Electric Current and Charge Flow Basic Concepts and Analogies

1. What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge? A vibration, also known as an oscillation, is a periodic wiggle in time. A wave is a wobble in both spatial and temporal. 2. What condition is necessary for the sustained flow of water in a pipe? What analogous condition is necessary for the sustained flow of charge in a wire? Something including a pump must maintain a differential in water pressure between the two ends of a pipe to keep water flowing. Similarly, something like a battery must preserve a difference in electric potential between two points ends of a wire to keep charges flowing. 3. Why are electrons, rather than protons, the principal charge carriers in metal wires? Because metals have mobile exterior electrons that are free to move inside the substance, electrons are the primary charge carriers in metals. 4. What exactly is an ampere? An amperage is a unit of electric current measurement. It's the same as one coulomb every second. 5. Name two kinds of practical “electric pumps.” A battery is a useful "electric pump" that can maintain a constant charge flow. Another example is a generator. 6. How much energy is supplied to each coulomb of charge that flows through a 12-V battery? Each equation of charge flowing through a 12-volt battery receives 12 J. As indicated on page 422, the work done on the charges equals charge multiplied by electric potential. 7. Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit? Each equation of charge flowing through a 12-volt battery receives 12 J. As indicated on page 422, the work done on the charges equals charge compounded by electric potential. 8. Will water flow more easily through a wide pipe or a narrow pipe? Will current flow more easily through a thick wire or a thin wire? A wide pipe allows water to flow more easily. A thick wire allows electric current to flow more readily. A short, wide straw is easier to drink water through than a long, narrow one. A short, thick wire will have less resistance than a long, thin wire. 9. Does heating a metal wire increase or decrease its electrical resistance? The resistance of a metal wire doubles when it is heated. When a metal is heated, its atoms jitter around more, obstructing electron motion and boosting electrical resistance. 10. What is the unit of electrical resistance? The ohm, abbreviated as, is the unit of electrical resistance. An upside down horse shoe like shape. 11. If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current? The flow is halved when the resistance increases while the voltage remains constant. 12. If the resistance of a circuit remains constant while the voltage across the circuit decreases to half its former value, what change occurs in the current? The current is halved when the voltage is halved although the resistance remains constant. 13. How does wetness affect the resistance of your body? When moist, the electrical resistance of the human body is reduced. 14. What is the function of the round third prong in a modern household electrical plug? The spherical third prong is "grounded," meaning it connects the appliance's casing to the ground. 15. Does a battery produce dc or ac? Does the generator at a power station produce dc or ac? Direct current, or dc, is produced by a battery. A power station's generator creates alternating current, or ac. 16. What does it mean to say that a certain current is 60 Hz? A 60 hertz (60 Hz) current is alternating current (ac), and the direction of the current changes constantly. In 1.0 second, sixty entire sine waves of electricity are completed. 17. What property of a diode enables it to convert ac to pulsed dc? A device is an electronic one-way valve that enables just one path of electricity to flow through it. 18. A diode converts ac to pulsed dc. What electrical device smoothest the pulsed dc to a smoother dc? A capacitor smooths down a pulsed direct current, making it "less jerky." 19. What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light? The mistake is that electrons in a line travel at "only" roughly 1200 times the speed of light, or millions of kilometers per hour. Their "drift velocity" is considerably slower. 20. Why does a wire that carries electric current become hot? Electrons clash with wire atoms in a current-carrying wire. Some of the energy of the electrons is converted into mechanical energy of the cable atoms, causing the wire to heat up. 21. What is meant by drift velocity? The effective frequency of the atoms that make up an electric current is called drift velocity. 22. A tipped domino sends a pulse along a row of standing dominoes. Is this a good analogy for the way electric current, sound, or both travel? Sound waves are the only thing that the domino analogy works for. 23. What is the error in saying that the source of electrons in a circuit is the battery or generator? The electrons previously present in the superconductor are the source of electrons in a circuit. 24. When you make your household electric payment at the end of the month, which are you billed for: voltage, current, power, or energy? You are charged in kWh, which is the amount of energy you used. 25. From where do the electrons originate that produce an electric shock when you touch a charged conductor? The atoms in your body are the ones that are making up the current when you touch a charged conductor and get shocked (now, why did you do that?!?). 