A&P General: Ohm's law, power and series-parallel circuits

Electricity is the biggest General subject in this site's weighting, and most of its questions are short calculations. The distractors are predictable: the answer you get by multiplying instead of dividing, or by adding parallel resistances as if they were in series. Learn the handful of formulas below and the sanity checks that go with them.

Questions and answers, explained

All 12 questions above, with the correct answer and why it is correct. Everything here is on a&p general: ohm's law, power and series-parallel circuits.

1. A 24-volt source is connected across a 6-ohm resistor. How much current flows?

   • A144 amperes
   • B4 amperesCorrect
   • C0.25 ampere
   • D18 amperes

   Why: Ohm's law: I = E / R = 24 V / 6 Ω = 4 A. Multiplying (144) or dividing the wrong way (0.25) are the two classic slips; write the formula before you plug in numbers.

   Reference AM.I.A.K7a · FAA-H-8083-30B, Ch. 12 (Ohm's Law)

2. A 28-volt DC landing light draws 5 amperes. What power does it consume?

   • A5.6 watts
   • B33 watts
   • C140 wattsCorrect
   • D700 watts

   Why: Watt's law: P = I × E = 5 A × 28 V = 140 W. Power in a DC circuit is simply current times voltage.

   Reference AM.I.A.K7c · FAA-H-8083-30B, Ch. 12 (Power)

3. Resistors of 10, 20, and 30 ohms are connected in series. What is the total resistance?

   • A60 ohmsCorrect
   • B5.45 ohms
   • C20 ohms
   • D6,000 ohms

   Why: In series, resistances simply add: 10 + 20 + 30 = 60 Ω. 5.45 Ω is what the same three resistors give in parallel.

   Reference AM.I.A.K11b · FAA-H-8083-30B, Ch. 12 (Series Circuits)

4. A 6-ohm and a 3-ohm resistor are connected in parallel. What is the total resistance?

   • A9 ohms
   • B4.5 ohms
   • C18 ohms
   • D2 ohmsCorrect

   Why: For two resistors in parallel, product over sum: (6 × 3) / (6 + 3) = 18 / 9 = 2 Ω. Total parallel resistance is always smaller than the smallest branch, which rules out 9 and 4.5 at a glance.

   Reference AM.I.A.K11c · FAA-H-8083-30B, Ch. 12 (Parallel Circuits)

5. Three 12-ohm resistors are connected in parallel. What is the total resistance?

   • A36 ohms
   • B4 ohmsCorrect
   • C12 ohms
   • D0.25 ohm

   Why: Equal resistors in parallel: divide one value by how many there are. 12 Ω / 3 = 4 Ω.

   Reference AM.I.A.K11d · FAA-H-8083-30B, Ch. 12 (Parallel Circuits)

6. Kirchhoff's current law states that at any junction in a circuit,

   • Athe voltage drops add up to the source voltage
   • Bthe current is the same in every branch
   • Ccurrent entering equals current leavingCorrect
   • Dresistance is inversely proportional to current

   Why: The current law is about junctions: the sum of currents flowing into a point equals the sum flowing out. Option A is Kirchhoff's voltage law, which applies around a closed loop.

   Reference AM.I.A.K7b · FAA-H-8083-30B, Ch. 12 (Kirchhoff's Laws)

7. A 28-volt series circuit has three loads. Two of them drop 10 volts and 12 volts. What is the drop across the third load?

   • A6 voltsCorrect
   • B22 volts
   • C28 volts
   • D9.3 volts

   Why: Kirchhoff's voltage law: the drops around a closed loop add up to the applied voltage. 28 − (10 + 12) = 6 V.

   Reference AM.I.A.K7b · FAA-H-8083-30B, Ch. 12 (Kirchhoff's Laws)

8. Lenz's law says that an induced current always flows in a direction that

   • Aadds to the change in flux that caused it
   • Bopposes the change in flux that caused itCorrect
   • Cfollows the conventional-flow direction
   • Dkeeps the circuit's resistance constant

   Why: Lenz's law: the magnetic field of an induced current opposes the change that produced it. It is why an inductor resists changes in current and why a generator gets harder to turn as load increases.

   Reference AM.I.A.K7e · FAA-H-8083-30B, Ch. 12 (Lenz's Law)

9. Using the right-hand motor rule, with the index finger pointing along the magnetic field and the second finger along the current, the thumb shows

   • Athe direction the conductor movesCorrect
   • Bthe polarity of the field coils
   • Cthe direction of the induced voltage
   • Dthe strength of the magnetic field

   Why: The FAA handbook gives the right-hand motor rule exactly this way: index finger along the field, second finger along the current, and the thumb shows which way the current-carrying wire is pushed. That push is what turns a motor's armature.

   Reference AM.I.A.K7f · FAA-H-8083-30B, Ch. 12 (DC Motors, Figure 12-294)

10. Capacitors of 4, 6, and 10 microfarads are connected in parallel. What is the total capacitance?

    • A1.94 microfarads
    • B6.67 microfarads
    • C20 microfaradsCorrect
    • D240 microfarads

    Why: Capacitors behave opposite to resistors. In parallel their capacitances add: 4 + 6 + 10 = 20 μF. In series you would use the reciprocal formula, which gives 1.94 μF.

    Reference AM.I.A.K3 · FAA-H-8083-30B, Ch. 12 (Capacitors in Parallel)

11. When capacitors are connected in series, the total capacitance is

    • Athe sum of the individual values
    • Bequal to the largest capacitor
    • Cthe average of the individual values
    • Dless than the smallest capacitorCorrect

    Why: Series capacitors combine like parallel resistors: 1/CT = 1/C1 + 1/C2 + …, so the total is always smaller than the smallest one. Connecting them in series effectively increases the plate spacing.

    Reference AM.I.A.K3 · FAA-H-8083-30B, Ch. 12 (Capacitors in Series)

12. Why will a transformer not work when steady DC is applied to its primary?

    • ADC voltage is too low to cross the core
    • BThe core saturates and reverses polarity
    • CDC flows only on the secondary side
    • DInduction needs a changing magnetic fieldCorrect

    Why: A transformer works by mutual induction, and voltage is induced only while the magnetic flux is changing (Faraday's law). Steady DC produces a constant field, so nothing is induced in the secondary after the instant of switching.

    Reference AM.I.A.K7d · FAA-H-8083-30B, Ch. 12 (Transformers; Faraday's Law)

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