More About Alternating Current Practice Test

Review the following concepts if needed:

• Inductance Help
• Inductive Reactance Help
• Capacitance Help
• Capacitive Reactance Help
• RLC Impedance Help

More About Alternating Current Practice Test

A good score is eight correct. Answers are given at the end.

1. Three 300-pF capacitors are connected in series. What is the reactance of the combination?

(a) −100 ohms

(b) −300 ohms

(c) −900 ohms

(d) We need more information to calculate it.

2. Three 300-pF capacitors are connected in parallel. What is the capacitance of the combination?

(a) 100 pF

(b) 300 pF

(c) 900 pF

(d) We need more information to calculate it.

3. The complex-number impedance of a 47-ohm pure resistance is

(a) 47 + j 0.

(b) 0 − j 47.

(c) 0 + j 47.

(d) 47 + j 47.

4. A component is specified as having a complex-number impedance of −25 + j 30. From this you can reasonably conclude that

(a) there is a typographical error in the document.

(b) the reactance is capacitive.

(c) the impedance is a pure resistance.

(d) the device is operating with dc.

5. A solid material with a high dielectric constant between the plates of a capacitor can

(a) decrease the capacitance compared with an air dielectric.

(b) increase the capacitance compared with an air dielectric.

(c) increase the frequency.

(d) decrease the frequency.

6. If the inductance of a coil is doubled, then X _L at any particular frequency

(a) becomes twice as large.

(b) becomes four times as large.

(c) becomes half as large.

(d) becomes one-fourth as large.

7. When a dc voltage is applied to an inductor, the reactance, in theory, is

(a) negative infinity.

(b) positive infinity.

(c) zero.

(d) dependent on the voltage.

8. The complex-number impedance vectors for a pure resistance of 30 ohms and a pure capacitance of 100 μF:

(a) are the same length.

(b) are perpendicular to each other.

(c) point in opposite directions.

(d) are none of the above.

9. As the frequency of an ac signal through a 33-pF capacitor increases,

(a) the capacitor offers less and less opposition to the ac.

(b) the capacitor offers more and more opposition to the ac.

(c) the opposition to the ac does not change.

(d) the opposition to the ac might either increase or decrease depending on how fast the frequency changes.

10. A complex-number impedance of 500 + j 0 represents:

(a) a pure resistance.

(b) a pure inductive reactance.

(c) a pure capacitive reactance.

(d) a short circuit.

Answers

1. d

2. c

3. a

4. a

5. b

6. a

7. c

8. b

9. a

10. a