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Alternating current and voltaage
1.
A sine wave has a periodic time of 0.25 ms. Which one of the following gives the frequency of the sine wave?
a) 250 Hz
b) 400 Hz
c) 2.5 kHz
d) 4 kHz.
2.
A sine wave has an r.m.s. value of 200V. Which one of the following gives the approximate peak-peak value of the voltage?
a) 141 V
b) 283 V
c) 400 V
d) 566 V.
3.
The reactance of a capacitor is 100 ohm at 50 Hz. What will the reactance be at 400 Hz?
a) 25 ohm
b) 50 ohm
c) 200 ohm
d) 400 ohm.
4.
The reactance of an inductor is 600 ohm at 2 kHz. What will the reactance be at 500 Hz?
a) 150 ohm
b) 300 ohm
c) 1.2 kohm
d) 2.4 kohm.
5.
The current flowing in a capacitor:
a) is in phase with the voltage across the capacitor
b) lags the voltage across the capacitor by 90 degrees
c) leads the voltage across the capacitor by 90 degrees
d) is 180 degrees out of phase with the voltage across the capacitor.
6.
A circuit consists of a resistance of 300 ohm connected in series with a reactance of 400 ohm. Which of the following gives the impedance of the circuit?
a) 100 ohm
b) 350 ohm
c) 500 ohm
d) 700 ohm.
7.
At resonance, the impedance of a series L-C-R tuned circuit will be:
a) zero
b) maximum
c) minimum
d) equal to R.
8.
A typical application for the component shown below is:
a) stepping down AC voltage
b) converting AC input to DC output
c) smoothing and regulating a DC supply
d) acting as a reservoir to store charge.
9.
A suitable material for use as the core of a power transformer is:
a) air
b) copper sheet
c) laminated steel
d) aluminium strip.
10.
A transformer has a turns ratio of 20:1. If the primary is fed from a 240V AC supply and a load of 30 ohms is connected to the secondary, which one of the following gives the primary current?
a) 20 mA
b) 50 mA
c) 400 mA
d) 500 mA.
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