Chapter 22: Generators
Discussion Questions:
13. Motors and generators are the same device, the distinction appearing in the operation rather than in the construction--mechanical power in and electrical power out and you have a generator; electrical power in and mechanical power out and you have a motor. It's reasonable, therefore, to ask, "Is a motor in some way a generator while it's turning?" Explain. If there is a generated back-current, how might it depend on the load? What do you think limits the speed of a free-turning motor? Describe the operation of the motor with a load attached. Why do many motors have open slots on their sides and little internal fan blades attached to their shafts? If you jam a motor--bind a drill or a blender so it's receiving current but not turning--it won't be long before you smell burning insulation. What's happening and why?
Problems:
32. We wish to design an ac generator using an armature on which is wound a flat tight rectangular coil (8.00 cm x 20.0 cm) of 150 turns. The geneerator output is to be sinusoidal with a maximum voltage of 20.0 V and a frequency of 50 Hz. How strong should the B-field be and at what angular speed should the coil turn?
54. A simple generator which consists of a flat rectangular coil (Figure 22.13) with a turning radius of 10 cm, is rotated about its central axis so that it moves perpendicularly across a 0.60-T magnetic field. The coil has an active length of 30 cm (that is, there is a total of 30 cm of wire cutting the field). What is the emf when the machine turns at 10 rev/s?
Chapter 23: AC and Electronics
Discussion Questions:
3. The transmission of electrical power via high-voltage dc is becoming increasingly attractive. List a few reasons why this might be the case.
4. Does the input to a transformer have to be AC in order for there to be an output?
Problems:
1. The maximum output voltage of an ac generator is +120 V at a time equal to 1/4 cycle. What will be the instantaneous terminal voltage : (a) 1/4 cycle later, and (b) 1/8 cycle later?
5. If the rms voltage read across a resistor by a voltmeter in a sinusoidal-ac circuit is 100 V, what is the maximum volage?
7. Determine the current drawn by a lit 100 W light bulb plugged into a 120-V wall outlet.
12. An 80 micro-F capacitor C is placed across the terminals of a 60-Hz sinusoidal-ac oscillator. What is the capacitive reactance of C?
16. Figure P16 depicts a simple circuit for studying the ac behavior of capacitors. What will the milliammeter read? If either capacitor is disconnected, what will happen to the current?
23. The circuit in Fig. P23 needs your help. Every time the switch is closed the fuse blows. What could be wrong? (Hint: One of the components is malfunctioning.)
26. A 1200-W hair dryer is plugged into a 120 V line. What's its resistance and how much current does it draw?
30. A sinusoidal oscillator with a terminal voltage of 125 V at a frequency of 55 Hz is put in seies with a capacitor of 100 micro-F. What effective current will be provided by the oscillator?