26. What is the relationship among electric power, current, and voltage? Electric power=(current)×(voltage) . 27. Which of these is a unit of power and which is a unit of energy: a watt, a kilowatt, a kilowatt-hour? A watt is a power unit equivalent to one joule per second. A kilowatt is a power unit that is equal to 1000 watts. A kilowatt-hour is a unit of energy that is equal to the power multiplied by the time. 28. How does the heat emitted by lamps affect their efÏciency? Because the goal of a lamp is to convert electrical energy into visible light, the more heat it produces (which is invisible to human sight), the less efÏcient it is. 29. In a circuit of two lamps in series, if the current through one lamp is 1 A, what is the current through the other lamp? Defend your answer. 1 Amp of electricity 30. If a voltage of 6 V is impressed across the circuit in the preceding question and the voltage across the first lamp is 2 V, what is the voltage across the second lamp? Defend your answer. 4 volts of electricity 31. In a circuit of two lamps in parallel, if there is a voltage of 6 V across one lamp, what is the voltage across the other lamp? 6 volts of electricity 32. How does the sum of the currents through the branches of a simple parallel circuit compare with the current in the voltage source? The total of the branch currents in a simple parallel circuit equals the total able to operate successfully by and flowing through the voltage source. 34. An electric cell is made by placing two plates made of different materials that have different afÏnities for electrons in a conducting solution. The voltage of a cell depends on the materials used and the solutions they are placed in, not on the size of the plates. (A cell is often called a battery, but strictly speaking, a battery is a series of cells—for instance, six cells in a 12-V car battery.) You can make a simple 1.5-V cell by placing a strip of copper and a strip of zinc in a tumbler of saltwater. An easy cell to construct is the citrus cell. Stick a straightened paper clip and a piece of copper wire into a lemon. Hold the ends of the wire close together, but not touching, and place the ends on your tongue. The slight tingle you feel and the metallic taste you experience result from a slight electric current from the citrus cell through the wires when your moist tongue closes the circuit. If you have a multimeter, you may check if your battery voltage is 1.5 volts. Put two of these batteries in series to test if the voltages total up to 3 volts. Is it a lemon since the battery doesn't work? 36. Send a letter to Grandma and convince her that whatever electric shocks she may have received over the years have been due to the movement of electrons already in her body—not electrons from somewhere else. Copper wire Lemon Paper clip Hello, Grandma. Do you recall how I made you lick the terminals of a 9-V battery and you received a tingling sensation? Please accept my apologies; I hope you have forgiven me. I recall believing that the battery's electrons were responsible for the shock. Here's some disturbing information: I was mistaken. When the electric current was generated, the electrons that were already in your cheek began to move, and THEY were the ones who caused you the shock. 38. When you touch your fingers (resistance 1000 Ω) to the terminals of a 6-V battery, show that the small current moving through your fingers is 0.006 A 0.006 Amps 40. An electric toy draws 0.5 A from a 120-V outlet. Show that the toy consumes 60 W of power. 600watts of electricity 42. What is the effect on the current in a wire if both the voltage across it and its resistance are doubled? If both are halved? There should be no effect in either case simply because the resistance changed cancels each other out. 44. Rearrange the equation current 5 voltage/resistance to express resistance in terms of current and voltage. Then solve the following: A certain device in a 120-V circuit has a current rating of 20 A. What is the resistance of the device (how many ohms)? Using the ohm symbol to represent that it is 6Ω . 46. The total charge that an automobile battery can supply without being recharged is given in terms of amperehours. A typical 12-V battery has a rating of 60 amperehours (60 A for 1 h, 30 A for 2 h, and so on). Suppose that you forget to turn the headlights off in your parked automobile. If each of the two headlights draws 3 A, how long will it be before your battery is “dead”? It will take approximately ten hours before the battery can be considered dead. 48. A 4-W night-light is plugged into a 120-V circuit and operates continuously for 1 year. Find the following: (a) the current it draws, (b) the resistance of its filament, (c) the energy consumed in a year. (d) Then show that for a utility rate of 15¢/kWh the cost for a year’s operation is $5.25. a. 130 amps, b. 3600 volts, c. 35 kWh, and d. $5.26. 50. Show in the preceding problem that 540 C of charge flows through the iron in 1 minute It would be the 540 C of charge. 