Physics 1060 - Spring 2017 - Midterm Exam 2 - Posted Version with Solutions
Problem 1:
The secondary coil of a large transformer near the street supplies your home with 120 volt alternating current. The transformer's primary coil has 50 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 6000 watts. The transformer's primary coil is being supplied with
(A) 6,000 volt alternating current and the current in that coil is 1 ampere. [65.4% picked]
(B) 6,000 volt alternating current and the current in that coil is 50 amperes [22.5% picked]
(C) 120 volt alternating current and the current in that coil is 50 amperes. [6.9% picked]
(D) 50 volt alternating current and the current in that coil is 120 amperes. [5.2% picked]
Answer: (A) 6,000 volt alternating current and the current in that coil is 1 ampere. [65.4% picked]
Why: TBA
Problem 2:
A racquetball court has rigid, immovable walls. You hit the rubber ball forward, toward the front wall of the court, and the ball bounces back at you. Before hitting the wall, the ball carried both energy and forward momentum. During its bounce, the ball transferred to the wall
(A) all of its energy and all its forward momentum. [7.4% picked]
(B) all of its energy and no momentum. [5.6% picked]
(C) no energy and more forward momentum than it had before the bounce. [55.8% picked]
(D) no energy and all of its forward momentum. [31.2% picked]
Answer: (C) no energy and more forward momentum than it had before the bounce. [55.8% picked]
Why: TBA
Problem 3:
You accidently pinch the cord of your desk lamp under your rocking chair and cut through one of the two wires in the lamp's cord. Only one wire now connects the lamp to the electric socket. If you switch on the lamp,
(A) 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. [0.0% picked]
(B) the normal amount of current will flow through the lamp, but only half as often, so the lamp will glow at half its normal brightness. [2.2% picked]
(C) no current will flow through either wire and the lamp will remain dark. [97.0% picked]
(D) the normal amount of current will flow through the one remaining wire and the lamp will glow at half its normal brightness. [0.9% picked]
Answer: (C) no current will flow through either wire and the lamp will remain dark. [97.0% picked]
Why: TBA
Problem 4:
The voltage at the start of an ordinary wire is 120 volts and a current of 10 amperes enters the wire. The voltage at the end of the wire is
(A) 120 volts and a current of 10 amperes exits the wire. [3.5% picked]
(B) more than 120 volts and a current of less than 10 amperes exits the wire. [1.3% picked]
(C) more than 120 volts and a current of 10 amperes exits the wire. [1.7% picked]
(D) less than 120 volts and a current of 10 amperes exits the wire. [93.5% picked]
Answer: (D) less than 120 volts and a current of 10 amperes exits the wire. [93.5% picked]
Why: TBA
Problem 5:
If you put a magnetic compass in a uniform magnetic field, the compass needle will experience a
(A) torque no matter which way the needle is pointing. [3.9% picked]
(B) force in the direction of the magnetic field. [6.5% picked]
(C) force in the direction opposite the magnetic field. [0.4% picked]
(D) torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field. [89.2% picked]
Answer: (D) torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field. [89.2% picked]
Why: TBA
Problem 6:
Which one of the following objects emits an electromagnetic wave that travels across the universe?
(A) An electrically charged ball that is traveling at constant velocity on top of a metal truck. [2.6% picked]
(B) An electrically charged ball that is motionless at the top of a tall metal building. [5.6% picked]
(C) An electrically charged ball that is traveling at constant velocity inside a glass elevator. [1.3% picked]
(D) An electrically charged ball on top of a flagpole that is waving back and forth in the wind. [90.5% picked]
Answer: (D) An electrically charged ball on top of a flagpole that is waving back and forth in the wind. [90.5% picked]
Why: TBA
Problem 7:
A sharp metal pin has a voltage of +1000 volts and it points toward a wide metal surface with a voltage of –1000 volts (see figure). What is the voltage midway between the surface and the pin?
