Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on the midterm examination.

Problem 1:

For an appliance to receive and consume electric power,

(A) a current must flow through it and it must lower the voltage of that current.
(B) it must polarize the current with its magnetic field.
(C) a current must flow through it and it must raise the voltage of that current.
(D) it must magnetize the current with its electric field.
Answer: (A) a current must flow through it and it must lower the voltage of that current.
Why: Power arrives in the energy carried by charges. If no charges are arriving (i.e. no current) or if those charges give up no energy (i.e. no voltage drop), then no power is being received and consumed by the appliance.

Suppose you cut one of the two wires leading from the electric outlet to your incandescent desk lamp and then used a diode to bridge the gap in that wire. Current might still flow when the lamp is turned on, but now that current would have to pass through the diode (see Figure #3 on the back page). With the diode in place,

(A) only half the normal amount of current will flow all the time, so the lamp will glow at about half its normal brightness.
(B) current will flow as usual and the lamp will glow at its normal brightness.
(C) a normal amount of current will flow but only half the time, so the lamp will glow at about half its normal brightness.
(D) no current will flow because diodes don't work with AC. The lamp will be dark.
Answer: (C) a normal amount of current will flow but only half the time, so the lamp will glow at about half its normal brightness.
Why: The diode allows current to flow only during one half of each alternating current cycle--when that current is trying to go the correct direction through the diode. During those half cycles when current is permitted to flow, it flows normally. Since power is reaching the filament of the lamp only half the time, it receives half the usual power and glows about half its usual brightness.

You have covered a grounded metal surface with a layer of photoconductor. Working in the dark, you sprinkle negative charge onto this surface. If you now expose only the left half of the photoconductor to light, you will find that

(A) the left half becomes neutral while the right half remains negatively charged.
(B) nothing happens because there is no changing magnetic field.
(C) negative charge flows from the right side of the photoconductor to the left and both sides become neutral.
(D) the right half becomes neutral while the left half remains negatively charged.
Answer: (A) the left half becomes neutral while the right half remains negatively charged.
Why: Wherever light strikes the photoconductor, it transforms from an insulator into a conductor. The charge then migrates through it and leaves its surface. By exposing the left half of the photoconductor to light, you allow its local charge to leave and it becomes neutral.

High voltage power lines are usually supported by glass insulators. An electric current can't flow through a piece of glass because

(A) glass does not contain any electrically charged particles.
(B) glass contains only positively charged particles.
(C) the electrons in the glass completely fill its valence levels and can't shift from one level to another to transport charge through the glass.
(D) glass contains only negatively charged particles.
Answer: (C) the electrons in the glass completely fill its valence levels and can't shift from one level to another to transport charge through the glass.
Why: The electrons in an insulator such as glass completely fill the valence levels. Since only two electrons, one spin-up and one spin-down, can be in any level, the valence can't accommodate any shift in electrons from one level to another. The electrons can't respond to any electric fields and they can't conduct electricity.

You're in the gym lifting weights up and down above your head. When are you doing (positive) work on the weights?

(A) When you hold them still but not when you raise them up or lower them down.
(B) When you raise them up but not when you hold them still or lower them down.
(C) When you raise them up or lower them down but not when you hold them still.
(D) When you lower them down but not when you hold them still or raise them up.
Answer: (B) When you raise them up but not when you hold them still or lower them down.
Why: For you to do work on the weights, they must move in the direction you push them. Since you are pushing up on the weights to keep them from falling, you only do work on them when you move them upward.

You shoot an arrow at a target and it heads for the bullseye. After the arrow has left the bow, which of the following most accurately describes the force pushing it forward during its flight?

(A) The arrow experiences no forward force.
(B) The forward force is constant all the way to the bullseye.
(C) The forward force increases to the midway point and then decreases to zero just as the arrow hits the bullseye.
(D) The forward force decreases steadily, reaching zero just as the arrow hits the bullseye.
Answer: (A) The arrow experiences no forward force.
Why: The arrow is coasting forward because of its inertia. It is not being pushed forward at all.

A resistor is essentially a poor conductor of electricity. When you send current through a resistor, that current always experiences a voltage drop, never a voltage rise. One way to understand this effect is that

(A) more current leaves a resistor than enters it, so the voltage must go down to compensate.
(B) it's easy to turn electric energy into thermal energy but not the other way around.
(C) less current leaves a resistor than enters it, so the voltage must go down as well.
(D) current can only flow forward through a resistor, never backward.
Answer: (B) it's easy to turn electric energy into thermal energy but not the other way around.
Why: Like all poor conductors, a resistor extracts energy from the charges that move through it so that their voltage drops. This energy is converted into thermal energy, so that the resistor becomes warm. For the charges passing through a resistor to experience a voltage rise, the resistor would have to give them energy. The only energy a resistor has to work with is its thermal energy, which it cannot convert into electric energy.