52. Rank the circuits illustrated according to the brightness of the identical bulbs, from brightest to dimmest. The brightness goes from A to B to C in the order or brightest to dimmest 54. All bulbs are identical in the circuits shown. An ammeter is connected next to the battery, as shown. Rank the current readings in the ammeter, from greatest to least. Greatest to the least would be bulb C then B then A 56. Consider the three parts of the circuit: A, the top branch with two bulbs; B, the middle branch with one bulb; C, the battery. a. Rank the currents through each, from greatest to least. b. Rank the voltages across each, from greatest to least. The current from greatest to least id A then B then C 58. What two things can be done to increase the amount of flow in a water pipe? Similarly, what two things can be done to increase the current in an electric circuit? To drive water flow, reduce the flow resistance (for example, by making the water pipe wider) and provide a large discrepancy in water pressure. Reduce the electrical resistance in a circuit (for example, by making the transmitÝng wires thicker) and use a higher voltage to generate the current. 60. Consider a circuit with a main wire that branches into two other wires. If the current is 10 A in the main wire and 4 A in one of the branches, how much current is in the other branch? There would be 6 amps in each wire unlike a single wire that carries all currents together. 62. What happens to the brightness of light emitted by a lightbulb when the current flowing through it increases? A lightbulb lights brighter when the current running through the filament is increased. 64. Your friend says that a battery supplies the electrons in an electric circuit. Do you agree or disagree? Defend your answer. I personally would disagree because they are not correct, this is because there statement is invalid. 66. Is a current-carrying wire electrically charged because of the electrons moving in it? No. Not ever in any case. 68. In which of the circuits shown does a current exist to light the bulb? Circuit 5 I believe. This is the only image where the light bulb lights up. 70. Something gets “used up” in a battery that eventually dies and goes flat. One friend says that current is used up. Another friend says that energy is used up. Who, if either, do you agree with, and why? When your acquaintance indicates that energy is depleted, he or she is correct. 72. Your friend says that, when jump-starting a dead battery, you should connect your live battery in parallel with the dead battery, which, in effect, replaces the dead one. Do you agree? The positive terminal of your friend's battery is connected to the positive of yours, and the negative terminal of his battery is connected to the negative terminal of yours. His battery effectively replaces yours and drives electricity through your starting circuit by connecting the two batteries "in parallel." 74. Why is the current in an incandescent bulb greater immediately after it is turned on than it is a few moments later? (That’s why bulbs usually burn out just as they are being turned on.) The low resistance of the room-temperature filament allows a significant current to flow through it, burning out the bulb. As the filament heats up, its electrical resistance rises, lowering the current. 76. Why are all compact fluorescent lamps more efÏcient than incandescent lamps? Compact fluorescent lamps (the one with a glowing filament) are more efÏcient than conventional lamps (the kind with a glowing filament) because so much of the energy they use is converted to visible light and less is used to heat the environment. 78. Why does the filament of a lightbulb glow but the connecting wires do not? The filament dissipates the majority of the energy. 80. What causes electric shock: current or voltage? Voltage causes a current to flow, which causes you to be shocked. As a result, the applied voltage is the "origin" of the shock. 82. Would you expect to find dc or ac in the filament of a lightbulb in your home? In the headlight of an automobile? Alternating current, or ac, is virtually commonly used in a home (in the United States, at a frequency of 60 Hz and 110-120 V). A automobile battery maintains a constant voltage and only allows current to flow in one direction. Direct current, or dc, would be expected in the headlight. 84. As more lines are opened at a fast-food restaurant, the resistance to the motion of people trying to get served is reduced. How is this similar to what happens when more branches are added to a parallel circuit? When more identical branches are introduced in both circumstances, the overall resistance decreases. 86. Which is the same for a 10-V and a 20-V resistor in series in a series circuit: current or voltage? The current will be the same. 88. The damaging effects of electric shock result from the amount of current that flows in the body. Why, then, do we see signs that read “Danger—High Voltage” rather than “Danger—High Current”? Voltage causes a current to flow, which causes you to be shocked. As a result, the high applied voltage is the "source" of the shock. Because current cannot flow unless a person is present to provide a conducting channel from the high-voltage device to the earth, the sign just serves as a warning. 