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Subel. L04
47 questions
Subelement L04
Inductors and Capacitors.
Section L04
How many microfarads is 1 000 000 picofarads?
  • 1000 microfarads
  • 0.001 microfarad
  • Correct Answer
    1 microfarad
  • 1 000 000 000 microfarads

Pico is a millionth of a millionth, micro is a millionth. Converting from picofarads to microfarads: from small units to larger units, requires fewer digits, decimal point moves to the left by SIX positions, a MILLION times less.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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An inductance of 10 000 microhenrys may be stated correctly as:
  • Correct Answer
    10 millihenrys
  • 100 millihenrys
  • 10 henrys
  • 1 000 henrys

Micro is a millionth, milli is a thousandth. Converting from microhenrys to millihenrys: from small units to larger units, requires fewer digits, decimal point moves to the left by three positions, a thousand times less.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If two equal-value inductors are connected in series, what is their total inductance?
  • Half the value of one inductor
  • The same as the value of either inductor
  • The value of one inductor times the value of the other
  • Correct Answer
    Twice the value of one inductor

key words: SERIES INDUCTORS. Inductors (coils) in combinations obey rules similar to resistors. In SERIES, the total value is the sum of the values. In PARALLEL combination with components of IDENTICAL values, the total value is the value of one component divided by the number in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If two equal-value inductors are connected in parallel, what is their total inductance?
  • The value of one inductor times the value of the other
  • Correct Answer
    Half the value of one inductor
  • Twice the value of one inductor
  • The same as the value of either inductor

key words: PARALLEL INDUCTORS. Inductors (coils) in combinations obey rules similar to resistors. In PARALLEL combination with components of IDENTICAL values, the total value is the value of one component divided by the number in the circuit. In SERIES, the total value is the sum of the values.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If two equal-value capacitors are connected in series, what is their total capacitance?
  • Twice the value of one capacitor
  • The same as the value of either capacitor
  • The value of one capacitor times the value of the other
  • Correct Answer
    Half the value of either capacitor

key words: SERIES CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. Capacitors add up in parallel combinations BUT the total value is less than the smallest in a series combination. With identical CAPACITORS in SERIES, the total value is the value of one component divided by the number in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If two equal-value capacitors are connected in parallel, what is their total capacitance?
  • The value of one capacitor times the value of the other
  • Half the value of one capacitor
  • Correct Answer
    Twice the value of one capacitor
  • The same as the value of either capacitor

key words: PARALLEL CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. With CAPACITORS in PARALLEL, the total value is the sum of the values. Picture in your head, the area of the plates growing as more and more capacitors are added in parallel. More plate area, more capacity.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What determines the inductance of a coil?
  • The coil diameter, the number of turns of wire used to wind the coil and the type of metal used for the wire
  • The core material, the coil diameter, the length of the coil and whether the coil is mounted horizontally or vertically
  • Correct Answer
    The core material, the coil diameter, the length of the coil and the number of turns of wire used to wind the coil
  • The core material, the number of turns used to wind the coil and the frequency of the current through the coil

Inductance in a coil is due to the interaction of the magnetic fields from one turn to the others. The ease of setting up a magnetic field through a suitable core material, the relative position of the turns (diameter and length) and the number of turns all contribute to inductance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What determines the capacitance of a capacitor?
  • The material between the plates, the area of one plate, the number of plates and the material used for the protective coating
  • Correct Answer
    The material between the plates, the surface area of the plates, the number of plates and the spacing between the plates
  • The material between the plates, the number of plates and the size of the wires connected to the plates
  • The number of plates, the spacing between the plates and whether the dielectric material is N type or P type

