Physics 1060 - Spring 2018 - Midterm 2 Exam - Posted Version with Solutions
Problem 1:
When your cellphone's battery is being recharged,
(A) voltage is changing through the battery from its lower current end to its higher current end. [2.2% picked]
(B) current is flowing through the battery from its lower voltage end to its higher voltage end. [19.7% picked]
(C) voltage is changing through the battery from its higher current end to its lower current end. [7.2% picked]
(D) current is flowing through the battery from its higher voltage end to its lower voltage end. [70.9% picked]
Answer: (D) current is flowing through the battery from its higher voltage end to its lower voltage end. [70.9% picked]
Why: When a battery is recharging, it is acting as a consumer of electric power. Current must enter the battery at higher voltage, drop off power in the battery, and leave the battery a lower voltage.
Problem 2:
A rocking chair has damaged the cord of your desk lamp. One of the two wires in the cord is completely cut in half and cannot carry any current. However, the other wire still connects the lamp to the electric socket. If you switch on the lamp,
(A) the normal amount of current will flow through both wires and the lamp will glow at its normal brightness. [0.0% picked]
(B) the normal amount of current will flow through the one remaining wire and the lamp will glow at half its normal brightness. [0.4% picked]
(C) no current will flow through either wire and the lamp will remain dark. [98.2% picked]
(D) half the normal amount of current will flow through the one remaining wire and the lamp will glow at a quarter of its normal brightness. [1.3% picked]
Answer: (C) no current will flow through either wire and the lamp will remain dark. [98.2% picked]
Why: The desk lamp requires a complete circuit to operate. Current must flow from the outlet to the lamp through the higher voltage wire, drop off power in the lamp, and return to the outlet through the lower voltage wire. If one of those wires is broken, then the current cannot complete this roundtrip. Instead, positive charge accumulates on the higher voltage part of the wiring and negative charge accumulates on the lower voltage part of the wiring. Once that has happened, no further current flows and power is not dropped off in the lamp. The lamp remains dark.
Problem 3:
The current passing through a metal wire is proportional to
(A) one divided by the voltage difference between the two ends of the wire. [3.1% picked]
(B) the net charge of that wire. [3.1% picked]
(C) one divided by the net charge of that wire. [1.3% picked]
(D) the voltage difference between the two ends of the wire. [92.4% picked]
Answer: (D) the voltage difference between the two ends of the wire. [92.4% picked]
Why: A wire obeys Ohms law, so the current passing through it is equal to the wire's electrical resistance times the voltage drop across the wire. The larger the voltage drop between the entry end of the wire and the exit end of the wire, the larger the electric field pushing current forward through the wire. The current that flows is proportional to that electric field and therefore to the voltage drop itself.
Problem 4:
You are arm-wrestling a friend. While you are gradually winning the competition, compare the torque that your arm exerts on your friend's arm to the torque your friend's arm exerts on your arm.
(A) The two torques are equal. [4.9% picked]
(B) Those two torques are equal in amount but opposite in direction. [40.8% picked]
(C) The torque you exert on your friend is less in amount than the torque your friend exerts on you. [0.0% picked]
(D) The torque you exert on your friend is greater in amount than the torque your friend exerts on you. [54.3% picked]
Answer: (B) Those two torques are equal in amount but opposite in direction. [40.8% picked]
Why: The two torques identified in the question are a Newton's third law pair and must be equal in amount but opposite direction. It doesn't matter how your arms are moving or even that they are moving at all. If your arm twists something, that something must twist your arm equally hard in the opposite direction, action/reaction. Those two torques act on different objects, one acts on the something and the other acts on your arm. What's typically more interesting in arm wrestling is whether your shoulder is twisting the pair of arms more strong than your friend's should is twisting the pair of arms. Those two torques act on one object (namely, the two arms) and determines who is winning.
