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The primary of an ideal 1:3 step-up transformer is connected to a source, and the secondary feeds a resistor R, dissipating power P. If instead R is connected directly to the same source, the power it dissipates is:

1. P/9
2. P/3
3. P
4. 3P

Correct answer: P/9

Solution

Let source voltage be Vs. Through the 1:3 step-up transformer, secondary voltage = 3Vs, so P = (3Vs)²/R = 9Vs²/R. Connected directly, the resistor sees Vs, giving power P' = Vs²/R. Therefore P' = P/9.

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