(A) 0 volts. [40.7% picked]
(B) A positive voltage less than +1000 volts. [22.9% picked]
(C) A positive voltage greater than +1000 volts. [3.9% picked]
(D) A negative voltage. [32.5% picked]
Answer: (D) A negative voltage. [32.5% picked]
Why: TBA
Problem 8:
A battery charger recharges a battery by
(A) pushing current through the battery from its negative terminal to its positive terminal. [9.5% picked]
(B) pushing current through the battery from its positive terminal to its negative terminal. [89.2% picked]
(C) connecting a wire between the battery's positive terminal and its negative terminal. [0.9% picked]
(D) removing positive charges from the battery's positive terminal and negative charges from its negative terminal. [0.4% picked]
Answer: (B) pushing current through the battery from its positive terminal to its negative terminal. [89.2% picked]
Why: TBA
Problem 9:
When they are far away from any external magnetic influences, two blocks of pure iron exert no magnetic forces on one another. Why not?
(A) Each iron block contains many highly magnetic domains, but those domains are oriented in semi-random directions. The domains' magnetic fields cancel so each block appears non-magnetic. [77.5% picked]
(B) Iron is a non-magnetic metal, meaning that it has no intrinsic magnetism, even at the atomic or molecular scale. [6.9% picked]
(C) Each iron block is highly magnetic but its north pole is equally strong as its south pole. The attractive forces between opposite poles of the two blocks are therefore cancelled by the repulsive forces between like poles of the two blocks. [7.4% picked]
(D) Each iron block contains as many north poles as it contains south poles. The net magnetic pole of each block is therefore zero and the blocks are completely non-magnetic. [8.2% picked]
Answer: (A) Each iron block contains many highly magnetic domains, but those domains are oriented in semi-random directions. The domains' magnetic fields cancel so each block appears non-magnetic. [77.5% picked]
Why: TBA
Problem 10:
Radio and microwave devices use tank circuits to help them move enough electric charge to produce strong electromagnetic waves. At its most basic level, a tank circuit consists of a capacitor (two surfaces that can store separated electric charge) and an inductor (an electromagnet that can carry current). When a tank circuit is helping a device produce an electromagnetic wave,
(A) the tank circuit carries current only in one direction so that the current flows up the antenna and becomes an electromagnetic wave. [2.2% picked]
(B) the tank circuit is accelerating up and down rhythmically, in sync with the electromagnetic wave. [8.2% picked]
(C) energy in the tank circuit is shifting back and forth rhythmically between the capacitor's electric field and the inductor's magnetic field. [88.7% picked]
(D) the tank circuit is turning AC electric power into DC electric power and thereby strengthening the electromagnetic wave. [0.9% picked]
Answer: (C) energy in the tank circuit is shifting back and forth rhythmically between the capacitor's electric field and the inductor's magnetic field. [88.7% picked]
Why: TBA
Problem 11:
A typical radio wave that is traveling from Charlottesville to Richmond has a vertical electric field and horizontal magnetic field. Suppose we used some filtering system to eliminate the wave's magnetic field, but made no effort to get rid of its electric field. What would happen to the wave?
(A) The wave would affect only vertical antennas in Richmond; without its horizontal magnetic field it would no longer be able to affect horizontal antennas. [2.6% picked]
(B) The wave could not exist without its magnetic field. [94.8% picked]
(C) The wave would carry only half as much power to Richmond as before we eliminated its magnetic field. [1.3% picked]
(D) The wave's polarization would change from horizontal to vertical. [1.3% picked]
Answer: (B) The wave could not exist without its magnetic field. [94.8% picked]
Why: TBA
Problem 12:
You are boating on a large lake and you are talking to a friend by radio. Each of you holds a small radio transmitter-receiver with its antenna pointing straight up toward the sky. If your friend is directly south of you, along which directions do the electric and magnetic fields of your friend's radio wave point?
