Page 382 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 29] ALTERNATING-CURRENT CIRCUITS 367

SOLVED PROBLEM 29.24
A loudspeaker whose voice coil has an impedance of 6 is to be used with an amplifier whose load
impedance is 7350 . Find the ratio of turns in a transformer that will match these impedances.
Let us use the subscripts 1 for the amplifier and 2 for the voice coil. From the formula for maximum power
transfer

7350 √
N 1 Z 1
= = = 1225 = 35
N 2 Z 2 6
Thus the ratio of turns is 35:1, and the transformer winding connected to the amplifier should have 35 times as many
turns as the winding connected to the voice coil.

Multiple-Choice Questions

29.1. The current in an ac circuit
(a) leads the voltage (c) is in phase with the voltage
(b) lags behind the voltage (d) is any of the above, depending on the circuit

29.2. The current and voltage cannot be exactly in phase in a circuit that contains
(a) only resistance (c) inductance and capacitance
(b) only inductance (d) resistance, inductance, and capacitance
29.3. A capacitor, an inductor, and a resistor are connected in series. If the capacitance is increased, the impedance of the
circuit
(a) decreases (c) decreases or increases
(b) increases (d) decreases, increases, or stays the same

29.4. In a certain circuit X C = X L . The power factor of the circuit
(a) is 0 (c) is 1
(b) is 0.5 (d) depends on the resistance in the circuit

29.5. A coil of negligible resistance is connected to a 60-V, 400-Hz ac source. If the current in the coil is 1.2 A, the
inductance of the coil is
(a) 8 µH (c) 20 mH
(b) 14 mH (d) 50 H

29.6. The current that flows when a 5.0-µF capacitor is connected to a 120-V, 60-Hz ac source is
(a) 0.16 A (c) 4.4 A
(b) 0.23 A (d) 64 kA

29.7. When a capacitor, an inductor, and a resistor are connected in series to an ac source, the effective voltages across
the circuit components are V C = 12.0V, V L = 8.0V,and V R = 6.0 V. The effective voltage of the source is
(a) 4.5 V (c) 21 V
(b) 7.2 V (d) 26 V

29.8. The phase angle in the circuit of Question 29.7 is
(a) −24 ◦ (c) +24 ◦
(b) −34 ◦ (d) +34 ◦