Electricity Exercises

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Exercises I. Essay Questions 1. What is Ohm’s Law? How does power transmitted through a circuit change with current? With voltage across the battery? 2. What is the difference between a series and a parallel circuit? What are the advantages of one over the other? Which type of wiring is more convenient in a house? 3. Describe the principal of operation of an electric motor and an electric generator. When do you use an AC or a DC motor. A synchronous and an asynchronous generator? 4. Define installed, peak, baseload, and reserve capacities. What kind of power plant is suitable to meet the needs for each of these? 5. Identify and describe the major components of a coal-burning electric power plant. How does it differ from a nuclear or a geothermal plant? 6. Describe the advantages and disadvantages of generating electricity using renewable sources of energy such as wind and solar. 7. What causes an electric shock? What are the basic safety precautions for avoiding electric shocks? II. Multiple Choice Questions: 1. Which of the following statements is not correct? a. Atoms are made of negatively charged electrons and positively charged nuclei. b. In neutral atoms, there are as many protons as there are electrons. c. The total number of charges in the universe is constant. d. Charge and mass are two properties of matter. e. Unlike charges which are always attractive, two masses separated at a distance can attract or repel each other. 2. A neutral atom with 10 protons must have a. 10 neutrons b. 10 electrons c. 10 neutrons and 10 electrons d. 10 neutrons and any number of electrons e. 5 neutrons and 5 electrons 3. A live wire (a wire in which current flows) is placed directly over a magnetic compass. As a result, the needle of the compass will a. Not move b. Point in the direction perpendicular to the wire c. Point in the direction parallel to the wire d. Fluctuate about an axis perpendicular to the wire e. Fluctuate about an axis parallel to the wire 4. According to Ohm’s Law, the current in a circuit is a. Independent of the voltage b. Linearly proportional to the voltage c. Proportional to the square of the voltage d. Inversely proportional to the voltage e. Inversely proportional to the square of the voltage 5. According to Ohm’s Law, the current in a circuit is a. Independent of the resistance b. Linearly proportional to the resistance c. Proportional to the square of the resistance d. Inversely proportional to the resistance e. Inversely proportional to the square of the resistance 6. Two light bulbs, a 100-W and a 200-W, are connected in parallel to the two terminals of a battery. a. Both light bulbs receive the same amount of current. b. The 100-W light bulb receives more current. c. The 200-W light bulb receives more current. d. The entire current passes through the 100-W bulb. e. The entire current passes through the 200-W bulb. 7. Two light bulbs, a 100-W and a 200-W, are connected in series to the two ends of a battery. a. Both light bulbs receive the same amount of current. 328 b. The 100-W light bulb receives twice the current. c. The 200-W light bulb receives twice the current. d. The 100-W light bulb receives four times the current. e. The 200-W light bulb receives four times the current. 8. Consider two lamps that are identical but for one exception; one has a thicker filament than the other. Which has the higher resistance? a. Thicker filament b. Thinner filament c. Both about the same d. It depends on the current e. It depends on the voltage across them 9. How many 100-W light bulbs can be simultaneously turned on in a household before the circuit breaker fuse burns out? The maximum current through the circuit breaker is 15 amperes and the supply line is at 120 volts. a. 1 b. 5 c. 6 d. 8 e. 18 10. Ten identical bulbs are placed in a series circuit. One light burns out. What will happen? a. The rest of the bulbs will be turned off. b. The rest of the bulbs will burn out. c. The rest of the bulbs get dimmer. d. The current in the circuit reduces by 10 percent. e. The rest of the bulbs keep glowing as if nothing has happened. 11. The kilowatt is a unit of a. Energy b. Power c. Voltage d. Torque e. Force 12. The kilowatt-hour is a unit of a. Energy b. Power c. Either power or energy d. Torque e. Electric potential 13. Which of the following represent(s) energy a. Joule b. BTU c. kWh d. eV e. All of the above 14. Which of the following represent(s) power a. Joule/min b. Barrels of oil per day c. Kilowatt d. Horsepower e. All of the above 15. What is the unit used by utility companies when they sell the electricity used in our homes? a. Kilowatts b. BTU c. Volts d. Kilowatt-hours e. Therms 16. Electrical energy can be produced by a. Mechanical energy b. Chemical energy c. Radiant energy d. Magnetic energy e. All of the above 17. Electrical power can be calculated by multiplying a. Current by voltage b. Current by resistance c. Current by resistance-squared d. Resistance by voltage e. Resistance by