(A) The electric field points north-south and the magnetic field points up-down. [0.9% picked]
(B) The electric field points up-down and the magnetic field points north-south. [12.1% picked]
(C) The electric field points up-down and the magnetic field points east-west. [83.1% picked]
(D) The electric field points east-west and the magnetic field points up-down. [3.9% picked]
Answer: (C) The electric field points up-down and the magnetic field points east-west. [83.1% picked]
Why: TBA
Problem 13:
An FM radio station has an official frequency of 102 MHz (102 million cycles per second) and plays music you like. What aspect of the station's radio wave is conveying the information your receiver needs to reproduce the music?
(A) The time interval between the radio wave's electric field peaks varies slightly to represent the music's air pressure variations. [85.7% picked]
(B) The radio wave's electric field varies in direction, toward you and away from you, to represent fluctuations up and down in the music's air pressure. [3.5% picked]
(C) The radio wave varies in height, moving up and down slightly, to represent fluctuations up and down in the music's air pressure. [7.4% picked]
(D) The radio wave's energy varies slightly between electric energy and magnetic energy to represent the music's air pressure fluctuations. [3.5% picked]
Answer: (A) The time interval between the radio wave's electric field peaks varies slightly to represent the music's air pressure variations. [85.7% picked]
Why: TBA
Problem 14:
You hold a positively charged stick near an electrically neutral glass ball and then near an electrically neutral metal ball. What force does the charged stick experience in these two cases?
(A) The charged stick is attracted to the glass ball but it is repelled by the metal ball. [7.4% picked]
(B) The charged stick is attracted to the metal ball but not the glass ball. [38.5% picked]
(C) The charged stick is attracted to both the glass ball and the metal ball. [45.0% picked]
(D) The charged stick is attracted to the glass ball but not to the metal ball. [9.1% picked]
Answer: (C) The charged stick is attracted to both the glass ball and the metal ball. [45.0% picked]
Why: TBA
Problem 15:
A particular AM radio station transmits a radio wave with a wavelength of 300 meters. It uses what is known as a simple monopole antenna: a vertical metal mast that rises upward above a horizontal conducting surface. The conducting surface below the mast is the ground itself. For that mast to be resonant at the station's transmission frequency and as efficient as possible at emitting its radio wave, the mast is approximately
(A) 300 meters tall. [2.6% picked]
(B) 75 meters tall. [86.1% picked]
(C) 150 meters tall. [10.4% picked]
(D) 3 meters in diameter. [0.9% picked]
Answer: (B) 75 meters tall. [86.1% picked]
Why: TBA
Problem 16:
When you connect an electromagnet to a source of alternating current, its magnetization reverses direction repeatedly. An electric charge near that AC electromagnet
(A) experiences a force directly from the AC electromagnet's magnetic field. [5.6% picked]
(B) experiences zero force because the AC electromagnet's magnetic field averages to zero. [6.1% picked]
(C) experiences a force because the AC electromagnet's changing magnetic field produces an electric field. [85.3% picked]
(D) experiences zero force because it is not affected by magnetic fields. [3.0% picked]
Answer: (C) experiences a force because the AC electromagnet's changing magnetic field produces an electric field. [85.3% picked]
Why: TBA
Problem 17:
When 120 volt AC power is connected to the primary coil of a particular transformer, the secondary coil of that transformer supplies 30 volt AC power to its secondary circuit. Suppose you modify the transformer's secondary coil, doubling the number of turns in that secondary coil. How does that alteration affect the coil and secondary circuit?