You were heading forward in your car before coming to a complete stop at a red light. The careless driver of the car behind you fails to stop and his car crashes into your car from behind. You suddenly find your head pressed deeply into the elastic cushion of your seat's headrest. During the period when your head is pressed into the cushion, the net force on your head is

(A) zero and it is not accelerating.
(B) backward and its acceleration is backward.
(C) backward and its acceleration is forward.
(D) forward and its acceleration is forward.
Answer: (D) forward and its acceleration is forward.
Why: While your head is denting the cushion, the two objects are pushing on one another--your head pushes back on the cushion to dent it and the cushion pushes forward on your head. That forward force on your head is the only unbalanced force on it (your neck balances out your head's weight). Thus the net force on your head is forward and it accelerates in the direction of that net force: forward.

A helicopter is hovering motionless above a disabled boat, while rescue workers use a rope to lift an injured sailor. While that sailor is being lifted upward, the net force on the motionless helicopter is

(A) downward and equal to the sailor's weight.
(B) zero.
(C) upward and equal to the sailor's weight.
(D) downward and equal to the sailor's weight plus the helicopter's weight.
Answer: (B) zero.
Why: The helicopter remains motionless, so it isn't accelerating. If it isn't accelerating, the net force on it must be zero.

A magnet factory is making bar magnets, each about the size and shape of an ordinary ruler. After forming each bar of metal, that bar must be magnetized. The bar is placed in a coil of wire and a huge pulse of current is sent through the coil. During the pulse, current is only sent in one direction through the coil-it's a pulse of direct current or DC. If, instead, the current reversed directions rapidly during the pulse-a pulse of alternating current or AC-then

(A) the bar magnet would end up with two south poles and no north poles.
(B) the poles of the bar magnet would also reverse rapidly and the bar wouldn't acquire a strong magnetization.
(C) the bar would become overmagnetized because AC is much more magnetic than DC.
(D) the bar magnet would end up with two north poles and no south poles.
Answer: (B) the poles of the bar magnet would also reverse rapidly and the bar wouldn't acquire a strong magnetization.
Why: The domains in the bar magnet would shift back and forth so that its magnetization would follow the rapidly reversing field from the alternating current in the coil. As this reversing field became weaker, the domains would gradually stop reversing and would be left somewhat randomized. The overall magnetization of the bar would be relatively weak because all its domains would not be aligned with one another.

A capacitor consists of two metal surfaces separated by an insulating layer. A new capacitor has no charge on either of its surfaces. If you begin transferring charge from one surface to the other, the first surface becomes negatively charged while the second surface becomes positively charged. As you transfer the charge, the voltage of

(A) the positively charged surface increases and the energy stored in the capacitor increases.
(B) the positively charged surface decreases but the energy stored in the capacitor increases.
(C) both surfaces increase but the energy stored in the capacitor decreases.
(D) both surfaces increase and the energy stored in the capacitor stays constant.
Answer: (A) the positively charged surface increases and the energy stored in the capacitor increases.
Why: It takes energy to transfer positive charge from the negative surface to the positive surface. Each transfer is harder than the last because there are more and more positive charges to repel each new positive charge being transferred. The energy per charge goes up and thus the voltage rises. The energy used for each transfer is stored in the capacitor, so that its energy increases.

When positive charge on its gate turns the channel of an n-channel MOSFET into n-type semiconductor, all three parts of the MOSFET (source, channel, and drain) are n-type. As a result,

(A) the MOSFET becomes highly magnetic, with its north pole on its source and its south pole on its drain.
(B) the MOSFET becomes highly magnetic, with its south pole on its source and its north pole on its drain.
(C) current flows easily through the MOSFET, from source to drain.
(D) no current can flow through the MOSFET.
Answer: (C) current flows easily through the MOSFET, from source to drain.
Why: When the channel portion of the MOSFET has been electrically transformed into n-type material, the entire MOSFET is n-type. Electrons in the conduction levels can thus move freely throughout the MOSFET structure because there are no p-n junctions left and no depletion regions. The MOSFET conducts electricity.

A transformer provides the 12,000 volts needed to operate the neon sign in a local convenience store. When AC current flows through the transformer's primary coil and experiences a voltage drop of 120 volts, current flows through the transformer's secondary coil and experiences a voltage rise of 12,000 volts (see Figure #4 on the back page). Based on this observation, it's likely that

(A) the currents in the two coils are about equal.
(B) the secondary coil has about 100 times as many turns in it as the primary coil.
(C) the current in the secondary coil is about 100 times as large as the current in the primary coil.
(D) the primary coil has about 100 times as many turns in it as the secondary coil.
Answer: (B) the secondary coil has about 100 times as many turns in it as the primary coil.
Why: Since the charges in the secondary coil pick up 100 times as much energy as the charges in the primary coil lose, the charges in the secondary coil must complete 100 times as many trips around the magnetic core.

