Conceptual Physics Guided Reading for Electric Current Chapter 23
Problem 1
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
Salamat A.
Numerade Educator
Problem 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?
Problem 3
Why are electrons, rather than protons, the principal charge carriers in metal wires?
Salamat A.
Numerade Educator
Problem 5
Name two kinds of practical "electric pumps."
Salamat A.
Numerade Educator
Problem 6
How much energy is supplied to each coulomb of charge that flows through a 12-V battery?
Problem 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?
Salamat A.
Numerade Educator
Problem 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?
Problem 9
Does heating a metal wire increase or decrease its electrical resistance?
Salamat A.
Numerade Educator
Problem 10
Name a material commonly used for making connection wires.
Problem 11
If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?
Salamat A.
Numerade Educator
Problem 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?
Problem 13
How does wetness affect the resistance of your body?
Salamat A.
Numerade Educator
Problem 14
What is the function of the round third prong in a modern household electrical plug?
Problem 15
Does a battery produce dc or ac? Does the generator at a power station produce dc or ac?
Salamat A.
Numerade Educator
Problem 16
What does it mean to say that a certain current is 60 Hz?
Problem 17
What property of a diode enables it to convert ac to pulsed dc?
Salamat A.
Numerade Educator
Problem 18
A diode converts ac to pulsed dc. What electrical device smoothes the pulsed dc to a smoother dc?
Problem 19
What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light?
Salamat A.
Numerade Educator
Problem 20
Why does a wire that carries electric current become hot?
Problem 21
What is meant by drift velocity?
Salamat A.
Numerade Educator
Problem 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?
Problem 23
What is the error in saying that the source of electrons in a circuit is the battery or generator?
Salamat A.
Numerade Educator
Problem 24
When you make your household electric payment at the end of the month, which are you billed for: voltage, current, power, or energy?
Problem 25
From where do the electrons originate that produce an electric shock when you touch a charged conductor?
Salamat A.
Numerade Educator
Problem 26
An electric bulb is labeled '100 W, 250 V'. What information does this convey?
Problem 27
Which of these is a unit of power and which is a unit of energy: a watt, a kilowatt, a kilowatt-hour?
Salamat A.
Numerade Educator
Problem 28
How does the heat emitted by lamps affect their efficiency?
Problem 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.
Salamat A.
Numerade Educator
Problem 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.
Problem 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?
Salamat A.
Numerade Educator
Problem 32
How does the sum of the currents through the branches of a simple parallel circuit compare with the current in the voltage source?
Problem 33
What is the function of fuses or circuit breakers in a circuit?
Salamat A.
Numerade Educator
Problem 34
An electric cell is made by placing two plates made of different materials that have different affinities 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.
Problem 35
Examine the electric meter in your house. It is probably in the basement or on the outside of the house. You will see that, in addition to the clocklike dials in the meter, there is a circular aluminum disk that spins between the poles of magnets when electric current goes into the house. The more electric current, the faster the disk turns. The speed of the disk is directly proportional to the number of watts used; for example, it spins 5 times as fast for 500 W as for 100 W. You can use the meter to determine how many watts an electrical device uses. First, make sure that all electrical devices in your home are disconnected (it is okay to ignore electric clocks and other 2-W devices, which will hardly be noticeable). The disk will be practically stationary. Then connect a 100-W bulb and note how many seconds it takes for the disk to make five complete revolutions. The black spot painted on the edge of the disk makes this easy. Disconnect the 100-W bulb and plug in a device of unknown wattage. Again, count the seconds for five revolutions. If it takes the same time, it's a 100-W device; if it takes twice the time, it's a 50-W device; half the time, a 200-W device; and so forth. In this way you can estimate the power consumption of devices fairly accurately.
Problem 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.
Problem 37
A toaster has a heating element of 15 ? and is connected to a 120-V outlet. Show that the current drawn by the toaster is 8 A.
Salamat A.
Numerade Educator
Problem 38
Calculate the resistance of a copper wire of $1-\mathrm{cm}$ length, 1 -mm radius, and $1.72 \times 10^{-8} \Omega$ -m resistivity.
Problem 39
An electric bulb draws a current of 1.5 A at 6.0 V. Find the resistance of the filament of the bulb when glowing.
Salamat A.
Numerade Educator
Problem 40
A 40-W bulb is used for 12.5 hours every day for 30 days. Calculate the consumption of electrical energy.
Problem 41
Calculate the power of a hair dryer that operates on 120 V and draws a current of 10 A.
Salamat A.
Numerade Educator
Problem 42
What happens to the resistance of a metal wire if its radius is halved?
Problem 43
The wattage marked on a lightbulb is not an inherent property of the bulb but depends on the voltage to which it is connected, usually 110 V or 120 V. How many amperes flow through a 60-W bulb connected in a 120-V circuit?
Salamat A.
Numerade Educator
Problem 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)?
