# Homework Help: Power transmission line cable

1. Oct 19, 2011

### titanium90

1. The problem statement, all variables and given/known data

As attached.

2. Relevant equations

-

3. The attempt at a solution

I think P is a step-up transformer while Q is a step-down transformer. Does the current and/or voltage change during XY? I know the answer is either B or C, but I do not the explanation. Can anyone please explain to me the correct answer.

Thank you very much and have a nice day ahead!

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2. Oct 19, 2011

### cepheid

Staff Emeritus
Any real wire (unlike the "ideal" wires you deal with in circuit problems) is going to have some non-zero resistance. What does that tell you?

3. Oct 19, 2011

### titanium90

So am i correct to say the current decreases along XY since it is going to meet some resistance. Thus, B is true. However, by Ohm's Law (V=RI), the voltage across at X should be greater than that across at Y, making C also true?

I am sorry if I misunderstood somewhere.

4. Oct 19, 2011

### cepheid

Staff Emeritus
Current does not decrease along a resistor. If the current at one end was greater than the current at the other end, then you'd have a "pile up" of charge somewhere (either that or you'd have charges disappearing inexplicably). This doesn't happen.

It's true that the current is less than it would have been if you had no resistance (or at least a much smaller resistance). But for a given resistance, the current value is determined and is the same everywhere along the transmission line.

5. Oct 19, 2011

### titanium90

So I know that option B is the correct answer to the question.

I have a last question: Am I correct to say that since current flows from a high potential (X) to low potential (Y), so the voltage across the power line at X is greater than the voltage across the line at Y?

Thank you so much!

6. Oct 19, 2011

### cepheid

Staff Emeritus
Yeah, your application of Ohm's Law was correct. The voltage drop across the resistor is always such that the potential is higher at the end where the current is entering the resistor than it is at the end where the current is leaving it.