(A) Twice as much current now flows through the secondary coil. The coil now supplies 15 volt AC power to its secondary circuit. [2.6% picked]
(B) Each charge moving through the secondary coil now has twice as much work done on it by the transformer's magnetic field. The coil now supplies 60 volt AC power to its secondary circuit. [19.5% picked]
(C) Twice as much current now flows through the secondary coil. The coil still supplies 30 volt AC power to its secondary circuit. [0.9% picked]
(D) Each charge moving through the secondary coil now has twice as much work done on it by the transformer's electric field. The coil now supplies 60 volt AC power to its secondary circuit. [77.1% picked]
Answer: (D) Each charge moving through the secondary coil now has twice as much work done on it by the transformer's electric field. The coil now supplies 60 volt AC power to its secondary circuit. [77.1% picked]
Why: TBA
Problem 18:
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) the correct amount of current flows through the hairdryer's heating element, but that current produces too many volts in the heating element and burns it up. [5.2% picked]
(B) the temperature of 230 volt alternating current is much higher than the hairdryer was designed to handle and it burns up the hairdryer. [5.6% picked]
(C) the correct amount of current flows through the hairdryer's heating element, but that current alternates too rapidly and overheats the heating element. [3.9% picked]
(D) the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element. [85.3% picked]
Answer: (D) the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element. [85.3% picked]
Why: TBA
Problem 19:
The electric power grid in the United States uses alternating current because
(A) alternating current is less likely to cause an electrical fire than direct current. [1.7% picked]
(B) transformers can move power from one circuit to another only if those circuits are carrying alternating currents. [94.4% picked]
(C) an alternating current carries more electric power than a direct current. [1.7% picked]
(D) 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]
Answer: (B) transformers can move power from one circuit to another only if those circuits are carrying alternating currents. [94.4% picked]
Why: TBA
Problem 20:
The current passing through a metal wire is proportional to
(A) the voltage difference between the two ends of the wire. [89.6% picked]
(B) one divided by the net charge of that wire. [1.7% picked]
(C) the net charge of that wire. [2.6% picked]
(D) one divided by the voltage difference between the two ends of the wire. [6.1% picked]
Answer: (A) the voltage difference between the two ends of the wire. [89.6% picked]
Why: TBA
Problem 21:
A twist-tie is a long, thin, sharp metal wire wrapped in paper. If you place a twist-tie in a microwave oven and turn the oven on, what will happen to the twist-tie?
(A) The twist tie will not be affected by the microwaves because it contains no water. [0.4% picked]
(B) The twist tie will remain cool, but electric charge will spray out of its sharp ends. [1.7% picked]
(C) An electric current will flow through it, heating it up, and electric charge will also spray out of its sharp ends. [97.0% picked]
(D) An electric current will flow through it, heating it up, but no charge will leave the wire. [0.9% picked]
Answer: (C) An electric current will flow through it, heating it up, and electric charge will also spray out of its sharp ends. [97.0% picked]
Why: TBA
Problem 22:
You are playing basketball and have managed to get your hands on the ball that an opponent is still holding. At this moment, the two of you are pulling the ball in opposite directions and you seem to be winning: the ball is moving toward you at constant velocity. At this moment, the net force on the ball
(A) points toward you. [5.2% picked]
(B) is zero. [94.4% picked]
(C) points straight down. [0.4% picked]
(D) points toward your opponent. [0.0% picked]
Answer: (B) is zero. [94.4% picked]
Why: TBA
Problem 23:
You have two bar magnets that are identical except that one is 10 times as heavy as the other. The net magnetic pole of the larger magnet is
(A) 10 times the net magnetic pole of the smaller magnet. [3.5% picked]
(B) 0.1 times the net magnetic pole of the smaller magnet. [0.4% picked]
(C) 100 times the net magnetic pole of the smaller magnet. [1.7% picked]
(D) equal to the net magnetic pole of the smaller magnet. [94.4% picked]
Answer: (D) equal to the net magnetic pole of the smaller magnet. [94.4% picked]
Why: TBA
Problem 24:
You are listening to a weak radio station as you walk through a city. You find that the reception depends on your location and that occasionally you get remarkably good reception. How are the structures around you acting to give you better reception than you would get if those structures weren't there?
(A) The iron in those structures is amplifying the radio wave from the station, strengthening its electric field. [6.9% picked]
(B) The iron in those structures is amplifying the radio wave from the station, strengthening its magnetic field. [9.1% picked]
(C) Portions of the radio wave that reflect from those structures are producing constructive interference at the antenna of your radio receiver. [81.8% picked]
(D) The structures are slowing the radio wave, giving your receiver more time to gather the wave with its antenna. [2.2% picked]
Answer: (C) Portions of the radio wave that reflect from those structures are producing constructive interference at the antenna of your radio receiver. [81.8% picked]
Why: TBA
Problem 25:
You are sitting in the lobby of a hotel, watching 4 people who are wearing backpacks. Which person is doing work on their backpack at this moment?