voltage-squared 18. To maximize power dissipation through a given resistance heater, we must a. Increase current b. Increase resistance c. Decrease current 329 Chapter 13 - Electricity d. Decrease resistance e. Increase both current and resistance 19. The relationship between voltage and current in an electric line is like a. Flow rate to volume of the fluid in a hydraulic line b. Pressure to flow rate in a hydraulic line c. Pressure to volume of the fluid in a hydraulic line d. Flow rate to pipe diameter in a hydraulic line e. Flow rate to pipe length in a hydraulic line 20. The relationship between electrical resistance and flow resistance is like a. Wire length to pipe length b. Wire diameter to pipe diameter c. Wire material to pipe material d. Current in the wire to flow rate in the pipe e. Energy carried by electricity in a wire to energy carried by water in a pipe 21. Most electrical power transmission is in the form of an alternating current because a. Direct currents cannot be transmitted in electrical wires b. It is cheaper to produce c. It is safer to use d. It can be transmitted with less transmission loss e. It can travel faster 22. In the United States, most electricity is generated by a. Burning fossil fuels b. Nuclear power c. Solar energy d. Hydroelectric e. Wind and biomass 23. The failure of California to deregulate energy was mainly due to a. Insufficient refineries and generating stations b. Rapid increase in number of trucks and sport utility vehicles c. Out-dated power plants d. Malicious conspiracy by energy suppliers, politicians, and operators e. All of the above 24. The agency responsible for regulating interstate electric commerce is a. Federal Energy Regulatory Commission b. Power Exchange c. Department of Energy d. Department of Commerce e. Independent System Operators 25. The baseload of daily energy consumption represents a. The fraction used in early morning b. Average energy consumption during the day c. The load supplied by nuclear or fossil fuel d. The load required to provide minimal needs of consumers e. None of the above 26. Baseload power plants are used a. Primarily during the nighttime b. Primarily during the daytime c. Day and night d. During peak times e. During emergencies 27. Gas turbines are best suited for power generation a. At all times b. Only during daytimes c. Only during nighttimes d. During peak hours and emergencies e. For baseload power production 28. In the US, when is the peak electricity demand in summer? a. 6:00 am - 9:00 am b. 9:00 am - noon c. Noon - 3:00 pm d. 3:00 pm – 6:00 pm e. 6:00 pm – 9:00 pm 29. The severity of an electric shock depends on a. The health and physical condition of the victim b. The amount of current and length of time it passes through the body 330 c. The path the current takes through the body d. The type of electricity e. All of the above 30. If you think of a battery as a pump charging electric current, then you can think of the battery voltage as a. The flow rate of fluid b. The flow velocity c. The pressure d. The water e. None of the above 31. Consider two lights, A and B, each screwed into 120-volt sockets. The two bulbs are identical except that the filament in B is thicker than the filament in A. Which statement is true? a. Both A and B have the same brightness. b. A will be brighter because it has the highest resistance. c. A will be brighter because it has the least resistance. d. B will be brighter because it has the highest resistance. e. B will be brighter because it has the least resistance. 32. The amount of current that passes through the body as a result of an electrical shock depends on a. The voltage in the line b. The resistance of the body and clothing c. The source of electricity d. The contact area and contact pressure with the live wire e. All of the above 33. Ultracapacitors are best suited when we need a. A lot of power over a very short time b. A lot of power over a long time c. A lot of energy over a very short time d. A lot of energy over a long time e. A lot of energy and power over a short time 34. A magnet that is moved inside a coil of wires produces a. A magnetic field b. Electricity c. Gravity d. A spark e. Nothing 35. How much of the energy of coal burned in a power plant is discarded as heat? a. An insignificant amount b. 10-20 percent c. 20-40 percent d. 40-60 percent e. More than 60 percent II. True or False? 1. Lightning is the result of static electricity. 2. Even currents as low as 50-100 mA can cause death, especially if the path of current is through the heart. 3. An alternating current results from the back and forth movement of electrons in a wire. 4. Power companies usually charge electricity in kilowatt-hours. 5. For the most economical way of transmitting power, it is best to use extremely high voltages. 6. Base-load energy is cheapest to produce; peak-load electricity is the most expensive. 7. Fuel cells are efficient devices for the direct conversion of solar energy to electricity. 8. Photovoltaic and thermophotovoltaics are two names for the same device. 