CHAPTER 17: Alternating-Current Power and Resonance

A transmission line operates at its best efficiency when

(a) the load impedance constitutes a pure resistance equal to the characteristic impedance of

the line.

(b) the load impedance constitutes a pure inductive reactance equal to the characteristic

impedance of the line.

(c) the load impedance constitutes a pure capacitive reactance equal to the characteristic

impedance of the line.

(d) the absolute-value impedance of the load equals the characteristic impedance of the line.

A

The ninth harmonic of 900 kHz is

(a) 100 kHz.

(b) 300 kHz.

(c) 1.20 MHz.

(d) 8.10 MHz.

D

A pure resistance dissipates or radiates

(a) complex power.

(b) imaginary power.

(c) true power.

(d) apparent power.

C

Suppose that we want to build a ó-wave dipole antenna designed to have a fundamental

resonant frequency of 14.3 MHz. How long should we make the antenna, as measured from end

to end in meters?

(a) 32.7 m

(b) 10.0 m

(c) 16.4 m

(d) We need more information to answer this question.

B

When a transmission line exhibits standing waves, we find a voltage maximum

(a) wherever we find a current maximum.

(b) wherever we find a current minimum.

(c) at the transmitter end of the line.

(d) at the load end of the line.

B

Standing waves on a transmission line (as compared with a line operating without any

impedance mismatch) increase the loss in the wire conductors at the

(a) current maxima.

(b) voltage maxima.

(c) current minima and voltage maxima.

(d) current minima and voltage minima.

A

When we take the cosine of the phase angle in an AC circuit or system that contains both

resistance and reactance, we get the

(a) true power.

(b) imaginary power.

(c) apparent power.

(d) power factor.

D

Which of the following parameters is an example of true power in an AC circuit or system?

(a) The AC that appears between the plates of a capacitor

(b) The AC that passes through a wire inductor

(c) The AC that dissipates as heat in a transmission line

(d) The AC that travels along a transmission line

C

Suppose that the apparent power in a circuit equals 40 W and the true power equals 30 W.

What's the power factor?

(a) 60%

(b) 75%

(c) 80%

(d) We need more information to calculate it.

B

Suppose that the true power in a circuit equals 40 W and the imaginary power equals 30 W.

What's the power factor?

(a) 60%

(b) 75%

(c) 80%

(d) We need more information to calculate it.

C

Consider a series circuit with a resistance of 24 Ω and an inductive reactance of 10 Ω.

What's the power factor?

(a) 42%

(b) 58%

(c) 92%

(d) 18%

C

Imagine that you encounter a series circuit with a resistance of 24 Ω and a capacitive

reactance of -10 Ω. What's the power factor?

(a) 42%

(b) 58%

(c) 92%

(d) 18%

C

Suppose that a circuit has 24 Ω of resistance and 10 Ω of inductive reactance in series.

A meter shows 100 W, representing the VA power. What's the true power?

(a) 18 W

(b) 34 W

(c) 85 W

(d) 92 W

D

Suppose that the true power equals 100 W in a circuit that consists of a resistance of

60.0 Ω in series with an inductive reactance of 80.0 Ω. What's the VA power?

(a) 167 W

(b) 129 W

(c) 60.0 W

(d) 36.0 W

A

Suppose that the true power equals 100 W in a circuit that consists of a resistance of

80.0 Ω in series with an inductive reactance of 60.0 Ω. ( The resistance and reactance

numbers here are transposed from the values in Question 14.) What's the VA power?

(a) 64.0 W

(b) 80.0 W

(c) 125 W

(d) 156 W

C

Suppose that we connect a coil and capacitor in series. The inductance is 36 μH and the

capacitance is 0.0010 μF. What's the resonant frequency?

(a) 36 kHz

(b) 0.84 MHz

(c) 2.4 MHz

(d) 6.0 MHz

B

What will happen to the resonant frequency of the circuit described in Question 16 if we

connect a 100 Ω resistor in series with the existing coil and capacitor?

(a) It will increase.

(b) It will stay the same.

(c) It will decrease.

(d) We need more information to answer this question.

B

Suppose that we connect a coil and capacitor in parallel, with L = 75 μH and C = 150 pF.

What's fo ?

(a) 1.5 MHz

(b) 2.2 MHz

(c) 880 kHz

(d) 440 kHz

A

What will happen to the resonant frequency of the circuit described in Question 18 if we

connect a 22-pF capacitor in parallel with the existing coil and capacitor?

(a) It will increase.

(b) It will stay the same.

(c) It will decrease.

(d) We need more information to answer this question.

C

We want to cut a .-wave section of transmission line for use at 18.1 MHz. The line has a

velocity factor of 0.667. How long should we make the section?

(a) 9.05 m

(b) 3.62 m

(c) 3.00 m

(d) 2.76 m

D