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Subel. ZLB
20 questions
Subelement ZLB

Basic Electrical Theory

Section ZLB09

Capacitors, Inductors, Resonance

The total capacitance of two or more capacitors in series is

  • Correct Answer
    always less than that of the smallest capacitor
  • always greater than that of the largest capacitor
  • found by adding each of the capacitances together
  • found by adding the capacitances together and dividing by their total number
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Filter capacitors in power supplies are sometimes connected in series to

  • Correct Answer
    withstand a greater voltage than a single capacitor can withstand
  • increase the total capacity
  • reduce the ripple voltage further
  • resonate the filter circuit
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A component is identified as a capacitor if its value is measured in

  • microvolts
  • millihenrys
  • megohms
  • Correct Answer
    microfarads
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Two metal plates separated by air form a 0.001 uF capacitor. Its value may be changed to 0.002 uF by

  • Correct Answer
    bringing the metal plates closer together
  • making the plates smaller in size
  • moving the plates apart
  • touching the two plates together
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The material separating the plates of a capacitor is the

  • Correct Answer
    dielectric
  • semiconductor
  • resistor
  • lamination
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Three 15 picofarad capacitors are wired in parallel. The value of the combination is

  • Correct Answer
    45 picofarad
  • 18 picofarad
  • 12 picofarad
  • 5 picofarad
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Capacitors and inductors oppose an alternating current. This is known as

  • resistance
  • resonance
  • conductance
  • Correct Answer
    reactance
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The reactance of a capacitor increases as the

  • frequency increases
  • Correct Answer
    frequency decreases
  • applied voltage increases
  • applied voltage decreases

Correct answer: frequency decreases

The reactance of a capacitor is given by:

\[ X_C = \frac{1}{2\pi f C} \]

where:

  • \(X_C\) is capacitive reactance (ohms)
  • \(f\) is frequency (Hz)
  • \(C\) is capacitance (farads)

From the formula, reactance is inversely proportional to frequency. As the frequency decreases, the denominator becomes smaller, so the reactance becomes larger.

  • frequency increases causes reactance to decrease, not increase.
  • applied voltage increases does not change reactance, reactance depends only on frequency and capacitance.
  • applied voltage decreases also has no effect on reactance.

Therefore, the reactance of a capacitor increases as the frequency decreases.

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The reactance of an inductor increases as the

  • Correct Answer
    frequency increases
  • frequency decreases
  • applied voltage increases
  • applied voltage decreases
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Increasing the number of turns on an inductor will make its inductance

  • decrease
  • Correct Answer
    increase
  • remain unchanged
  • become resistive
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The unit of inductance is the

  • farad
  • Correct Answer
    henry
  • ohm
  • reactance
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Two 20 uH inductances are connected in series. The total inductance is

  • 10 uH
  • 20 uH
  • Correct Answer
    40 uH
  • 80 uH
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Two 20 uH inductances are connected in parallel. The total inductance is

  • Correct Answer
    10 uH
  • 20 uH
  • 40 uH
  • 80 uH
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A toroidal inductor is one in which the

  • Correct Answer
    windings are wound on a closed ring of magnetic material
  • windings are air-spaced
  • windings are wound on a ferrite rod
  • inductor is enclosed in a magnetic shield
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A transformer with 100 turns on the primary winding and 10 turns on the secondary winding is connected to 230 volt AC mains. The voltage across the secondary is

  • 10 volt
  • Correct Answer
    23 volt
  • 110 volt
  • 2300 volt
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An inductor and a capacitor are connected in series. At the resonant frequency the resulting impedance is

  • maximum
  • Correct Answer
    minimum
  • totally reactive
  • totally inductive

Correct answer: minimum

For a series LC circuit, the inductive reactance \(X_L\) and capacitive reactance \(X_C\) are:

\[ X_L = 2\pi f L \]

\[ X_C = \frac{1}{2\pi f C} \]

At the resonant frequency, these reactances are equal in magnitude and opposite in sign:

\[ X_L = X_C \]

They cancel each other, so the total reactive component becomes zero. The remaining impedance is only the small resistive losses in the circuit, making the total impedance minimum.

  • maximum applies to a parallel resonant circuit, not a series circuit.
  • totally reactive is incorrect because the reactive components cancel at resonance.
  • totally inductive is incorrect because there is no net reactance at resonance.

Therefore, at resonance a series LC circuit has minimum impedance.

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An inductor and a capacitor are connected in parallel. At the resonant frequency the resulting impedance is

  • Correct Answer
    maximum
  • minimum
  • totally reactive
  • totally inductive
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An inductor and a capacitor form a resonant circuit. The capacitor value is increased by four times. The resonant frequency will

  • increase by four times
  • double
  • Correct Answer
    decrease to half
  • decrease to one quarter
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An inductor and a capacitor form a resonant circuit. If the value of the inductor is decreased by a factor of four, the resonant frequency will

  • increase by a factor of four
  • Correct Answer
    increase by a factor of two
  • decrease by a factor of two
  • decrease by a factor of four

Correct answer: increase by a factor of two

The resonant frequency of an LC circuit is given by:

\[ f = \frac{1}{2\pi\sqrt{LC}} \]

If the inductance \(L\) is decreased by a factor of four, the new inductance is:

\[ L_{new} = \frac{L}{4} \]

Substituting into the formula:

\[ f_{new} = \frac{1}{2\pi\sqrt{(L/4)C}} \]

\[ f_{new} = \frac{1}{2\pi\left(\frac{1}{2}\sqrt{LC}\right)} = 2f \]

So reducing the inductance by four causes the square root term to halve, which makes the frequency double.

  • increase by a factor of four would require the inductance to decrease by a factor of sixteen.
  • decrease by a factor of two is the opposite of what happens.
  • decrease by a factor of four is also incorrect because reducing inductance raises frequency.

Therefore, decreasing the inductance by a factor of four causes the resonant frequency to increase by a factor of two.

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A "high Q" resonant circuit is one which

  • carries a high quiescent current
  • Correct Answer
    is highly selective
  • has a wide bandwidth
  • uses a high value inductance
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