Inductors and Capacitors
B-005-1-4 (3) How many microfarads is 1 000 000 picofarads?
1. 1 000 000 000 microfarads
2. 1000 microfarads
3. 1 microfarad
4. 0.001 microfarad
> 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.
B-005-1-11 (4) An inductance of 10 000 microhenrys may be stated
correctly as:
1. 100 millihenrys
2. 10 henrys
3. 1 000 henrys
4. 10 millihenrys
> 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.
B-005-9-1 (4) If two equal-value inductors are connected in series, what is
their total inductance?
1. Half the value of one inductor
2. The same as the value of either inductor
3. The value of one inductor times the value of the other
4. 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.
B-005-9-2 (4) If two equal-value inductors are connected in parallel, what
is their total inductance?
1. Twice the value of one inductor
2. The same as the value of either inductor
3. The value of one inductor times the value of the other
4. Half the value of one 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.
B-005-9-3 (4) If two equal-value capacitors are connected in series, what is
their total capacitance?
1. Twice the value of one capacitor
2. The same as the value of either capacitor
3. The value of one capacitor times the value of the other
4. 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.
B-005-9-4 (2) If two equal-value capacitors are connected in parallel, what
is their total capacitance?
1. The same as the value of either capacitor
2. Twice the value of one capacitor
3. The value of one capacitor times the value of the other
4. Half the value of one 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.
B-005-9-5 (3) What determines the inductance of a coil?
1. The core material, the number of turns used to wind the core and the frequency of
the current through the coil
2. The core diameter, the number of turns of wire used to wind the coil and the type
of metal used for the wire
3. The core material, the core diameter, the length of the coil and the
number of turns of wire used to wind the coil
4. The core material, the core diameter, the length of the coil and whether the coil is
mounted horizontally or vertically
> 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.
B-005-9-6 (1) What determines the capacitance of a capacitor?
1. The material between the plates, the area of one side of one plate, the
number of plates and the spacing between the plates
2. The material between the plates, the number of plates and the size of the wires
connected to the plates
3. The number of plates, the spacing between the plates and whether the dielectric
material is N type or P type
4. The material between the plates, the area of one plate, the number of plates and
the material used for the protective coating
> 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.
B-005-9-7 (4) If two equal-value capacitors are connected in parallel, what
is their capacitance?
1. The same value of either capacitor
2. The value of one capacitor times the value of the other
3. Half the value of either capacitor
4. Twice 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.
B-005-9-8 (2) To replace a faulty 10 millihenry choke, you could use two:
1. Two 20 millihenry chokes in series
2. Two 5 millihenry chokes in series
3. Two 30 millihenry chokes in parallel
4. Two 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.
B-005-9-9 (3) Three 15 microfarad capacitors are wired in series. The total
capacitance of this arrangement is:
1. 45 microfarads
2. 12 microfarads
3. 5 microfarads
4. 18 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.
B-005-9-10 (2) Which series combinations of capacitors would best replace
a faulty 10 microfarad capacitor?
1. two 10 microfarad capacitors
2. two 20 microfarad capacitors
3. twenty 2 microfarad capacitors
4. ten 2 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 ]
B-005-9-11 (3) The total capacitance of two or more capacitors in series is:
1. found by adding each of the capacitors together and dividing by the total number
of capacitors
2. found by adding each of the capacitors together
3. always less than the smallest capacitor
4. always greater than the largest 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.
B-005-10-1 (3) How does a coil react to AC?
1. As the amplitude of the applied AC increases, the reactance decreases
2. As the amplitude of the applied AC increases, the reactance increases
3. As the frequency of the applied AC increases, the reactance increases
4. As the frequency of the applied AC increases, the reactance decreases
> 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.
B-005-10-2 (1) How does a capacitor react to AC?
1. As the frequency of the applied AC increases, the reactance decreases
2. As the frequency of the applied AC increases, the reactance increases
3. As the amplitude of the applied AC increases, the reactance increases
4. As the amplitude 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.
B-005-10-3 (2) The reactance of capacitors increases as:
1. applied voltage increases
2. AC frequency decreases
3. applied voltage decreases
4. AC 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.
B-005-10-4 (3) 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:
1. resonance
2. inductance
3. impedance
4. capacitance
> 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 ]
B-005-10-5 (1) Capacitive reactance:
1. decreases as frequency increases
2. applies only to series RLC circuits
3. increases as frequency increases
4. increases with the time constant
> 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.
