3. Electrical fundamentals - Q&A

A transformer should always have its

• primary shorted or off load.
• secondary shorted or on load.
• secondary open circuit or on load.

Which of the following would be used to calculate a transformer's transformation ratio?

• Secondary turns / primary turns.
• Primary turns * secondary turns.
• Primary turns / secondary turns.

A potentiometer varies

• resistance.
• current.
• voltage.

A 50 microFarad capacitor is fed with a current of 25 milliAmpere. How long will it take to charge it to 100Volts?

• 1 second.
• 0.2 seconds.
• 12 minutes.

One revolution of a three phase generator will produce

• 1 cycle.
• 3 cycles.
• 2 cycles.

The temperature of a pure metal can greatly affect the resistance of it. What temperature scale is used?

• Absolute.
• Centigrade.
• Fahrenheit.

The method of ascertaining the voltage of a standard aircraft lead-acid battery is by checking

• the voltage with rated load switched ON.
• the voltage immediately after charging.
• the voltage off load.

In a resonant parallel circuit, current is

• zero.
• minimum.
• maximum.

In what equipment is a photon radiated when an electron leaves a hole?

• Photo-cell.
• LED.
• Photo diode.

The circuit shown is at resonance. If the value of the inductance is trebled, the impedance will

• stay the same.
• treble.
• reduce.

In a 4:1 step up transformer, what would give a reflected impedance of 1 kOhm?

• 1 kOhm.
• 16 kOhm.
• 4 kOhm.

The input is 115V and is applied across the 300 turn portion of the autotransformer shown. What is the output?

• 115V
• 28V
• 460V

A transformer has 2000V, 500 turns on the primary and 50 turns on the secondary. What would be the line voltage if the transformer were Delta/Star connected?

• 200V.
• 450V.
• 346V.

A 48-volt direct current source is required to furnish 192 watts to a parallel circuit consisting of three resistors of equal value. What is the value of each resistor?

• 4 ohm.
• 36 ohm.
• 12 ohm.

The instantaneous value of the EMF induced in a rotating wire loop is

• directly proportional to the sine of the angle between the loop and the field.
• indirectly proportional to the sine of the angle between the loop and the field.
• directly proportional to the cosine of the angle between the loop and the field.

A capacitor is fully charged after 25 seconds to a battery voltage of 20 Volts. The battery is replaced with a short circuit. What will be the voltage across the capacitor after one time constant?

• 0 volts.
• 7.36 volts.
• 12.64 volts.

An autotransformer is

• primary and secondary coils.
• primary coil only.
• primary and secondary coils on single bar.

A paramagnetic material has a relative permeability of

• zero.
• less than unity.
• greater than unity.

An increase in operating temperature in most electrical devices carrying current results in

• a decrease in resistance and an increase in current.
• no effect on the resistance and current.
• an increase in resistance and a decrease in current.

The poles of a DC generator are laminated to

• reduce eddy current losses.
• reduce flux losses.
• reduce hysteresis losses.

What polarity do interpoles take in a DC motor?

• Either, provided they lay on the MNA.
• The same as the next main pole ahead in the direction of rotation.
• The same as the next main pole behind in the direction of rotation.

A 10:1 step up power transformer has 120 V, and 3A flowing through the primary, and 0.285 A flowing through the secondary what is the efficiency?

• 90%.
• 95%.
• 85%.

An atom contains

• hydrogen.
• molecules.
• electrons.

If a capacitor 1 uF and an inductor 10mH are connected in parallel what is their approximate resonant frequency?

• 1.6 kHz.
• 62 Hz.
• 3.2 MHz.

Three inductors 10 mH, 5 mH and 20 mH are connected in parallel. What is the total inductance?

• Without knowing the coupling factor the total inductance cannot be found.
• 2.86mH.
• 1.35mH.