4. An overheated resistor is usually a symptom of a problem rather than its cause.

Transcription

1 TRUE/FALSE 1. Voltage can exist only where there is a current path. Page: 1 2. An open circuit condition is one where R =. 3. One ampere equals 1 joule per second. 4. An overheated resistor is usually a symptom of a problem rather than its cause. 5. One horsepower is equal to 746 kw. FILL-IN-THE-BLANK 6. In a resistive circuit, current is proportional to applied voltage and proportional to resistance. 7. The symbol for source voltage is, and the symbol for load voltage is. 8. When determining the polarity of voltage drop across a resistor, draw an arrow indicating the direction of current through the resistor and place the plus sign at the (head, tail) of the current arrow. 9. Overrating the power of a transistor by or more times provides a safety margin against overheating. MULTIPLE CHOICE 10. A circuit has a fixed voltage E applied to it. If you measure I = 4 A at first, then reduce the value of R by one-half, what will be the value of I? a. 1 A b. 2 A c. 8 A d. 16 A 11. If a circuit has a fixed applied voltage, what happens to its current level if the resistance is multiplied by ten? a. The current will rise to ten times its original value. b. The current will drop 10% of its original value. c. The current level is not changed. d. The current will drop to 90% of its original value. 12. Which one of the following equations for power is incorrect? a. P = IV b. P = IE c. P = IR d. P = W/t

2 Page: The direction of energy transfer is always a source and a resistor. a. from, from b. from, into c. into, from d. into, into 14. To convert from units of watts to horsepower,. a. multiply watts by 746 b. divide watts by 746 c. multiply kilowatts by 746 d. multiply kilowatt-hours by 746 SHORT ANSWER 15. Given the following values for I and R, use Ohm's law to determine V: (a) I = 2 A, R = 100 Ω, (b) I = 20 ma, R = 470 Ω, (c) I = 0.2 A, R = 4.7 kω, (d) I = 0.22 ma, R = 470 kω, (e) I = 150 µa, R = 2.2 MΩ. 16. Given the following values for V and R, use Ohm's law to determine I: (a) V = 24 V, R = 100 Ω, (b) V = 400 mv, R = 470 Ω, (c) V = 200 V, R = 4.7 kω, (d) V = 1.2 kv, R = 470 kω, (e) V = 800 µv, R = 15 Ω. 17. Given the following values for V and I, use Ohm's law to determine R: (a) V = 24 V, I = 10 A, (b) V = 400 mv, I = 470 µa, (c) V = 200 V, I = 4.7 ma, (d) V = 1.2 kv, I = 200 ma, (e) V = 800 µv, I = 15 µa. 18. A 20-Ω resistor is connected to a 10-V battery. What is the current? 19. A 470-Ω resistor is connected to a 12-V source. What is the current? 20. Use Ohm's law to determine the amount of resistance in a circuit when a 1.5-V battery causes 10 mω to flow. 21. An electrical test shows that 250 mω of current is flowing through a resistive circuit when 12.5 V is applied. What is the resistance of the circuit? 22. Use Ohm's Law to determine the amount of voltage across a 220-Ω resistor when 100 mω is flowing through it. 23. A resistor with color code yellow, violet, yellow is connected to a 100-V source. How much current is flowing through the resistor? 24. A resistor with color code brown, black, red has 100 mω flowing through it. How much voltage is being applied? 25. What is the wattage rating of a device that is doing 1 kj of work per second?