90. Is the following label on a household product cause for concern? “Caution: This product contains tiny, electrically charged particles moving at speeds in excess of 100,000,000 kilometers per hour.” It's a nerdy physics joke on the label. 92. Why are the wingspans of birds a consideration in determining the spacing between parallel wires in a power line? If parallel wires are wider together than a bird's length in a power supply system, the bird may mistakenly connect two nearby wires. 94. If electrons flow very slowly through a circuit, why doesn’t it take a noticeably long time for a lamp to glow when you turn on a distant switch? The electric field inside the wire and the circulation are established at thousands of miles per hour when the switch is closed. This speed is not the same as the conduction electrons' drift velocity. 96. Consider a pair of flashlight bulbs connected to a battery. Will they glow brighter if they are connected in series or in parallel? Will the battery run down faster if the bulbs are connected in series or in parallel? When bulbs are connected in parallel, they will glow brighter. If the bulbs are interconnected, the battery will drain faster. 98. If several bulbs are connected in series to a battery, they may feel warm to the touch but not visibly glow. What is your explanation? The supply voltage for two or more devices connected in series is the SUM of the voltage drops throughout the devices. Each duplicate bulb "feels" only a portion of the battery's voltage, resulting in lower current flow than usual. The filament isn't heated enough to shine at such a lower voltage (emit light). 100. As more and more bulbs are connected in series to a flashlight battery, what happens to the brightness of each bulb? Assuming that heating inside the battery is negligible, what happens to the brightness of each bulb when more and more bulbs are connected in parallel? The bulbs will dim and continue to stay dim for the mean time. 102. Why is there no effect on other branches in a parallel circuit when one branch of the circuit is opened or closed? When two or more devices are linked in parallel, the voltage across each device is the same. On page 444, you'll find Rule 1 for parallel connection, which is detailed on pages 443-444. Figure 23.18 illustrates this as well. Regardless of what happens to parallel branches, each branch receives the full power source voltage. 104. When two identical resistors are connected in parallel, which of the following is the same for both resistors: (a) voltage across each, (b) power dissipated in each, (c) current through each? Do any of your answers change if the resistors are different from each other? When two identical resistors are wired in parallel, the results are similar. Because they are resistors in parallel, the voltage across them is always the same. If the resistors aren't identical, the one with the higher resistance will have less current and less power dissipated. 106. Are these three circuits equivalent? Why or why not? When two identical resistors are wired in parallel, the results are similar. Because they are resistors in parallel, the voltage across them is always the same. If the resistors aren't identical, the one with the higher resistance will have less current and less power dissipated. 108. A simple lie detector consists of an electric circuit, often from one finger to another. A sensitive meter shows the current that flows when a small voltage is applied. How does this technique indicate that a person is lying? (And when doesn’t this technique indicate when someone is lying?) Yes, each of the three circuits is equivalent. 110. A car’s headlights dissipate 40 W on low beam and 50 W on high beam. Is there more or less resistance in the high-beam filament? When a person lies down, he or she may sweat, which causes the body's resistance between the two fingers to vary owing to the moist skin. The electrical lie detector will not operate if the person lies but does not perspire while doing so. 112. To connect a pair of resistors so that their combined (equivalent) resistance will be less than the resistance of either one, should you connect them in series or in parallel? Connect them in parallel 114. Your friend says that the equivalent (combined) resistance of resistors connected in series is always greater than the resistance of the largest resistor. Do you agree? Personally, I do agree with the statement they made. 116. Your electronics friend needs a 20-V resistor but has only 40-V resistors. He tells you that he can combine them to produce a 20-V resistor. How Make a parallel connection between two 40-ohm resistors. 118. Is the resistance of a 100-W bulb greater or less than the resistance of a 60-W bulb? Assuming the filaments in each bulb are the same length and made of the same material, which bulb has the thicker filament? It's less than. The filament has a power rating of 100 watts. 120. (a) What single resistor would be equivalent to the resistors in this arrangement? (b) If 24 volts were applied across points A and B, how much current would there be in the circuit? (c) How much current in the 12-ohm resistor? The corresponding resistance is 4 in A, 6 in B, and 2 in C.