# Transformers, RMS values NEED HELP!

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#1
Electrical power is transmitted through cables of total resistance 1.8 ohms operated with AC at an rms voltage of 11kV. The power supplied to the input of the cables is 960kW. Calculate:
(i) the peak value of the current in the cables,

So I was thinking of using P = VI, by then finding the peak voltage firstly by using Vrms = V/sqrt(2) where then rearranging to get V which is the peak voltage, giving me a value of 1.56*10^4 volts. Plugging those values into the I = P/V to give me 61.5 A.

That answer however is incorrect as the mark scheme says it is 123 or 120 A. I do not understand why they worked it out this way by doing Irms = P/Vrms, then using Irms=I/sqrt(2) to work out peak current. Why is my method of working incorrect? Thanks in advance, sorry for the long paragraph.
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5 years ago
#2
Electrical power is transmitted through cables of total resistance 1.8 ohms operated with AC at an rms voltage of 11kV. The power supplied to the input of the cables is 960kW. Calculate:
(i) the peak value of the current in the cables,

So I was thinking of using P = VI, by then finding the peak voltage firstly by using Vrms = V/sqrt(2) where then rearranging to get V which is the peak voltage, giving me a value of 1.56*10^4 volts. Plugging those values into the I = P/V to give me 61.5 A.

That answer however is incorrect as the mark scheme says it is 123 or 120 A. I do not understand why they worked it out this way by doing Irms = P/Vrms, then using Irms=I/sqrt(2) to work out peak current. Why is my method of working incorrect? Thanks in advance, sorry for the long paragraph.
Your assumption is that if you have peak voltage, you'll have peak current. This is not necessarily true. Think about P=IV, if V increases, I must decrease.

The formula P(rms) = I(rms) * V(rms) holds, but I don't really think the formula P(peak) = I(peak) * V(peak) does.
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#3
(Original post by kingaaran)
Your assumption is that if you have peak voltage, you'll have peak current. This is not necessarily true. Think about P=IV, if V increases, I must decrease.

The formula P(rms) = I(rms) * V(rms) holds, but I don't really think the formula P(peak) = I(peak) * V(peak) does.
Ahh right, thanks man.
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