GROL General Radiotelephone 1 — Questions and Answers
Question 1: What formula would determine the inductive reactance of a coil if frequency and coil inductance are known?
- XL = πf L
- XL = 2πf L (Correct answer)
- XL = 1 / 2πf C
- XL = 1 / R2+X2
Correct answer: XL = 2πf L
The inductive reactance (XL) of a coil is directly proportional to both the frequency (f) of the alternating current and the inductance (L) of the coil. The formula XL = 2πf L mathematically expresses this relationship, where 2π represents the angular frequency in radians per second. This formula is fundamental for calculating impedance in AC circuits.
Question 2: Permeability is defined as:
- The magnetic field created by a conductor wound on a laminated core and carrying current.
- The ratio of magnetic flux density in a substance to the magnetizing force that produces it. (Correct answer)
- Polarized molecular alignment in a ferromagnetic material while under the influence of a magnetizing force.
- None of these.
Correct answer: The ratio of magnetic flux density in a substance to the magnetizing force that produces it.
Permeability is a fundamental magnetic property of a material, quantifying its ability to support the formation of a magnetic field within itself. It is precisely defined as the ratio of the magnetic flux density (B) produced in the substance to the magnetizing force (H) that produces it. This property determines how easily a material can be magnetized.
Question 3: Which of these will be most useful for insulation at UHF frequencies?
- Rubber
- Mica (Correct answer)
- Wax impregnated paper
- Lead
Correct answer: Mica
Mica is an excellent electrical insulator, particularly effective at high frequencies like UHF, due to its low dielectric loss and high dielectric strength. Unlike materials such as rubber or wax-impregnated paper, mica maintains its superior insulating properties consistently across a wide range of frequencies. This makes it ideal for high-frequency electronic components where minimal energy loss is critical.
Question 4: Which of the 4 groups of metals listed below are the best low-resistance conductors?
- Gold, silver, and copper (Correct answer)
- Stainless steel, bronze, and lead
- Iron, lead, and nickel
- Bronze, zinc, and manganese
Correct answer: Gold, silver, and copper
Gold, silver, and copper are widely recognized as the best low-resistance conductors due to their atomic structures, which allow electrons to move freely. Silver boasts the highest electrical conductivity, followed closely by copper, and then gold. Their superior conductivity makes them indispensable in electrical wiring, contacts, and electronic components where efficient current flow is paramount.
Question 5: How does the input impedance of a field-effect transistor compare with that of a bipolar transistor?
- A FET has a high input impedance; a bipolar transistor has a low input impedance. (Correct answer)
- One cannot compare input impedance without first knowing the supply voltage.
- An FET has a low input impedance; a bipolar transistor has a high input impedance.
- The input impedance of FETs and bipolar transistors is the same.
Correct answer: A FET has a high input impedance; a bipolar transistor has a low input impedance.
Field-Effect Transistors (FETs) are voltage-controlled devices where the gate is electrically isolated from the channel, resulting in a very high input impedance, typically in the megaohms range. In contrast, Bipolar Junction Transistors (BJTs) are current-controlled devices, and their forward-biased base-emitter junction leads to a relatively low input impedance, usually in the kilohms range. This difference is a key characteristic distinguishing their operation.
Question 6: What is the description of a square wave?
- A wave with only 300 degrees in one cycle.
- A wave whose periodic function is always negative.
- A wave whose periodic function is always positive.
- A wave that abruptly changes back and forth between two voltage levels and stays at these levels for equal amounts of time. (Correct answer)
Correct answer: A wave that abruptly changes back and forth between two voltage levels and stays at these levels for equal amounts of time.
A square wave is a non-sinusoidal periodic waveform characterized by instantaneous transitions between two distinct voltage levels. It maintains each level for an equal duration, resulting in a symmetrical, rectangular shape when plotted over time. This distinct characteristic makes it fundamental in digital electronics and signal processing, representing binary states like 'on' and 'off'.
Question 7: Which of the following groups is correct for listing common materials in order of descending conductivity?
- Silver, copper, aluminum, iron, and lead (Correct answer)
- Lead, iron, silver, aluminum, and copper
- Iron, silver, aluminum, copper, and silver
- Silver, aluminum, iron, lead, and copper
Correct answer: Silver, copper, aluminum, iron, and lead
Electrical conductivity measures a material's ability to conduct electric current, with higher values indicating better conduction. Silver is the most electrically conductive metal, followed closely by copper, which is widely used due to its excellent conductivity and affordability. Aluminum, iron, and lead follow in descending order of conductivity, reflecting their differing atomic structures and electron mobility.
Question 8: Which of the following Ohm Law formulas is incorrect?
- I = E / R
- I = R / E (Correct answer)
- E = I x R
- R = E / I
Correct answer: I = R / E
Ohm's Law describes the fundamental relationship between voltage (E), current (I), and resistance (R) in an electrical circuit. The correct formulations are E = I x R, I = E / R, and R = E / I. The option I = R / E incorrectly places resistance in the numerator and voltage in the denominator, which would yield an incorrect current value.
Question 9: The second harmonic of a 380 kHz frequency is:
- 2 MHz
- 760 kHz (Correct answer)
- 190 kHz
- 144.4 GHz
Correct answer: 760 kHz
A harmonic is an integer multiple of a fundamental frequency. To find the second harmonic, you simply multiply the fundamental frequency by two. Therefore, for a fundamental frequency of 380 kHz, the second harmonic is 380 kHz * 2, which equals 760 kHz.
Question 10: At 150 degrees, what is the amplitude of a sine wave having a peak value of 5 volts?
- -4.3 volts
- -2.5 volts
- +2.5 volts (Correct answer)
- +4.3 volts
Correct answer: +2.5 volts
The amplitude of a sine wave at a specific angle can be calculated using the formula: Amplitude = Peak Value × sin(angle). For a peak value of 5 volts at 150 degrees, the calculation is 5V × sin(150°). Since sin(150°) equals 0.5, the amplitude at 150 degrees is 5V × 0.5 = 2.5 volts.
Question 11: In a circuit where the AC voltage and current are out of phase, how can the true power be determined?
- By multiplying the apparent power times the power factor. (Correct answer)
- By subtracting the apparent power from the power factor.
- By dividing the apparent power by the power factor.
- By multiplying the RMS voltage times the RMS current.
Correct answer: By multiplying the apparent power times the power factor.
In AC circuits where voltage and current are out of phase, apparent power (measured in VA) represents the total power delivered, while true power (measured in Watts) is the actual power consumed by the load. True power is determined by multiplying the apparent power by the power factor. The power factor accounts for the phase difference, indicating how effectively electrical power is converted into useful work.
GROL General Radiotelephone 1
What formula would determine the inductive reactance of a coil if frequency and coil inductance are known?