Chapter 3 · Effective value of AC & the transformer: practice questions

The effective (root-mean-square, rms) value of an alternating current is best defined as:

• Athe maximum instantaneous value reached by the current during a cycle
• Bthe value of a steady direct current that produces the same heating effect in the same resistance during the same time
• Cthe average value of the current taken over one complete cycle
• Dthe value indicated by a moving-coil ammeter

The periodic time of an alternating current is 0.02 s. Its frequency is equal to:

• A50 Hz
• B20 Hz
• C100 Hz
• D0.02 Hz

In a step-up transformer, the number of turns of the secondary coil compared with that of the primary coil is...

• Aequal
• Bgreater
• Calways exactly double
• Dsmaller

The average value of a sinusoidal alternating current taken over one complete cycle is equal to:

• AI_max
• BI_max / √2
• C2 I_max / π
• Dzero

The instantaneous induced e.m.f. of an AC generator equals zero at the instant when:

• Athe magnetic flux is changing at its greatest rate
• Bthe magnetic flux linking the coil is zero
• Cthe coil's plane is parallel to the magnetic field lines
• Dthe magnetic flux linking the coil is maximum

The primary coil of an ideal transformer has 1200 turns and is connected to a 240 V alternating source. To obtain an effective output voltage of 20 V, the number of turns of the secondary coil must be...

• A144 turns
• B14400 turns
• C100 turns
• D60 turns

A transformer of efficiency 90 % is supplied with an electric power of 2000 W at its primary coil. The power lost (dissipated) inside the transformer equals...

• A2000 W
• B20 W
• C200 W
• D1800 W

The primary coil of an ideal transformer has 1100 turns and is connected to a 220 V alternating supply. The secondary coil feeds a lamp rated '12 V, 36 W', which operates normally. The number of secondary turns and the current in the primary coil are…

• A6 turns and 0.30 A
• B60 turns and 0.16 A
• C600 turns and 0.16 A
• D60 turns and 3.0 A

An AC generator produces a peak e.m.f. of 200 V. The instantaneous value of the e.m.f. after one sixth of the periodic time has elapsed, measured from the zero position, is equal to:

• A200 V
• B141 V
• C100 V
• D173 V

A power station delivers 20 kW of electric power to a transmission line of total resistance 10 ohm at an effective voltage of 1000 V. The power lost as heat in the line equals...

• A4 kW
• B0.4 kW
• C10 kW
• D2 kW