Problem 45
Using the formula power $=$ current $\times$ voltage, find the current drawn by a $1200-W$ toaster connected to 120 $\mathrm{V}$ . Then, using the method from the preceding problem, show that the resistance of the toaster is 12$\Omega .$
Salamat A.
Numerade Educator
Problem 46
The total charge that an automobile battery can supply without being recharged is given in terms of ampere-hours. A typical 12-V battery has a rating of 60 ampere-hours (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"?
Problem 47
Show that operating a 100-W lamp continuously for 1 week when the power utility rate is 15¢/kWh costs $2.52.
Salamat A.
Numerade Educator
Problem 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.
Problem 49
An electric iron connected to a 110-V source draws 9 A of current. Show that the amount of heat it generates in a minute is nearly 60,000 J.
Salamat A.
Numerade Educator
Problem 50
Show in the preceding problem that 540 C of charge flows through the iron in 1 minute.
Problem 51
In periods of peak demand, power companies lower their voltage. This saves them power (and saves you money!). To see the effect, consider a 1200-W coffeemaker that draws 10 A when connected to 120 V. Suppose the voltage is lowered by 10% to 108 V. By how much does the current decrease? By how much does the power decrease? (Caution: The 1200-W label is valid only when 120 V is applied. When the voltage is lowered, the current is lowered, so the power output of the toaster's heating coils drops, which
cools the coil and lowers its resistance. However, for this problem assume the change in resistance is slight.)
Salamat A.
Numerade Educator
Problem 52
Rank the circuits illustrated according to the brightness of the identical bulbs, from brightest to dimmest.
Problem 53
The bulbs shown are identical. An ammeter is placed in different locations, as shown. Rank the current readings in the ammeter, from greatest to least.
Aparna S.
Numerade Educator
Problem 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.
Problem 55
All bulbs are identical in the circuits shown. A voltmeter is connected across a single bulb to measure the voltage drop across it. Rank the voltage readings from greatest to least.
Salamat A.
Numerade Educator
Problem 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.
Problem 57
What condition is necessary for a sustained flow of electric charge through a conducting medium?
Salamat A.
Numerade Educator
Problem 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?
Problem 59
Consider a water pipe that branches into two smaller pipes. If the flow of water is 10 gallons per minute in the main pipe and 4 gallons per minute in one of the branches, how much water per minute flows in the other branch?
Salamat A.
Numerade Educator
Problem 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?
Problem 61
One example of a water system is a garden hose that waters a garden. Another is the cooling system of an automobile. Which of these systems exhibits behavior more analogous to an electric circuit? Explain.
Salamat A.
Numerade Educator
Problem 62
What happens to the brightness of light emitted by a lightbulb when the current flowing through it
increases?
Problem 63
The circuit within an incandescent lamp is shown. Which of the battery arrangements will light the lamp?
Salamat A.
Numerade Educator
Problem 64
Your friend says that a battery supplies the electrons in an electric circuit. Do you agree or disagree? Defend your answer.
Problem 65
How much energy is given to each coulomb of charge passing through a 6-V battery?
Salamat A.
Numerade Educator
Problem 66
Is a current-carrying wire electrically charged because of the electrons moving in it?
Problem 67
Your tutor tells you that an ampere and a volt really measure the same thing and that the different terms only serve to make a simple concept seem confusing. Why should you consider getting a different tutor?
Salamat A.
Numerade Educator
Problem 68
In which of the circuits shown does a current exist to light the bulb?
Problem 69
Does more current flow out of a battery than into it? Does more current flow into a lightbulb than out of it? Explain.
Eric M.
Numerade Educator
Problem 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?
Problem 71
Suppose you leave your car lights on while you see a movie. When you return, your battery is too "weak" to start your car. A friend gives you a jump-start with his battery and battery cables. What physics is occurring here?
Christopher D.
Numerade Educator
Problem 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?
Problem 73
An electron moving in a wire collides repeatedly with atoms and travels an average distance between collisions called the mean free path. If the mean free path is less in some metals, what can you say about the resistance of these metals? For a given conductor, what can be done to lengthen the mean free path?
Salamat A.
Numerade Educator
Problem 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.)
Problem 75
Only a small percentage of the electric energy fed into a common lightbulb is transformed into light. What happens to the remaining energy?
Salamat A.
Numerade Educator
Problem 76
Why are all compact fluorescent lamps more efficient than incandescent lamps?
Problem 77
Why are thick wires rather than thin wires usually used to carry large currents?
Salamat A.
Numerade Educator
Problem 78
Why does the filament of a lightbulb glow but the connecting wires do not?
Problem 79
Will a lamp with a thick filament draw more current or less current than a lamp with a thin filament?
Salamat A.
Numerade Educator
Problem 80
What causes electric shock: current or voltage?
Problem 81
If a current of one- or two-tenths of an ampere were to flow into one of your hands and out the other, you would probably be electrocuted. But if the same current were to flow into your hand and out the elbow above the same hand, you would survive even though the current might be large enough to burn your flesh. Explain.