Problem 5:
The secondary coil of a large transformer near the street supplies your home with 120-volt alternating current electric power. The transformer's primary coil has 20 times as many turns as its secondary coil. At this moment, the transformer is supplying electric power only to your home and that power is 2400 watts. The transformer's primary coil is being supplied with
(A) 20-volt alternating current and the current in that coil is 120 amperes. [2.7% picked]
(B) 2,400-volt alternating current and the current in that coil is 20 amperes [12.6% picked]
(C) 120-volt alternating current and the current in that coil is 20 amperes. [8.5% picked]
(D) 2,400-volt alternating current and the current in that coil is 1 ampere. [76.2% picked]
Answer: (D) 2,400-volt alternating current and the current in that coil is 1 ampere. [76.2% picked]
Why: Because the transformer's primary coil has 20 times the turns of the secondary coil, the transformer's electric field does 20 times as much negative work on each charge passing through the primary coil as it does positive work on each charge passing through the secondary coil. As a result, the voltage drop in the primary coil is 20 times the voltage rise in the secondary coil. With a 120 volt AC voltage rise in the secondary coil, the voltage drop in the primary coil must be 2,400 volts AC. The power that the primary coil transfers to the secondary coil is 2,400 watts, so the primary current must supply 2,400 watts to the primary coil. Given the 2,400 volt AC volt drop in that coil, a current of 1 ampere is required to supply 2,400 watts.
Problem 6:
The electric power grid in the United States uses alternating current because
(A) an alternating current carries more electric power than a direct current. [0.4% picked]
(B) an alternating current delivers both positive and negative charges, while a direct current delivers only positive charges. That difference makes alternating current twice as efficient as direct current. [2.2% picked]
(C) alternating current is less likely to cause an electrical fire than direct current. [0.4% picked]
(D) transformers can move power from one circuit to another only if those circuits are carrying alternating current. [96.9% picked]
Answer: (D) transformers can move power from one circuit to another only if those circuits are carrying alternating current. [96.9% picked]
Why: Transformers can more power from a primary circuit to a secondary circuit, but only if those two currents are changing with time. Changing currents produce electric fields and those electric fields do work or negative work on currents flowing in those circuits. If the currents don't change with time, they produce constant magnetic fields and constant magnetic fields produce no electric fields. Power therefore cannot move between the circuits.
Problem 7:
When a light wave is travelling through empty space, midway between a distant star and your eye, that wave consists of
(A) a magnetic field and either an electric field or an electric charge. [1.8% picked]
(B) an electric field and a magnetic field, but no electric charges or magnetic poles. [89.2% picked]
(C) an electric field and either a magnetic field or a magnetic pole. [4.5% picked]
(D) either an electric field or an electric charge, and either a magnetic field or a magnetic pole. [4.5% picked]
Answer: (B) an electric field and a magnetic field, but no electric charges or magnetic poles. [89.2% picked]
Why: Electromagnetic waves are simple disturbances in space and can move through vacuum (empty space) easily. As they do, they consist only of electric and magnetic fields. To emit or absorb an electromagnetic wave, electric charge or magnetic pole is required. One the wave is travelling, however, it doesn't need charges or poles.
Problem 8:
A 120-volt AC electrical outlet in the United States has two slots, known as neutral and power. Which slot sometimes has a voltage of +130 volts and which slot sometimes has a voltage of –130 volts?
(A) Neutral can be +130 volts and neutral can be –130 volts. [4.0% picked]
(B) Power can be +130 volts and neutral can be –130 volts. [11.2% picked]
(C) Power can be +130 volts and power can be –130 volts. [42.6% picked]
(D) Neither slot ever has those voltages. [42.2% picked]
Answer: (C) Power can be +130 volts and power can be –130 volts. [42.6% picked]
Why: The US National Electrical Code requires that the neutral wire have nearly the same voltage as the voltage of the Earth itself. The power or hot wire undergoes nearly all of the voltage change. Although the nominal voltage of the outlet is 120 volts AC, the power or hot wire has a voltage that varies sinusoidally (according to the trigonometric sine function with time) and reaches values of +170 and –170.
Problem 9:
At the point where a current of 1 ampere enters an ordinary wire, the voltage is 100 volts. The wire is electrically isolate from everything else. At the point where current exits that same wire, the voltage is
(A) less than 100 volts and a current of 1 ampere exits the wire. [89.7% picked]
(B) greater than 100 volts and a current of 1 ampere exits the wire. [0.9% picked]
(C) 100 volts and a current of less than 1 ampere exits the wire. [3.1% picked]
(D) less than 100 volts and a current of less than 1 ampere exits the wire. [6.3% picked]
Answer: (A) less than 100 volts and a current of 1 ampere exits the wire. [89.7% picked]
Why: Ordinary electrical conductors do not carry currents for free, they require electric fields to push those current forward. In a wire, that electric field is produced by a voltage drop, so that current enters the wire at a higher voltage and exits the wire at a lower voltage. Since charge is conserved, all the charge that enters the wire must eventually exit the wire. Thus the current exiting the wire is the same as the current entering the wire.