(A) The person who is walking at constant velocity across the lobby floor. [14.7% picked]
(B) The person at the reception desk, who is lowering a backpack toward the floor at constant velocity. [37.7% picked]
(C) The person who is riding upward at constant velocity in a glass elevator. [45.0% picked]
(D) The person who is riding down an escalator at constant velocity toward the lobby. [2.6% picked]
Answer: (C) The person who is riding upward at constant velocity in a glass elevator. [45.0% picked]
Why: TBA
Problem 26:
Electric power is transmitted long distances as a relatively small current at a relatively high voltage. Why is that arrangement used?
(A) Reducing the current in each wire reduces the resistances of those wires and causes their voltages to increase. [4.8% picked]
(B) Decreasing the current in each wire reduces the power those wires consume. Increasing the voltage difference between the wires allows those small currents to transmit large amounts of power. [82.3% picked]
(C) Increasing the voltage difference between the wires reduces the resistance of those wires and causes their currents to decrease. [2.2% picked]
(D) Increase the voltage differences between the wires reduces the power wasted by those wires. Decreasing the current in those wires allows those large voltages to transmit large amounts of power. [10.8% picked]
Answer: (B) Decreasing the current in each wire reduces the power those wires consume. Increasing the voltage difference between the wires allows those small currents to transmit large amounts of power. [82.3% picked]
Why: TBA
Problem 27:
Compared to an ordinary AA battery, an ordinary D battery has
(A) the same voltage and can supply the same total electrical energy. [0.9% picked]
(B) a higher voltage but can supply the same total electrical energy. [6.1% picked]
(C) the same voltage but can supply more total electrical energy. [79.2% picked]
(D) a higher voltage and can supply more total electrical energy. [13.9% picked]
Answer: (C) the same voltage but can supply more total electrical energy. [79.2% picked]
Why: TBA
Problem 28:
A junkyard uses a huge electromagnet to pick up old steel automobiles. That electromagnet only binds magnetically to steel when it is supplied with electricity. What is it about electricity that causes that attraction between the electromagnet and the steel?
(A) Electric charges are magnetic and produce magnetic fields. [4.3% picked]
(B) Differences in voltage produce magnetic field. [0.9% picked]
(C) Electric currents produce magnetic fields. [94.8% picked]
(D) Separated positive and negative charges produce magnetic fields. [0.0% picked]
Answer: (C) Electric currents produce magnetic fields. [94.8% picked]
Why: TBA
Problem 29:
When you place a ceramic mug of coffee in a microwave oven and turn that oven on, the coffee's temperature increases. What is it about coffee that causes it to heat in the microwave oven?
(A) Coffee contains water molecules and each water molecule has a large magnetic dipole (a north end and a south end). [10.0% picked]
(B) Coffee has a positive electric charge and the microwaves magnetize that electric charge. [0.4% picked]
(C) The microwave oven blows hot air across the mug of coffee and allows heat to flow into the coffee. [0.4% picked]
(D) Coffee contains water molecules and each water molecule has a large electric dipole (a positive end and a negative end). [89.2% picked]
Answer: (D) Coffee contains water molecules and each water molecule has a large electric dipole (a positive end and a negative end). [89.2% picked]
Why: TBA
Problem 30:
Your pet hamster chews through 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 operate properly, but the cord would become hot. [4.3% picked]
(B) The cord would remain cool, but your lamp would become hot. [0.4% picked]
(C) Both your lamp and the cord would become hot. [3.5% picked]
(D) Your lamp would receive approximately zero power and the cord would become hot. [91.8% picked]
Answer: (D) Your lamp would receive approximately zero power and the cord would become hot. [91.8% picked]
Why: TBA
