AC Fundamentals Questions and Answers

Answer: C

Statement 1 is true: direct current does not alternate, so its frequency is zero. Statement 2 is false: a transformer works by changing flux, so it cannot step DC up or down, and DC is changed in voltage with converters. Statement 3 is true: corona loss is lower in DC transmission for the same peak voltage. Statement 4 is true: the Ferranti effect, the voltage rise at the open end of a long AC line caused by line capacitance, does not occur on DC. So the true statements are 1, 3 and 4, option (C), and the other options include statement 2.

Answer: D

The rms value of a sine wave is the peak value divided by the square root of 2, which is the same as 0.707 times the peak.

\[ V_{rms} = \frac{V_{peak}}{\sqrt{2}} \]

\[ V_{rms} = \frac{100}{1.414} = 70.7\ \mathrm{V} \]

So the rms value is 70.7 V, option (D). The value 141.4 V comes from multiplying the peak by root 2, 282.4 V is twice that, and 35.4 V is half of the correct value, so each of them applies the factor in the wrong way.