Problem 10:
Suppose both coils of an ordinary transformer are carrying constant electric currents. Why can't the transformer transfer electric power from one coil to the other?
(A) Each coil's constant current produces no magnetic or electric field and therefore cannot do work or negative work on charges in the other coil's current. [8.1% picked]
(B) The two coils are electrically insulated from one another, so the transformer cannot transfer electric power between its coils, regardless of what types of currents those coils are carrying. [21.5% picked]
(C) Each coil's constant current produces a constant magnetic field, which produces no electric field and therefore cannot do work or negative work on charges in the coils' currents. [43.0% picked]
(D) Each coil's constant current produces a constant electric field, which produces no magnetic field and therefore cannot do work or negative work on charges in the other coil's current. [27.4% picked]
Answer: (C) Each coil's constant current produces a constant magnetic field, which produces no electric field and therefore cannot do work or negative work on charges in the coils' currents. [43.0% picked]
Why: Constant electric currents produce constant magnetic fields. With no change in their magnetic fields, the currents in the transformer produce no electric fields and do no work or negative work on one another. No power is transferred between the currents and the transformer is pointless.
Problem 11:
The power outlet in your room supplies 120-volt AC electric power. That outlet has two metal slots through which power is provided. One slot is longer than the other. Which of the following statements is true about those two metal slots?
(A) The two slots have the same voltage, but that voltage increases and decreases alternately with the passage of time. [17.0% picked]
(B) The voltage of the longer slot is always greater than the voltage of the shorter slot. [2.7% picked]
(C) The voltage of the longer slot is always less than the voltage of the shorter slot. [0.4% picked]
(D) There are moments during which the voltage difference between those two slots is zero. [79.8% picked]
Answer: (D) There are moments during which the voltage difference between those two slots is zero. [79.8% picked]
Why: The longer slot is "neutral," meaning that its voltage should be close to Earth ground. The shorter slot is "power" or "hot," with a voltage that varies sinusoidally in time. As the voltage of "power" fluctuates up and down about the voltage of "neutral," there are moments when the voltage of "power" is equal to the voltage of "neutral."
Problem 12:
You are sitting on your porch, listening to a radio station. Suppose that station's radio wave is traveling toward you horizontally and you are looking directly at that oncoming radio wave. If the wave's electric field is directed vertically (up and down), in which direction is the wave's magnetic field?
(A) Horizontally, left and right. [78.5% picked]
(B) Vertically, up and down. [8.1% picked]
(C) Horizontally, toward you and away from you. [13.5% picked]
(D) Because this wave already has an electric field, it has no magnetic field. [0.0% picked]
Answer: (A) Horizontally, left and right. [78.5% picked]
Why: The radio wave's electric field, magnetic field, and direction of propagation are all mutually perpendicular. Since the wave's electric field is vertical, its direction of propagation and its magnetic field must both be horizontal. With the wave coming toward you horizontally, the magnetic field must be directed horizontally left and right.
Problem 13:
The transformer has 120 turns in its primary coil and 24 turns in its secondary coil. With 120 volt AC electric power connected to the primary coil and a current of 1 ampere flowing through that primary coil, the transformer's secondary coil is supplying
(A) 24 volt AC electric power and a current of 5 amperes is flowing through that secondary coil. [80.3% picked]
(B) 24 volt AC electric power and a current of 6 amperes is flowing through that secondary coil. [14.8% picked]
(C) 5 volt AC electric power and a current of 24 amperes is flowing through that secondary coil. [2.7% picked]
(D) 5 volt AC electric power and a current of 6 amperes is flowing through that secondary coil. [2.2% picked]
Answer: (A) 24 volt AC electric power and a current of 5 amperes is flowing through that secondary coil. [80.3% picked]
Why: Since the primary coil and secondary coil experience the same electric field, the work done on charges moving through those coils must be proportional to their numbers of turns. Looking at the 120-turn primary coil, which has a voltage drop of 120 volts AC, the electric field must be doing enough negative work on the current to cause a voltage drop of 1 volt AC per turn. Current in the secondary coil must be experiencing a voltage rise of 1 volt AC per turn, so the entire 24-turn second coil has a voltage rise of 24 volts AC. The primary coil has 1 ampere of AC current flowing through it, so it is consuming 120 volts AC times 1 ampere AC, or 120 watts of electric power. The secondary coil must be supplying that same 120 watts of electric power to the secondary circuit. An AC current of 5 amperes at 24 volts AC provides 120 watts of power.