B-005-10-6 (4) Inductive reactance may be increased by:
1. a decrease in the applied frequency
2. a decrease in the supplied current
3. an increase in the applied voltage
4. an increase in the applied 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.
B-005-10-7 (2) A choke coil of 4.25 microhenrys is used in a circuit at a
frequency of 200 MHz. Its reactance is approximately:
1. 5 740 ohms
2. 5 340 ohms
3. 7 540 ohms
4. 4 750 ohms
> 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. XL = 2 * 3.14 *
200000000 Hz * 0.00000425 H = 2 * 3.14 * 200 Hz * 4.25 uH = 5338
B-005-10-8 (1) The capacitive reactance of a 25 microfarad capacitor
connected to a 60 hertz line is:
1. 106.1 ohms
2. 9 420 ohms
3. 2.4 ohms
4. 1 500 ohms
> Reactance is opposition. XC = 1 over ( 2 * PI * f * C ). Capacitive Reactance = 1
over the product of 'two times PI times frequency in Hertz times capacitance in
Farads'. XC = 1 / ( 2 * 3.14 * 60 Hz * 0.000025 F ) = 1 / ( 0.00942 ) = 106.16
B-005-10-9 (4) A power-supply filter has a capacitor of 10 microfarad. What
is the capacitive reactance of this capacitor to a frequency of 60 hertz?
1. 200 ohms
2. 100 ohms
3. 500 ohms
4. 265 ohms
> Reactance is opposition. XC = 1 over ( 2 * PI * f * C ). Capacitive Reactance = 1
over the product of 'two times PI times frequency in Hertz times capacitance in
Farads'. XC = 1 / ( 2 * 3.14 * 60 Hz * 0.000010 F ) = 1 / ( 0.003768 ) = 265.39
B-005-10-10 (1) What is the approximate inductive reactance of a 1 henry
choke coil used in a 60 hertz circuit?
1. 376 ohms
2. 3760 ohms
3. 188 ohms
4. 1888 ohms
> 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. XL = 2 * 3.14 *
60 Hz * 1 H = 376.8
B-005-10-11 (1) In general, the reactance of inductors increases with:
1. increasing AC frequency
2. decreasing AC frequency
3. decreasing applied voltage
4. increasing applied voltage
> 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.
B-005-11-1 (1) If no load is attached to the secondary winding of a
transformer, what is current in the primary winding called?
1. Magnetizing current
2. Direct current
3. Excitation current
4. Stabilizing 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".
B-005-11-2 (1) A transformer operates a 6.3 volt 2 ampere light bulb from
its secondary winding. The power consumed by the primary winding is
approximately:
1. 13 watts
2. 6 watts
3. 8 watts
4. 3 watts
> The Power Law: P = E * I, Power is Voltage times Current. 6.3 Volts * 2 Amperes
= 12.6 Watts
B-005-11-3 (4) A transformer has a 240 volt primary that draws a current of
250 mA from the mains supply. Assuming no losses, what current would be
available from a 12 volt secondary?
1. 215 amperes
2. 25 amperes
3. 50 amperes
4. 5 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.
B-005-11-4 (2) In a mains power transformer, the primary winding has 250
turns, and the secondary has 500. If the input voltage is 110 volts, the likely
secondary voltage is:
1. 440 V
2. 220 V
3. 560 V
4. 24 V
> A 'step-up' transformer, the secondary uses twice as many turns as the primary,
voltage is doubled ( exactly per the turns ratio ).
B-005-11-5 (3) The strength of the magnetic field around a conductor in air
is:
1. inversely proportional to the diameter of the conductor
2. directly proportional to the diameter of the conductor
3. directly proportional to the current in the conductor
4. inversely proportional to the voltage on 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. "1" and "2" allude to the conductor's diameter which has no influence. "4"
alludes to voltage which would only be of concern for an electrical field.
B-005-11-6 (1) Maximum induced voltage in a coil occurs when:
1. current is going through its greatest rate of change
2. the current through the coil is of a DC nature
3. current is going through its least rate of change
4. 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.