A simple capacitor is two plates next to one another. The material used as a dielectric to insulate the two plates and the distance between the plates influence the importance of the electric field that can be set-up. The area and number of plates multiply the capacitance effect.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If two equal-value capacitors are connected in parallel, what is their capacitance?
  • The value of one capacitor times the value of the other
  • Half the value of either capacitor
  • Correct Answer
    Twice the value of either capacitor
  • The same value of either capacitor

key words: PARALLEL CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. With CAPACITORS in PARALLEL, the total value is the sum of the values. Picture in your head, the area of the plates growing as more and more capacitors are added in parallel. More plate area, more capacity.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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To replace a faulty 10 millihenry choke, you could use two:
  • Correct Answer
    5 millihenry chokes in series
  • 20 millihenry chokes in series
  • 30 millihenry chokes in parallel
  • 5 millihenry chokes in parallel

key words: SERIES INDUCTORS. Inductors (coils) in combinations obey rules similar to resistors. In SERIES, the total value is the sum of the values.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Three 15 microfarad capacitors are wired in series. The total capacitance of this arrangement is:
  • 12 microfarads
  • 18 microfarads
  • Correct Answer
    5 microfarads
  • 45 microfarads

key words: SERIES CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. Capacitors add up in parallel combinations BUT the total value is less than the smallest in a series combination. With identical CAPACITORS in SERIES, the total value is the value of one component divided by the number in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Which series combinations of capacitors would best replace a faulty 10 microfarad capacitor?
  • Two 10 microfarad capacitors
  • Twenty 2 microfarad capacitors
  • Ten 2 microfarad capacitors
  • Correct Answer
    Two 20 microfarad capacitors

key words: SERIES CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. Capacitors add up in parallel combinations BUT the total value is less than the smallest in a series combination. With identical CAPACITORS in SERIES, the total value is the value of one component divided by the number in the circuit. [ capacitors in series might be useful to augment the overall voltage rating ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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The total capacitance of two or more capacitors in series is:
  • always greater than the largest capacitor
  • Correct Answer
    always less than the smallest capacitor
  • found by adding each of the capacitors together and dividing by the total number of capacitors
  • found by adding each of the capacitors together

key words: SERIES CAPACITORS. Capacitors behave OPPOSITE TO INDUCTORS. Capacitors add up in parallel combinations BUT the total value is less than the smallest in a series combination. With identical CAPACITORS in SERIES, the total value is the value of one component divided by the number in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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How does a coil react to AC?
  • As the frequency of the applied AC increases, the reactance decreases
  • Correct Answer
    As the frequency of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance decreases
  • As the amplitude of the applied AC increases, the reactance increases

Reactance is opposition. XL = 2 * PI * f * L. Inductive reactance = two times PI (i.e., 3.14) times frequency in hertz times inductance in henrys. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, inductive reactance goes up. Intuitively, the higher the frequency (i.e., rate of change), the more significant become the counter-currents induced in adjacent turns.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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How does a capacitor react to AC?
  • As the frequency of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance decreases
  • Correct Answer
    As the frequency of the applied AC increases, the reactance decreases

Reactance is opposition. XC = 1 over ( 2 * PI * f * C ). Capacitive Reactance = 1 over the product of 'two times PI (i.e., 3.14) times frequency in hertz times capacitance in farads'. A behaviour opposite to inductors. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, capacitive reactance goes down. Intuitively, the more frequent the change of polarity (AC changes polarity every half-cycle), the more incessant becomes the charge/discharge current, current never seems to stop, less apparent opposition to current flow.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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The reactance of capacitors increases as:
  • Correct Answer
    frequency decreases
  • applied voltage increases
  • applied voltage decreases
  • frequency increases

Reactance is opposition. XC = 1 over ( 2 * PI * f * C ). Capacitive Reactance = 1 over the product of 'two times PI (i.e., 3.14) times frequency in hertz times capacitance in farads'. A behaviour opposite to inductors. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, capacitive reactance goes down. Intuitively, the more frequent the change of polarity (AC changes polarity every half-cycle), the more incessant becomes the charge/discharge current, current never seems to stop, less apparent opposition to current flow.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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In inductances, AC may be opposed by both resistance of winding wire and reactance due to inductive effect. The term which includes resistance and reactance is:
  • inductance
  • capacitance
  • Correct Answer
    impedance
  • resonance