Problem 14:
One end of a AAA battery has a voltage of +0.75 volts and the other end has a voltage of –0.75 volts. Suppose you have a small positive charge located at 0 volts. What work is required to move that positive charge to each of end of the battery?
(A) Negative work is required to move the charge to the +0.75 volts end and positive work is required to move the charge to the –0.75 volts end. [28.3% picked]
(B) Negative work is required to move the charge to the +0.75 volts end and negative work is required to move the charge to the –0.75 volts end. [5.4% picked]
(C) Positive work is required to move the charge to the +0.75 volts end and negative work is required to move the charge to the –0.75 volts end. [54.7% picked]
(D) Positive work is required to move the charge to the +0.75 volts end and positive work is required to move the charge to the –0.75 volts end. [11.7% picked]
Answer: (C) Positive work is required to move the charge to the +0.75 volts end and negative work is required to move the charge to the –0.75 volts end. [54.7% picked]
Why: The higher its voltage, the more energy a positive charge has. Moving a positive charge to higher voltage thus requires positive work. Moving a positive charge to lower voltage releases energy, so it entails negative work.
Problem 15:
You have two bar magnets that are identical except that one has twice the mass of the other. The net magnetic pole of the larger magnet is
(A) equal to the net magnetic pole of the smaller magnet. [95.1% picked]
(B) half the net magnetic pole of the smaller magnet. [0.4% picked]
(C) four times the net magnetic pole of the smaller magnet. [3.1% picked]
(D) twice the net magnetic pole of the smaller magnet. [1.3% picked]
Answer: (A) equal to the net magnetic pole of the smaller magnet. [95.1% picked]
Why: All bar magnets, like every object ever seen, have zero net magnetic pole. A bar magnet has a north magnetic pole at one end and a south pole at the other end, but the sum of those two poles is exactly zero.
Problem 16:
At this moment in time, a water bottle is touching only one thing: the top of a table. The downward force the water bottle is exerting on the table right now
(A) is greater than the water bottle's weight. [2.7% picked]
(B) is greater than, less than, or equal to the water bottle's weight, depending on the situation. [41.7% picked]
(C) is equal to the water bottle's weight. [54.3% picked]
(D) is less than the water bottle's weight. [1.3% picked]
Answer: (B) is greater than, less than, or equal to the water bottle's weight, depending on the situation. [41.7% picked]
Why: Nothing is said about the bottle's motion, so it could have been dropped on the table. During that impact with the table, the bottle would exert a whole range of forces on the table. When it first came into contact with the table, it would push gently downward on the table. That force would increase as the bottle moves more deeply into the table's surface and would soon be equal to the bottle's weight. But the bottle would continue moving downward until its force on the table exceeded its weight. Overall, the bottle would bounce from table and during that bounce it would go from accelerating downward (while falling) to accelerating upward (near the bottom of the bounce) to accelerating downward (while falling for the second time).
Problem 17:
Like the back of a credit card, your ID card has a magnetic strip on it. The pattern of magnetization on that strip represents information about you. The magnetic strip is made from a material that is
(A) easy to magnetize and easy to demagnetize. [7.2% picked]
(B) easy to magnetize and difficult to demagnetize. [20.2% picked]
(C) difficult to magnetize and difficult to demagnetize. [68.2% picked]
(D) difficult to magnetize and easy to demagnetize. [4.5% picked]
Answer: (C) difficult to magnetize and difficult to demagnetize. [68.2% picked]
Why: TBA
Problem 18:
A recycling plant uses a magnet to separate steel scraps from aluminum scraps. Why does the magnet attract steel but not aluminum?
