General exam: electrical principles
Reactance, impedance, resonance, decibels, transformers and series/parallel circuits. 40 questions, 10 at a time, starting with ones you haven't seen.
Rules to remember10 key electrical principles rulesShow the rules
- Reactance is the opposition that inductors and capacitors present to alternating current. (p. 48)
- An inductor opposes alternating current through inductive reactance, measured in ohms. (p. 48)
- Admittance is the reciprocal of impedance and is measured in siemens. (p. 49)
- Reactance is an opposition to current, so like resistance and impedance it is measured in ohms. (p. 49)
- Reactance is written with the letter X, with XL for inductive and XC for capacitive reactance. (p. 50)
- At resonance the inductive and capacitive reactances are equal and opposite, so they cancel each other out. (p. 50)
- An unmodulated carrier has constant amplitude, so its peak envelope power equals its average power and the ratio is 1.00. (p. 52)
- The changing magnetic field from the primary links the secondary winding; this mutual inductance induces the secondary voltage. (p. 52)
- Capacitances add when capacitors are connected in parallel, so adding a capacitor in parallel increases the total capacitance. (p. 55)
- Inductances add when inductors are connected in series, so adding an inductor in series increases the total inductance. (p. 55)
Taken from the explanations to checked practice questions; page numbers link to the official handbook.
Sample electrical principles questions
1.What is the peak-to-peak voltage of a sine wave with an RMS voltage of 120 volts?
- A84.8 volts
- B240.0 volts
- C169.7 volts
- D339.4 volts
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✓ D. 339.4 volts
Peak voltage is RMS × 1.414, so 120 V RMS is about 169.7 V peak, and peak-to-peak is twice that, about 339.4 volts.
2.How does the total current relate to the individual currents in a circuit of parallel resistors?
- AIt equals the average of the branch currents
- BIt decreases as more parallel branches are added to the circuit
- CIt equals the sum of the currents through each branch
- DIt is the sum of the reciprocal of each individual voltage drop
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✓ C. It equals the sum of the currents through each branch
In a parallel circuit each branch draws its own current from the same voltage, and the total current is the sum of all the branch currents.
3.Which of the following is opposition to the flow of alternating current in an inductor?
- AReactance
- BAdmittance
- CConductance
- DReluctance
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✓ A. Reactance
An inductor opposes alternating current through inductive reactance, measured in ohms. Conductance, reluctance and admittance are different quantities.
4.What is impedance?
- AThe product of current and reactance
- BThe product of current and voltage
- CThe ratio of voltage to current
- DThe ratio of current to voltage
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✓ C. The ratio of voltage to current
Impedance is the total opposition to AC, found as voltage divided by current (Z = E / I), and it combines resistance and reactance.