Second-year review quiz

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15 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.

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1. A 120 V (RMS) supply has a peak voltage of about…

   A 85 V B 120 V C 170 V D 240 V
   Answer and why

   C. Peak = RMS × √2 = 120 × 1.414 ≈ 170 V. AC fundamentals.

2. How long does one cycle last at 60 Hz?

   A 60 ms B 16.7 ms C 1.67 ms D 6 ms
   Answer and why

   B. Period = 1 ÷ f = 1 ÷ 60 = 0.0167 s = 16.7 ms.

3. An inductor opposes…

   A any voltage B DC current once it is steady C nothing on AC D changes in the current through it
   Answer and why

   D. A coil’s field pushes back against any change in its current. Once DC is steady there is no change, so only the wire’s resistance is left. Inductance.

4. A 10 kΩ resistor in series with a 100 µF capacitor has a time constant of…

   A 0.1 s B 1 s C 10 s D 100 s
   Answer and why

   B. τ = R × C = 10,000 × 0.0001 = 1 s. After about five time constants (5 s) the capacitor is practically fully charged. Capacitance.

5. What is the inductive reactance of a 0.1 H coil at 60 Hz?

   A 6 Ω B 26,5 Ω C 37,7 Ω D 377 Ω
   Answer and why

   C. XL = 2πfL = 2 × 3.1416 × 60 × 0.1 = 37.7 Ω. Reactance and impedance.

6. A circuit has 30 Ω of resistance and 40 Ω of net reactance in series. Its impedance is…

   A 10 Ω B 70 Ω C 1 200 Ω D 50 Ω
   Answer and why

   D. Resistance and reactance add at right angles: Z = √(30² + 40²) = √2,500 = 50 Ω.

7. A load draws 5,000 VA at a power factor of 0.8. How many watts?

   A 4 000 W B 3 000 W C 6 250 W D 5 000 W
   Answer and why

   A. P = S × PF = 5,000 × 0.8 = 4,000 W. The reactive power is √(5,000² − 4,000²) = 3,000 VAR. Power factor.

8. A 3 kVA single-phase transformer is 480 V to 120 V. Its full-load secondary current is…

   A 25 A B 6,25 A C 4 A D 360 A
   Answer and why

   A. I = VA ÷ V = 3,000 ÷ 120 = 25 A. The primary carries 3,000 ÷ 480 = 6.25 A. Single-phase transformers.

9. In a wye system with 120 V from each line to neutral, the line-to-line voltage is about…

   A 120 V B 208 V C 240 V D 360 V
   Answer and why

   B. In wye, V line = √3 × V phase = 1.732 × 120 ≈ 208 V. Intro to three-phase.

10. Synchronous speed of a 2-pole motor on 60 Hz?

    A 1 200 rpm B 1 800 rpm C 3 600 rpm D 7 200 rpm
    Answer and why

    C. Ns = 120 × f ÷ P = 120 × 60 ÷ 2 = 3,600 rpm. Magnetism and motors.

11. Controlling one light from four places takes…

    A four 3-way switches B one 3-way and three 4-ways C four single-pole switches D two 3-ways and two 4-ways
    Answer and why

    D. Always two 3-ways at the ends; every place in between adds one 4-way. Lighting circuits and controls.

12. On a 120/240 V service, 18 A of 120 V load is on L1 and 10 A is on L2. The neutral carries about…

    A 28 A B 18 A C 8 A D 0 A
    Answer and why

    C. The two sides are opposite, so the neutral carries the difference: 18 − 10 = 8 A. Services and panels.

13. Which is a raceway rather than a cable?

    A NMD90 / NM-B B AC90 / MC C TECK90 D EMT
    Answer and why

    D. EMT is tubing you install empty and pull conductors into. The others arrive with the conductors inside. Cables and raceways.

14. Practice figures: 15 A, 80 ft one way, 2.5 Ω per 1,000 ft, single-phase. The voltage drop is…

    A 3 V B 6 V C 1,5 V D 12 V
    Answer and why

    B. VD = 2 × I × R × L = 2 × 15 × 0.0025 × 80 = 6 V. Voltage drop.

15. With power on, a closed switch in a series circuit shows the full supply voltage across it. That switch is…

    A open inside B working normally C shorted D overloaded
    Answer and why

    A. A good closed contact shows nearly 0 V. The full supply voltage appears across the open. A troubleshooting method.