(A) Steel has magnetic domains that align with the magnetic field, but aluminum has no magnetic domains. [79.8% picked]
(B) Steel is magnetized, but aluminum is polarized. [6.7% picked]
(C) Steel is thick, but aluminum is thin. [0.0% picked]
(D) Steel is an electrical conductor, but aluminum is an electrical insulator. [13.5% picked]
Answer: (A) Steel has magnetic domains that align with the magnetic field, but aluminum has no magnetic domains. [79.8% picked]
Why: TBA
Problem 19:
A long bar magnet (a permanent magnet) has a north pole at its red-painted end and a south pole at its white-painted end. You break that magnet exactly in half, resulting in a red-painted half and a white-painted half. Which of the following correctly describes the two halves?
(A) Each half has both a north pole and a south pole, and the net pole of each half is exactly zero. [95.5% picked]
(B) Each half has both a north pole and a south pole, however, the red-painted half has a stronger north pole than south pole and the white-painted half has a stronger south pole than north pole. [2.7% picked]
(C) The red-painted half has only a north pole and the white-painted half has only a south pole. [0.4% picked]
(D) The two halves have no magnetic poles at all. [1.3% picked]
Answer: (A) Each half has both a north pole and a south pole, and the net pole of each half is exactly zero. [95.5% picked]
Why: TBA
Problem 20:
An electromagnet is a coil of wire that becomes magnetic due to electricity. What aspect of electricity makes it magnetic?
(A) Electric polarization is magnetic. [3.6% picked]
(B) Electric voltage is magnetic. [0.9% picked]
(C) Electric current is magnetic. [88.8% picked]
(D) Electric charge is magnetic. [6.7% picked]
Answer: (C) Electric current is magnetic. [88.8% picked]
Why: TBA
Problem 21:
Your roommate hammers the extension cord supplying power to your lamp and produces a short circuit: the two metal wires in the cord become electrically connected to one another. If no safety mechanisms existed, what would happen to the extension cord and your lamp?
(A) Your lamp would receive approximately zero power and the cord would become hot. [93.7% picked]
(B) Both your lamp and the cord would become hot. [2.2% picked]
(C) Your lamp would operate properly, but the cord would become hot. [4.0% picked]
(D) The cord would remain cool, but your lamp would become hot. [0.0% picked]
Answer: (A) Your lamp would receive approximately zero power and the cord would become hot. [93.7% picked]
Why: TBA
Problem 22:
As shown in the figure below, a sharp metal pin has a voltage of +1000 volts and it points toward a wide metal surface with a voltage of –1000 volts. Where in this system can you find a position where the voltage is 0 volts?
(A) To the right of the midway point. [62.8% picked]
(B) To the left of the midway point. [10.8% picked]
(C) There is no position in this system at which the voltage is 0 volts. [16.6% picked]
(D) At the midway point. [9.9% picked]
Answer: (A) To the right of the midway point. [62.8% picked] [NOTE: I originally selected (B) as the correct answer, but it is not correct.]
Why: The metal surface is huge compared to the pin, so that surface's voltage dominates most of the region between the two objects. If you were a bug located at the Midway point, the surface would appear large and the pin would appear tiny. Consequently, the voltage at the Midway point is nearly –1000 volts. It isn't until you moved quite near the pin that it would appear large enough to rival the surface. Thus the 0 volts point is located just to the left of the pin and well to the right of the Midway point.
Problem 23:
When electric power is transmitted long distances, it is usually carried through the wires by a small current at a large voltage. Power is transmitted that way because
(A) it does not require the use of a complete circuit. [0.4% picked]
(B) it does not require the use of transformers. [1.8% picked]
(C) a smaller current wastes less power in the wires. [92.8% picked]
(D) a larger voltage wastes less power in the wires. [4.9% picked]
Answer: (C) a smaller current wastes less power in the wires. [92.8% picked]
Why: TBA
Problem 24:
You are pushing a desk across a tile floor. The desk is moving in a straight line at a steady speed. Compare the force you exert on the desk to the frictional force the floor exerts on the desk. [Assume that those two forces are the only horizontal forces acting on the desk.]