9. Ultracapacitors are best when we want a lot of energy, but little power. 10. Both high currents and high voltages present danger to shocks. Safety Quiz: 11. You should not touch a person who was electrocuted unless you have unplugged the appliance. 331 Chapter 13 - Electricity 12. It is only current that can kill. Voltage does not matter. 13. During a storm, a tree standing alone in the middle of an open field is more vulnerable to lightning than a tree in the middle of a city. 14. You can get a shock by touching just one live conductor, or even a charged object such as a fence. 15. You need not worry about electric shock if you work around low voltage equipment. 16. The best approach to electrical safety is to add resistance to the path by wearing rubber gloves, boots, and other safety gear. 17. It is best to run extension cords under a carpet so no one trips over them. 18. You should not unscrew a light bulb with a wet hand, no matter the wattage. III. Fill-in the Blanks 1. The study of charges at rest is called _________ _________. 2. The product of electric potential across a resistance and the current passing through the resistance gives the __________ dissipated by that resistance. 3. When resistances are placed in parallel, _________ are added. 4. To transmit power to long distances, it is best to use ___________ current. 5. Shock severity depends on the length of time, the type of tissues involved and the magnitude of the ____________ passing through the body. IV. Match the List 1. Match the list of variables from the column to the right with the units given in the column to the left. a. Force watt b. Charge ohm c. Power joule d. Weight volt e. Mass coulomb f. Electric Potential ampere g. Electrical Resistance pascal h. Current kilogram i. Heat newton j. Pressure newton 2. Match letters on the diagram with components in the figure: ND1 OB F G H IK C D3 D2 MEJ L A a. Ambient air i. Fan b. Water j. Generator c. Steam k. Transformer d. Boiler l. Heat source e. Condenser m. Heat sink f. Pump n. Low-voltage line g. Turbine o. High voltage lines h. Cooling tower IV. Project I – Electrical Power Generating Station The migration of new industries and increased population requires the construction of new power generation facilities with a capacity of 10 MWe near your city. A panel consisting of consumer groups, oil and gas company representatives, coal miners, economists, scientists, business leaders, regulatory agencies, electric utilities, and environmentalists are meeting to debate the merit of various proposals. Several options are being considered: a. Purchase necessary additional power from a neighboring state or country. b. Use a combination of renewable energy alternatives such as wind, wave, and solar. c. Expand the existing power plant facilities using 332 coal, oil, or natural gas. d. Build a new fossil plant in the city suburbs, away from major population centers. e. Construct a 15 MW combined heat and power (CHP) geothermal power plant. f. Construct a nuclear fission power plant. You are assigned the task of assuming the role of a reporter who is to report on various arguments being made in favor of or against each option. Please detail the findings by writing a report summarizing the following: 1. What are the geographical considerations that preclude using one or more of these options? 2. Which remaining options make the most economic sense? Consider the initial cost of construction, cost of producing and distributing power, maintenance costs, and the potential increase in employment. 3. Which option makes the most environmental sense? 4. What are the political implications of this decision, if any? Project II - Health Hazards associated with Power lines and mobile phones. There has been a wide array of studies on possible risks associated with the long-term health effects from electrical systems which was previously assumed to be safe. Some studies have claimed an increase rate of cancer among those who live in proximity to the high-voltage power lines, or using mobile phones frequently. None of these studies are conclusive, in part because of the difficulty in carrying laboratory experiments, or isolating the effect from many other factors which may contribute to the health risks. In case of mobile phones, data are limited, as these devices found their way in our daily life only about a decade ago. In light of the long latency period for developing tumors, the linkage of these devices to cancer can only be marginally documented. In this project you are asked to search the internet for scientific studies that link or unlink the electromagnetic radiation to safety and health. Write a one-page report outlining the recent findings and answer the following questions: 1. How does electricity and magnetism interact with matter, specifically to various human tissues? 2. Would you expect a greater effect at higher currents? At higher voltages? 3. Are there needs for concern? If so, are there steps that can be taken to minimize the health risks, what are they?