PEAK voltage?

The input power is the product of the input voltage and the input current, so your value of 1250 Amps looks reasonable -- keep in mind that it is a 500 mega-Watt system!

A) Just look at the rms definition for a sine wave, and draw on a paper the sine wave and its rms value.
B) Looks like you have to find the resistance of the load. It's a voltage source and two resistors. Then play with the formulas you wrote.

You are dealing with several megawatts, it's rather normal you get high current values.