Questions for Review
1. In an insulator, all the outer shell atomic electrons are utilized
in bonding the atoms together,while in a conductor the atoms bond while leaving
some electrons in the "conduction band," from whence they are "free" to
move in response to an applied voltage.
2. Voltage is defined as "energy per charge," while power is a measure of how much energy is delivered per unit time. When a 9-Volt battery is connected to a circuit, for instance, it supplies nine Joules of energy per coulomb of charge that passes through it. Power is measured in Watts, and is the product of voltage times current. If that same nine Volt battery drives a current of 100 milliAmperes, or 0.1 Amperes, it delivers a power of 0.1 Amp x 9 Volt = 0.9 Watts.
3. Silicon is the basis for semiconductors and it has 4 valence electrons that contribute to bonding atoms together into solid state silicon. When "doped" with atoms that have 3 valence electrons, such as gallium, the electron deficiency is equivalent to an abundance of positively charge "holes" that can carry current through the semiconductor, creating a "p-type" (p for positive) semiconductor. Similarly, when doped with atoms that have 5 valence electrons, such as arsenic, there would be an abundance of electrons that wouldn't participate in bonding and would be free to carry a current, creating an n-type (n for negative) semiconductor.
4. Sound waves correspond to pressure fluctuations that alternate above and below atmospheric pressure. AC voltage is a useful way to represent and process acoustic information because it corresponds to a voltage that alternates between positive and negative, with respect to some reference voltage.
5. The resistance of a metal wire depends upon three factors: resistivity, length and cross sectional area. Resistivity is an intrinsic property, depending upon the atomic structure of the metal and the extent to which electrons are scattered by the lattice of bonded atoms as they respond to an applied voltage across the wire. Resistance is directly proportional to resistivity. The other two factors are geometric and can be understood by analogy to the flow of water in a pipe. Suppose a stream is funneled into a pipe. The longer the pipe, the more the stream flow would be inhibited. The larger the opening of the pipe, the less the stream flow would be inhibited. Similarly, the longer a piece of wire, the greater its resistance to current flow, and the larger its cross section, the less its resistance.
6. In selecting the length and thickness of wires for connecting a speaker to an amplifier, one would want to ensure that the total resistance of the wires was significantly less than the output resistance of the amplifier and the speaker resistance. If the speakers were quite far from the amplifier, one would need to choose a relatively thick wire to achieve this condition.
7. As described above, voltage is a measure of the energy imparted per unit charge, in Joues per Coulomb, as, for example, charges flow in a circuit. Current is a measure of the rate at which charges flow in a circuit and is measured in Coulombs per second.
8. In a plastic coated wire the current is carried by the copper inside the plastic because copper is a conductor that has conduction electrons that can freely move under the influence of an applied voltage. The plastic sheath serves as an insulator to keep current in the copper wire from going where it is not desired, for example, if the wire were to touch a conducting pipe in the room.
9. An electrostatic speaker consists of a pair of large, planar electrodes that have a thin sheet of conducting mylar in between. A large bias voltage is applied between the two conducting plates and the thin sheet to generate an electric field. When the AC audio signal is placed across the two plates, fluctuations in the amount of charge on the plates cause disruptions in the electric field between them that then exert forces on the thin sheet. Thus the sheet vibrates in response to the applied AC voltage, and given it slenderness and large area it is able to move a considerable amount of air to generate sound waves. Holes drilled in the conducting plates allow them to store charge while transmitting the sound waves to the room. This type of speaker distorts less than a cone speaker because a small amplitude motion corresponds to loud sound generation, given the large area of the sheet. It is also efficient because the diaphragm (sheet) is lightweight and the large area effectively couples sound waves from the diaphragm to the room.
10. A cross-over network sends different bands of frequencies in an audio signal to different speakers to improve the efficiency of sound generation. Low frequencies are sent to a large speaker called a woofer, middle frequencies are sent to a medium-sized speaker called a midrange, and high frequencies are sent to a small speaker called a tweeter. A simple way to build a cross-over is as follows: an inductor is placed in series with the woofer, a capacitor in series with the tweeter, and a series combination of an inductor and capacitor in series with a midrange. The inductor has low impedance at low frequencies and transmits them to the woofer, while the capacitor has low impedance at high frequencies and transmits these to the tweeter. The midrange combination of inductor and capacitor transmits above a certain "cut off" frequency and below a higher frequency "cut off." The frequencies at which these various elements transmit or block AC current depends upon the values of the capacitance, inductance and the resistances in the circuit.
Exercises
1. B. 1/5 times as large, or 5 times smaller.
2. D. 16 volts.
3. B. resistance decreases when resistors are in parallel.
4. D. be turned off.
5. A. 24 Ohms.
8. B. current flows through a wire when there is a voltage between its
two ends.
9. A. power is proportional to voltage squared, so the power
quadruples.
10. B. 2.5 Amperes, since Power = I2*R.
11. B. 2 Ohms.
12. A. 26 Watts (assuming all 52 Watts is delivered to the
speakers)
13. C. the current through each speaker is the same, but the 16-ohm
speaker receives double the power that the 8-ohm speaker receives.
14. B. 0.72 watt
15. A. 8/3 ohms
16. E. there should be three groups in series, with each having three
speaker in parallel. Each parallel combination has an equivalent
resistance of 8/3 ohms, as in #15 above, and the sum of three such
combinations has a resistance of 8/3 + 8/3 + 8/3 = 8 ohms.
17. A. impedance of a capacitor is large when the current frequency is
low.
18. A. alternating current
19. A. thick wires offer less resistance to the current.
20. E. all of them
21. A. low frequency currents will reach the speaker without being
reduced in amplitude
22. D. 3/4 times larger
23. A. 72 watts, 0.9 watts
24. B. thick wires offer less resistance to the current
25. C. 500 Hz AC current
26. C. a neutral atom has the same number of electrons and protons
27. A. like charges repel, unlike charges attract
28. D. bass and treble frequencies
29. B. current through the capacitor increases as the frequency is
increased
30. C. the force between charged objects becomes weaker as they are
moved further apart