Impedance is measured in ohms. It is the combined effect of reactance(s) and resistance. Resistance affects DC and AC equally. Reactance is a property only present under AC. [ DC = direct current, AC = alternating current ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Capacitive reactance:
  • applies only to series RLC circuits
  • increases as frequency increases
  • increases with the time constant
  • Correct Answer
    decreases as frequency increases

Reactance is opposition. XC = 1 over ( 2 * PI * f * C ). Capacitive Reactance = 1 over the product of 'two times PI (i.e., 3.14) times frequency in hertz times capacitance in farads'. A behaviour opposite to inductors. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, capacitive reactance goes down. Intuitively, the more frequent the change of polarity (AC changes polarity every half-cycle), the more incessant becomes the charge/discharge current, current never seems to stop, less apparent opposition to current flow.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Inductive reactance may be increased by:
  • an increase in the applied voltage
  • Correct Answer
    an increase in the applied frequency
  • a decrease in the applied frequency
  • a decrease in the supplied current

Reactance is opposition. XL = 2 * PI * f * L. Inductive reactance = two times PI (i.e., 3.14) times frequency in hertz times inductance in henrys. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, inductive reactance goes up. Intuitively, the higher the frequency (i.e., rate of change), the more significant become the counter-currents induced in adjacent turns.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What property allows a coil wound on a ferrite core to mitigate the effects of an offending radio signal?
  • Low reactance at audio frequencies
  • High reactance at audio frequencies
  • Correct Answer
    High reactance at radio frequencies
  • Low reactance at radio frequencies

The coil (inductor) when dealing with an offending radio signal: chokes-off radio frequency (high reactance), but passes audio frequencies (low reactance). Recall that the opposition of a coil to AC current flow (inductive reactance) grows as frequency increases.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What property allows an RF bypass capacitor on an audio circuit to divert an offending radio signal?
  • High reactance at audio frequencies
  • Correct Answer
    Low reactance at radio frequencies
  • High reactance at radio frequencies
  • Low reactance at audio frequencies

The bypass capacitor must provide a low impedance path for an offending signal without affecting lower frequency signals: low reactance for radio frequency, high reactance for audio. Recall that the opposition of a capacitor to AC current flow (capacitive reactance) decreases as frequency goes up.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What property allows an RF bypass capacitor to have little effect on an audio circuit?
  • High reactance at high frequencies
  • Low reactance at low frequencies
  • Correct Answer
    High reactance at low frequencies
  • Low reactance at high frequencies

The bypass capacitor must provide a low impedance path for an offending signal without affecting lower frequency signals: low reactance for radio frequency, high reactance for audio. Recall that the opposition of a capacitor to AC current flow (capacitive reactance) decreases as frequency goes up.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What property allows an RF choke coil to have little effect on signals meant to flow through the coil?
  • Low reactance at high frequencies
  • High reactance at high frequencies
  • Correct Answer
    Low reactance at low frequencies
  • High reactance at low frequencies

The coil (inductor) when dealing with an offending radio signal: chokes-off radio frequency (high reactance), but passes audio frequencies (low reactance). Recall that the opposition of a coil to AC current flow (inductive reactance) grows as frequency increases.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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In general, the reactance of inductors increases with:
  • decreasing applied voltage
  • increasing applied voltage
  • Correct Answer
    increasing AC frequency
  • decreasing AC frequency

Reactance is opposition. XL = 2 * PI * f * L. Inductive reactance = two times PI (i.e., 3.14) times frequency in hertz times inductance in henrys. Reactance (opposition) is not influenced by the amplitude of the applied voltage. If frequency goes up, inductive reactance goes up. Intuitively, the higher the frequency (i.e., rate of change), the more significant become the counter-currents induced in adjacent turns.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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If no load is attached to the secondary winding of a transformer, what is current in the primary winding called?
  • Direct current
  • Latent current
  • Stabilizing current
  • Correct Answer
    Magnetizing current

Even if no current is drawn from the secondary of the transformer, the primary winding remains an inductor. It lets some AC current through despite its reactance. This minimal current is called "Magnetizing current".