3 Page: How much energy is used by burning a 100-W light bulb for 24 hours? 27. What is the wattage rating of a load that draws 1.5 Ω of current from a 120-V source? 28. The instruments at a radio transmitter show the output is delivering 6 Ω at 1200 V. What is the output power level? 29. An office appliance is rated at 1200 W at 220 V. What is the current demand for this appliance? 30. A home appliance is rated at 1200 W at 120 V. Determine (a) the resistance of the appliance, and (b) the current demand of the appliance. 31. What is the highest wattage rating for an appliance that can be operated from a 120-V source that is fused at 15 Ω? 32. An electrical load has a resistance of 10 Ω. Determine the amount of voltage and current required for operating this load at 800 W. 33. When 12 V is initially applied to the resistive filament of an incandescent lamp, the resistance is 1.2 Ω. Five seconds later, the increase in operating temperature takes the resistance to 16 Ω. What is the current demand and power dissipation (a) the instant power is applied to the lamp, and (b) when the operating temperature has stabilized? 34. Indicate the largest amount of voltage that can be applied to the following 1/2-W resistors without causing heat damage: (a) 47 Ω, (b) 270 Ω, (c) 2.2 kω, (d) 470 kω, (e) 1.1 MΩ. 35. Indicate the largest amount of current that can flow through the following 1/2-W resistors without causing heat damage: (a) 47 Ω, (b) 270 Ω, (c) 2.2 kω, (d) 470 kω, (e) 1.1 MΩ. 36. A semiconductor cooling device can get rid of heat at the rate of 180 J/hr. What is the corresponding wattage rating? 37. What is the required wattage rating of a small swimming pool heater that must be capable of delivering 600 kj of energy in 30 minutes? How much will it cost to run this heater for 12 hours if the utility company is charging 10 cents per kwh? 38. Compare the cost of operating (a) a 60-W table lamp for 12 hours each day and (b) a 10-W night light for 24 hours each day. The cost of electricity in both instances is $0.08 per kwh.

4 Page: What is the efficiency of an electrical device that has input power of 120 W and output power of 100 W? 40. What is the efficiency of a motor where the input is 100 W and the output is 0.1 hp? 41. Determine the power output of a device, in watts and hp, where η = 86% and P in = 600 W. 42. What is the required efficiency of a circuit that has input given by 1.2 A at 120 V, and an output given as 12.6 V at 10.5 A? 43. The output of a motor-gearbox system is 8.5 hp. If the gearbox is 90% efficient and the motor is 80% efficient, what is the input power rating of the motor in watts? The current rating at 220 V? 44. List the three terms in Ohm's law, indicating the unit of measure, the abbreviation for the unit of measure, and the symbol for each term. 45. What percentage of power is lost in a device that requires a 140-W input for a 120-W output? 46. Given the power equation P = W/t, list the name, SI unit of measure, and abbreviation for the unit of measure for each of the three terms. 47. Describe the meaning of the following equation and show how it is derived. P Losses = P out 1 η 1

5 Answer Key Page: 1 1. False 2. True 3. False 4. True 5. False 6. directly, inversely [in the order shown] 7. E, V [in the order shown] 8. tail 9. two 10. c 11. b 12. c 13. b 14. b 15. (a) V = 200 V, (b) V = 9.4 V, (c) V = 940 V, (d) V = 103 V, (e) V = 330 V 16. (a) I = 240 ma, (b) I = 851 µa, (c) I = 42.6 ma, (d) I = 2.55 ma, (e) I = 53.3 µa 17. (a) R = 2.4 Ω, (b) R = 851 Ω, (c) R = 42.6 kω, (d) R = 6 kω, (e) R = 53.3 Ω Ω Ω or 25.5 mω Ω Ω V µω V kw MJ W kw Ω 30. (a) R = 12 Ω, (b) I = 10 Ω W 32. V = 89.4 V, I = 8.94 Ω 33. (a) I = 10 Ω, P = 120 W, (b) I = 0.75 Ω, P = 9 W 34. (a) V = 4.85 V, (b) V = 11.6 V, (c) V = 33.2 V, (d) V = 485 V, (e) V = 742 V 35. (a) I = 103 mω, (b) I = 43 mω, (c) I = 15 mω, (d) I = 1.03 mω, (e) I = 674 µω W kw, $ (a) 5.76 cents per day, (b) 1.92 cents per day 39. η = 83.3% 40. η = 74.6% 41. P out = 698 W = hp

6 42. η = 91.9% 43. P in = 8.81 kw, I = 40 A Answer Key Page: Current, amps, Ω, I Voltage, volts, V, E Resistance, ohms, Ω, R % 46. P = power, watts, W W = work, joules, J t = time, seconds, s 47. The meaning of the equation is that you can determine the power losses of a device, given the input power and efficiency. The equation is determined: (1) P out + P losses = P in, and η = P out /P in (2) P out + P losses = P out /η (3) P losses = (P out /η) P out = P out [(1/ η) 1]