(A) The force you exert desk is greater in amount than the frictional force the floor exerts on the desk. [12.6% picked]
(B) The force you exert desk is less in amount than the frictional force the floor exerts on the desk. [0.4% picked]
(C) The force you exert desk is equal to the frictional force the floor exerts on the desk. [20.6% picked]
(D) The force you exert desk is equal in amount but opposite in direction to the frictional force the floor exerts on the desk. [66.4% picked]
Answer: (D) The force you exert desk is equal in amount but opposite in direction to the frictional force the floor exerts on the desk. [66.4% picked]
Why: TBA
Problem 25:
Your hairdryer was designed to operate on 120-volt alternating current, but you are traveling in Europe and your friend connects it to 230-volt alternating current. The hairdryer quickly burns out because
(A) while the correct amount of current flows through the hairdryer's heating element, that current produces too many volts in the heating element and burns it up. [4.5% picked]
(B) while the correct amount of current flows through the hairdryer's heating element, that current alternates too rapidly and overheats the heating element. [4.5% picked]
(C) the temperature of 230-volt alternating current is much higher than the hairdryer was designed to handle and it burns up the hairdryer. [4.0% picked]
(D) the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element. [87.0% picked]
Answer: (D) the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element. [87.0% picked]
Why: TBA
Problem 26:
You have three batteries: a 12.0-volt battery (from a car) and two 1.5-volt batteries (from a flashlight). Using clips and wires, you can link these batteries together in various chains. The possible voltage rises that you can obtain with these batteries are
(A) 1.5 volts, 3.0 volts, 10.5 volts, 12.0 volts, and 13.5 volts. [4.5% picked]
(B) 1.5 volts, 3.0 volts, 12.0 volts, 13.5 volts, and 15.0 volts. [7.2% picked]
(C) 1.5 volts, 3.0 volts, 9.0 volts, 10.5 volts, 12.0 volts, 13.5 volts, and 15.0 volts. [85.7% picked]
(D) 1.5 volts and 12.0 volts. [2.7% picked]
Answer: (C) 1.5 volts, 3.0 volts, 9.0 volts, 10.5 volts, 12.0 volts, 13.5 volts, and 15.0 volts. [85.7% picked]
Why: TBA
Problem 27:
Your hairdryer is consuming 1800 watts of electric power. That power traveled to Charlottesville in high-voltage transmission wires carrying a total of 180 million watts (180,000,000 watts). What fraction of the electric charges passing through those transmission wires also pass through your toaster?
(A) 1 part in 1,000,000 [4.5% picked]
(B) 1 part in 100,000. [17.5% picked]
(C) 1 part in 10,000. [3.6% picked]
(D) zero. [74.4% picked]
Answer: (D) zero. [74.4% picked]
Why: TBA
Problem 28:
The electric field near a positive charge points
(A) toward the charge and becomes weaker with increasing distance from the charge. [7.2% picked]
(B) toward the charge and does not depend on the distance from the charge. [0.5% picked]
(C) away from the charge and becomes weaker with increasing distance from the charge. [90.5% picked]
(D) away from the charge and does not depend on the distance from the charge. [1.8% picked]
Answer: (C) away from the charge and becomes weaker with increasing distance from the charge. [90.5% picked]
Why: TBA
Problem 29:
The electricity powering your clock radio consists of a 2-ampere current that experiences a voltage drop of 5 volts. How much electric power is your clock radio consuming?
(A) 2.5 watts [15.7% picked]
(B) 5 watts [4.5% picked]
(C) 0.4 watts [4.5% picked]
(D) 10 watts [75.3% picked]
Answer: (D) 10 watts [75.3% picked]
Why: TBA
Problem 30:
Two billiard balls are rolling forward on a horizontal table, but the green ball is traveling twice as fast as the red ball. The two balls roll off the edge of the table simultaneously. The green ball hits the ground
(A) at the same time as the red ball, but the green ball lands twice as far from the table as the red ball. [89.7% picked]
(B) before the red ball. [0.0% picked]
(C) after the red ball. [1.3% picked]
(D) at the same time as the red ball, but the green ball lands four times as far from the table as the red ball. [9.0% picked]
Answer: (A) at the same time as the red ball, but the green ball lands twice as far from the table as the red ball. [89.7% picked]
Why: TBA