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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A transformer operates a 6.3 volt 2 ampere light bulb from its secondary winding. The input power to the primary winding is approximately:
  • 3 watts
  • Correct Answer
    13 watts
  • 6 watts
  • 8 watts

The Power Law: P = E * I, power is voltage times current. 6.3 volts * 2 amperes = 12.6 watts

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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A transformer has a 240 volt primary that draws a current of 250 milliamperes from the mains supply. Assuming no losses and only one secondary, what current would be available from the 12 volt secondary?
  • Correct Answer
    5 amperes
  • 215 amperes
  • 25 amperes
  • 50 amperes

As work is performed at a lower voltage on the secondary side, current on the secondary is larger. The turns ratio is '20 to 1' ( 240 volts to 12 volts ), the current ratio follows the inverse of that ratio: 20 * 0.25 amperes = 5 amperes. Method B: Primary consumes 60 watts ( 240 volts * 0.25 amperes ), secondary must draw that same power (discounting losses). What is the secondary current for 60 watts at 12 volts ? I = P / E (derived from P = E * I), I = 60 watts / 12 volts = 5 amperes.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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In a mains power transformer, the primary winding has 250 turns, and the secondary has 500. If the input voltage is 120 volts, the likely secondary voltage is:
  • 26 V
  • Correct Answer
    240 V
  • 480 V
  • 610 V

A 'step-up' transformer, the secondary uses twice as many turns as the primary, voltage is doubled ( exactly per the turns ratio ).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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The strength of the magnetic field around a conductor in air is:
  • inversely proportional to the voltage on the conductor
  • Correct Answer
    directly proportional to the current in the conductor
  • inversely proportional to the diameter of the conductor
  • directly proportional to the diameter of the conductor

Current and magnetism are closely related: current in a conductor sets up a magnetic field, dropping a conductor through magnetic lines of force creates a current. Voltage which would only be of concern for an electrical field. Reference to the conductor's diameter is a useless clue.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Maximum induced voltage in a coil occurs when:
  • Correct Answer
    current is going through its greatest rate of change
  • the current through the coil is of a DC nature
  • current is going through its least rate of change
  • the magnetic field around the coil is not changing

For induction to take place in a wire, a conductor must be subjected to a moving magnetic field (no movement, no induction). Either the conductor must move in the magnetic field OR the magnetic field must move if the conductor is immobile. If current changes drastically within a short period of time ('rate of change'), the magnetic field around the conductor changes rapidly, induction is maximized.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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The voltage induced in a conductor moving in a magnetic field is at a maximum when the movement is:
  • made in a clockwise direction
  • Correct Answer
    perpendicular to the lines of force
  • made in a counter clockwise direction
  • parallel to the lines of force

For induction to be maximum, the conductor must "cut" through the lines of magnetic force. Dropping through perpendicularly (at 90 degrees) through the magnetic field maximizes induction.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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A 100% efficient transformer has a turns ratio of 1/5. If the secondary current is 50 milliamperes, the primary current is:
  • 2 500 mA
  • 0.01 A
  • 0.25 mA
  • Correct Answer
    0.25 A

A turns ratio of '1 to 5' indicates a 'step-up' transformer, primary current will be larger than the secondary current by the inverse of that ratio. In this example, primary current is 5 * times 50 mA = 250 milliamperes = 0.25 amperes. Transformers do not "create" power out of nothing, the power ( E * I ) flowing into the primary equals the power drawn by the secondary plus losses (which are ignored for the sake of simplicity). For power to remain "comparable" on both sides of the transformer, current goes up if voltage increases and vice-versa.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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The fact that energy transfer from primary to secondary windings in a power transformer is not perfect is indicated by:
  • large secondary currents
  • high primary voltages
  • Correct Answer
    warm iron laminations
  • electrostatic shielding