B-005-11-7 (3) The voltage induced in a conductor moving in a magnetic
field is at a maximum when the movement is:
1. made in a counterclockwise direction
2. parallel to the lines of force
3. perpendicular to the lines of force
4. made in a clockwise direction
> 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.
B-005-11-8 (3) A 100% efficient transformer has a turns ratio of 1/5. If
the secondary current is 50 mA, the primary current is:
1. 2 500 mA
2. 0.01 A
3. 0.25 A
4. 0.25 mA
> 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.
B-005-11-11 (3) The fact that energy transfer from primary to secondary
windings in a power transformer is not perfect is indicated by:
1. electrostatic shielding
2. large secondary currents
3. warm iron laminations
4. high primary voltages
> Heating of the core laminations is a symptom of one of the losses in a real-life
transformer.
B-005-12-1 (1) Resonance is the condition that exists when:
1. inductive reactance and capacitive reactance are equal
2. inductive reactance is the only opposition in the circuit
3. the circuit contains no resistance
4. resistance is equal to the reactance
> 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.
B-005-12-2 (4) Parallel tuned circuits offer:
1. low impedance at resonance
2. zero impedance at resonance
3. an impedance equal to resistance of the circuit
4. very high 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.
B-005-12-3 (4) Resonance is an electrical property used to describe:
1. an inductor
2. a set of parallel inductors
3. the results of tuning a varicap (varactor)
4. the frequency characteristic of a coil and capacitor circuit
> 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.
B-005-12-4 (4) A tuned circuit is formed from two basic components. These
are:
1. resistors and transistors
2. directors and reflectors
3. diodes and transistors
4. inductors and capacitors
> 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.
B-005-12-5 (1) 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:
1. resonant frequency
2. impedance frequency
3. inductive frequency
4. reactive 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.
B-005-12-6 (4) In a parallel-resonant circuit at resonance, the circuit has a:
1. low impedance
2. low mutual inductance
3. high mutual inductance
4. high impedance
> 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.
B-005-12-7 (1) In a series resonant circuit at resonance, the circuit has:
1. low impedance
2. high impedance
3. low mutual inductance
4. high 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.
B-005-12-8 (4) A coil and an air-spaced capacitor are arranged to form a
resonant circuit. The resonant frequency will remain the same if we:
1. increase the area of plates in the capacitor
2. replace the air dielectric with oil in the capacitor
3. wind more turns on the coil
4. add a resistor to the circuit
> Resonance is affected exclusively by Inductance (L in Henrys for inductors) and
Capacitance ( C in Farads for capacitors ). "1" and "2" affect C. "3" affects L. Only
"4" has no impact on L nor C.
B-005-12-9 (2) Resonant circuits in a receiver are used to:
1. filter direct current
2. select signal frequencies
3. increase power
4. adjust voltage levels
> Resonance is about choosing a frequency (or narrow range of frequencies) over
others, either to eliminate it or favour it.
B-005-12-10 (1) Resonance is the condition that exists when:
1. inductive reactance and capacitive reactance are equal and opposite in
sign
2. inductive reactance is the only opposition in the circuit
3. the circuit contains no resistance
4. resistance is equal to the reactance
> 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.
B-005-12-11 (3) When a series LCR circuit is tuned to the frequency of the
source, the:
1. line current lags the applied voltage
2. line current leads the applied voltage
3. line current reaches maximum
4. impedance is maximum
> 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.
B-006-6-4 (3) When will a power source deliver maximum output to the
load?
1. When air wound transformers are used instead of iron-core transformers
2. When the power-supply fuse rating equals the primary winding current
3. When the impedance of the load is equal to the impedance of the source
4. When the load resistance is infinite
> 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.
B-006-6-5 (2) What happens when the impedance of an electrical load is
equal to the internal impedance of the power source?
1. The electrical load is shorted
2. The source delivers maximum power to the load
3. No current can flow through the circuit
4. The source delivers minimum power to the load
> 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.
B-006-6-6 (4) Why is impedance matching important?
1. So the load will draw minimum power from the source
2. To ensure that there is less resistance than reactance in the circuit
3. To ensure that the resistance and reactance in the circuit are equal
4. So the source can deliver maximum power to the load
> 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.