Salamat A.
Numerade Educator
Problem 82
Would you expect to find dc or ac in the filament of a lightbulb in your home? In the headlight of an
automobile?
Problem 83
Are automobile headlights wired in parallel or in series? What is your evidence?
Salamat A.
Numerade Educator
Problem 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?
Problem 85
What unit signifies (a) joule per coulomb, (b) coulomb per second, (c) watt?second?
Salamat A.
Numerade Educator
Problem 86
Which is the same for a $10-\Omega$ and a $20-\Omega$ resistor in series in a series circuit: current or voltage?
Problem 87
Which is the same for a $10-\Omega$ and a $20-\Omega$ resistor in parallel in a parallel circuit: current or voltage?
Salamat A.
Numerade Educator
Problem 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"?
Problem 89
Comment on the warning sign shown in the sketch.
Salamat A.
Numerade Educator
Problem 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."
Problem 91
Which will do less damage: plugging a 110-V hairdryer into a 220-V circuit or plugging a 220-V hairdryer into a 110-V circuit? Defend your answer.
Salamat A.
Numerade Educator
Problem 92
Why are the wingspans of birds a consideration in determining the spacing between parallel wires in a power line?
Problem 93
Estimate the number of electrons that a power company delivers annually to the homes of a typical town of 40,000 people.
Salamat A.
Numerade Educator
Problem 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?
Problem 95
Why is the speed of an electrical signal so much greater than the speed of sound?
Salamat A.
Numerade Educator
Problem 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?
Problem 97
What happens to the brightness of bulb A when the switch is closed and bulb B lights up?
Salamat A.
Numerade Educator
Problem 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?
Problem 99
In the circuit shown, how do the brightnesses of the identical lightbulbs compare? Which bulb draws the
Salamat A.
Numerade Educator
Problem 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?
Problem 101
What changes occur in the line current when more devices are introduced in a series circuit? In a parallel
circuit? Why are your answers different?
Salamat A.
Numerade Educator
Problem 102
Why is there no effect on other branches in a parallel circuit when one branch of the circuit is opened or closed?
Problem 103
When a pair of identical resistors are connected in series, 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?
Salamat A.
Numerade Educator
Problem 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?
Problem 105
Batteries do have internal resistance, which is not always negligible. It shows when the current a battery supplies increases, whereupon the voltage it supplies decreases. Taking internal resistance of the battery into consideration, will the brightness of many bulbs diminish when connected in parallel? Defend your answer.
Salamat A.
Numerade Educator
Problem 106
Are these three circuits equivalent? Why or why not?
Problem 107
Figure 23.19 shows a fuse placed in a household circuit. In what other locations might a fuse be placed
in this circuit to be useful, melting only if a problem arises?
Salamat A.
Numerade Educator
Problem 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?)
Problem 109
It is commonly said that a certain resistor draws a certain current. Does this mean that the resistor "attracts" the current? Defend your answer.
Salamat A.
Numerade Educator
Problem 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?
Problem 111
To connect a pair of resistors so that their combined (equivalent) resistance will be greater than the resistance of either one, should you connect them in series or in parallel?
Salamat A.
Numerade Educator
Problem 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?
Problem 113
If a glowing incandescent lightbulb is jarred and oxygen leaks inside, the bulb will momentarily brighten considerably before burning out. Putting excess current through a lightbulb will also burn it out. What physical change occurs when a lightbulb burns out?
Salamat A.
Numerade Educator
Problem 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?
Problem 115
Your friend says that the equivalent (combined) resistance of resistors connected in parallel is always less than the resistance of the smallest resistor. Do you agree?
Salamat A.
Numerade Educator
Problem 116
Your electronics friend needs a $20-\Omega$ resistor but has only $40-\Omega$ resistors. He tells you that he can combine them to produce a $20-\Omega$ resistor. How?
Problem 117
Your electronics friend needs a $10-\Omega$ resistor but has only $40-\Omega$ ones. How can he combine them to produce an equivalent resistance of 10$\Omega$ ?
Salamat A.
Numerade Educator
Problem 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?
Problem 119
If a 60-W bulb and a 100-W bulb are connected in series in a circuit, across which bulb will there be a
greater voltage drop? How about if they are connected in parallel?
Salamat A.
Numerade Educator
Problem 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?
Problem 121
Three 6-ohm resistors are arranged in a triangle as shown. A voltage source of 12 V is impressed across one of the resistors.
(a) How much current is in that resistor?
(b) How much current is in each of the other two resistors?
(c) How much current is in the voltage source?
(d) What is the equivalent resistance of the circuit (what single resistor connected to the voltage source could replace the three resistors)?
Salamat A.
Numerade Educator
Conceptual Physics Guided Reading for Electric Current Chapter 23
Source: https://www.numerade.com/books/chapter/electric-current/
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