Heating of the core laminations is a symptom of one of the losses in a real-life transformer.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Resonance is the condition that exists when:
  • inductive reactance is the only opposition in the circuit
  • the circuit contains no resistance
  • resistance is equal to the reactance
  • Correct Answer
    inductive reactance and capacitive reactance are equal

Resonance is the condition where Inductive Reactance (XL) is equal in value to Capacitive Reactance (XC). For a given Inductance (L, a coil or inductor) and Capacitance (C, a capacitor), resonance happens at one frequency: the resonant frequency. At resonance, the two reactances cancel each other, only resistance is left in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Parallel tuned circuits offer:
  • an impedance equal to resistance of the circuit
  • Correct Answer
    high impedance at resonance
  • low impedance at resonance
  • zero impedance at resonance

key words: PARALLEL, TUNED. Question refers to Resonance. The one frequency at which Inductive Reactance cancels Capacitive Reactance. In a PARALLEL circuit, Impedance (Z) at resonance is HIGH ( series circuit will be the opposite ). As a memory aid, try to visualize the PARALLEL circuit as a tub or tank, signals get trapped at resonance. Try to visualize the SERIES circuit as a slim tube, signals slip right through at resonance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Resonance is an electrical property used to describe:
  • the results of tuning a varicap (varactor)
  • Correct Answer
    the frequency characteristic of a coil and capacitor circuit
  • an inductor
  • a set of parallel inductors

Resonance is the condition where Inductive Reactance (XL) is equal in value to Capacitive Reactance (XC). For a given Inductance (L, a coil or inductor) and Capacitance (C, a capacitor), resonance happens at one frequency: the resonant frequency. At resonance, the two reactances cancel each other, only resistance is left in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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A tuned circuit is formed from two basic components. These are:
  • Correct Answer
    inductors and capacitors
  • resistors and transistors
  • directors and reflectors
  • diodes and transistors

A 'tuned' circuit is a synonym for a 'resonant' circuit. Resonance is the condition where Inductive Reactance (XL) is equal in value to Capacitive Reactance (XC). Inductors and Capacitors alone determine the resonant frequency of a circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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When a parallel coil-capacitor combination is supplied with AC of different frequencies, there will be one frequency where the impedance will be highest. This is the:
  • reactive frequency
  • Correct Answer
    resonant frequency
  • impedance frequency
  • inductive frequency

key words: COIL, CAPACITOR. A 'tuned' circuit. Question refers to Resonance. The one frequency at which Inductive Reactance cancels Capacitive Reactance. In a PARALLEL circuit, Impedance (Z) at resonance is HIGH ( series circuit will be the opposite ). As a memory aid, try to visualize the PARALLEL circuit as a tub or tank, signals get trapped at resonance. Try to visualize the SERIES circuit as a slim tube, signals slip right through at resonance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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In a parallel-resonant circuit at resonance, the circuit has a:
  • Correct Answer
    high impedance
  • low impedance
  • low mutual inductance
  • high mutual inductance

key words: PARALLEL, RESONANT. Question refers to Resonance. The one frequency at which Inductive Reactance cancels Capacitive Reactance. In a PARALLEL circuit, Impedance (Z) at resonance is HIGH ( series circuit will be the opposite ). As a memory aid, try to visualize the PARALLEL circuit as a tub or tank, signals get trapped at resonance. Try to visualize the SERIES circuit as a slim tube, signals slip right through at resonance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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In a series resonant circuit at resonance, the circuit has:
  • high mutual inductance
  • Correct Answer
    low impedance
  • high impedance
  • low mutual inductance

key words: SERIES, RESONANT. Question refers to Resonance. The one frequency at which Inductive Reactance cancels Capacitive Reactance. In a SERIES circuit, Impedance (Z) at resonance is LOW ( parallel circuit will be the opposite ). If Impedance is low (little total opposition), current will be high. As a memory aid, try to visualize the SERIES circuit as a slim tube, signals slip right through at resonance. Try to visualize the PARALLEL circuit as a tub or tank, signals get trapped at resonance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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A coil and an air-spaced capacitor are arranged to form a resonant circuit. The resonant frequency will remain the same if we:
  • wind more turns on the coil
  • Correct Answer
    add a resistor to the circuit
  • increase the area of plates in the capacitor
  • insert Mylar sheets between the plates of the capacitor

Resonance is affected exclusively by Inductance (L in henrys for inductors) and Capacitance ( C in farads for capacitors ). Capacitance is affected by the area of the plates and the choice of dielectric. Inductance is affected by the number of turns in a coil.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Resonant circuits in a receiver are used to:
  • Correct Answer
    select signal frequencies
  • filter direct current
  • increase power
  • adjust voltage levels

Resonance is about choosing a frequency (or narrow range of frequencies) over others, either to eliminate it or favour it.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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Resonance is the condition that exists when:
  • resistance is equal to the reactance
  • Correct Answer
    inductive reactance and capacitive reactance are equal and opposite in sign
  • inductive reactance is the only opposition in the circuit
  • the circuit contains no resistance

Resonance is the condition where Inductive Reactance (XL) is equal in value to Capacitive Reactance (XC). For a given Inductance (L, a coil or inductor) and Capacitance (C, a capacitor), resonance happens at one frequency: the resonant frequency. At resonance, the two reactances cancel each other, only resistance is left in the circuit.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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When a series LCR circuit is tuned to the frequency of the source, the:
  • line current leads the applied voltage
  • impedance is maximum
  • Correct Answer
    line current reaches maximum
  • line current lags the applied voltage

key words: SERIES, TUNED. Question refers to Resonance. The one frequency at which Inductive Reactance cancels Capacitive Reactance. In a SERIES circuit, Impedance (Z) at resonance is LOW ( parallel circuit will be the opposite ). If Impedance is low (little total opposition), current will be high. As a memory aid, try to visualize the SERIES circuit as a slim tube, signals slip right through at resonance. Try to visualize the PARALLEL circuit as a tub or tank, signals get trapped at resonance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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When will a power source deliver maximum output to the load?
  • When the load resistance is infinite
  • Correct Answer
    When the impedance of the load is equal to the impedance of the source
  • When air wound transformers are used instead of iron-core transformers
  • When the power-supply fuse rating equals the primary winding current

Impedance Match: maximum power transfer occurs when the impedance of the load matches the internal impedance of the source. For example, a transmitter designed to work into an impedance of 50 ohms, will delivered maximum power if the antenna system offers an impedance of 50 ohms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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What happens when the impedance of an electrical load is equal to the internal impedance of the power source?
  • The source delivers minimum power to the load
  • Correct Answer
    The source delivers maximum power to the load
  • The electrical load is shorted
  • No current can flow through the circuit

Impedance Match: maximum power transfer occurs when the impedance of the load matches the internal impedance of the source. For example, a transmitter designed to work into an impedance of 50 ohms, will delivered maximum power if the antenna system offers an impedance of 50 ohms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is impedance matching important?
  • Correct Answer
    So the source can deliver maximum power to the load
  • So the load will draw minimum power from the source
  • To ensure that there is less resistance than reactance in the circuit
  • To ensure that the resistance and reactance in the circuit are equal

Impedance Match: maximum power transfer occurs when the impedance of the load matches the internal impedance of the source. For example, a transmitter designed to work into an impedance of 50 ohms, will delivered maximum power if the antenna system offers an impedance of 50 ohms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

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