Your flashlight has three identical 1.5 volt batteries in it, arranged in a chain to give a total of 4.5 volts (see Figure #2 on the back page). Current passes first through battery (a), then through battery (b), then through battery (c), on its way to the bulb. When you operate the flashlight, the batteries provide power to the current and they gradually use up their chemical potential energy. Which battery will run out of chemical potential energy first?

(A) All three will run out at the same time.
(B) Battery (a) will run out first.
(C) Battery (b) will run out first.
(D) Battery (c) will run out first.
Answer: (A) All three will run out at the same time.
Why: The same current flows through all three batteries so that each battery transfers exactly the same number of charges each second. Since each battery gives the charges it transfers the same amount of energy, the three batteries get used up at exactly the same rate and die simultaneously.

Why can't you float one permanent magnet directly above another permanent magnet indefinitely by turning their north poles toward one another?

(A) Two north poles attract one another and the two magnets will pull together until they touch.
(B) That arrangement is unstable-the upper magnet will fall to the side or flip over.
(C) The repulsive forces between magnets cannot overcome the gravitational forces between them, so the upper magnet cannot float.
(D) Because they are stationary, the two magnets exert no forces on one another and the top magnet falls.
Answer: (B) That arrangement is unstable-the upper magnet will fall to the side or flip over.
Why: It can be proven mathematically that a stationary permanent magnet cannot float above a second stationary permanent magnet, no matter how hard you try. The arrangement is always unstable.

Your new toaster has two separate toasting units, each of which consumes 600 watts of power when it's in use. When you operate one unit, a current of 5 amperes flows through the wiring in your home and the wires waste about 1 watt of power handling that current. If you operate both toasting units at once, your toaster consumes 1200 watts and the current flowing through the wiring in your home doubles to 10 amperes. How much power will the wires in your home waste now?

(A) about 0.5 watts.
(B) about 4 watts.
(C) about 2 watts.
(D) about 1 watt.
Answer: (B) about 4 watts.
Why: Doubling the current in the wires causes a quadrupling of the power consumption. That's because the voltage drop in the wires doubles when you double the current, so that twice as many charges each second are losing twice as much energy each, for an overall factor of 4 increase in energy loss per second.

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 turn on the lamp,

(A) the normal amount of current will flow through both wires and the lamp will glow at its normal brightness.
(B) the normal amount of current will flow through the one remaining wire and the lamp will glow at half its normal brightness.
(C) no current will flow through either wire and the lamp will remain dark.
(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.
Answer: (C) no current will flow through either wire and the lamp will remain dark.
Why: With only one wire connecting the lamp to the power company, the circuit is incomplete. Charges cannot enter of leave your lamp without giving it a net charge, so they stop flowing. No current reaches the lamp, so it remains dark.

You are jumping on a trampoline and it occurs to you that energy is being transferred as you bounce. (...sounds like your usual thoughts, doesn't it?) You realize that sometimes you do work on the trampoline and sometimes it does work on you. The time when you do work on the trampoline is

(A) whenever you are touching its surface.
(B) when you are touching its surface and that surface is moving downward.
(C) when you are touching its surface and that surface is moving upward.
(D) during the second half of the trampoline's movement downward and the first half of the trampoline's movement upward.
Answer: (B) when you are touching its surface and that surface is moving downward.
Why: To do work on the trampoline, you have to push down on its surface and that surface must move in the direction of your force--downward.

You are throwing a ball straight up and then catching it as it returns to your hand. When the ball leaves your hand, its momentum is in the upward direction but when it returns to your hand, its momentum is in the downward direction. During its flight above your hand, what happens to the ball's initial upward momentum?

(A) The upward momentum is converted into gravitational potential energy.
(B) The upward momentum is converted into kinetic energy.
(C) The upward momentum is converted into thermal energy.
(D) The upward momentum is transferred to the earth.
Answer: (D) The upward momentum is transferred to the earth.
Why: As the ball rises, the earth pulls down on it and it pulls up on the earth. This pulling gradually transfers the ball's upward momentum to the earth and the ball's upward velocity diminishes.

Suppose that you reverse the two batteries in an ordinary flashlight and that they make good contact with the flashlight's wires. If you now turn on the flashlight,

(A) current will run in its usual direction through the flashlight's circuit but the bulb will remain dark.
(B) no current will run through the flashlight's circuit but the bulb will light up normally.
(C) current will run backward through the flashlight's circuit but the bulb will light up normally.
(D) no current will run through the flashlight's circuit and the bulb will remain dark.
Answer: (C) current will run backward through the flashlight's circuit but the bulb will light up normally.
Why: Reversing the batteries causes them to pump charge in the opposite direction. Since the circuit is still intact and since it contains nothing that prevents current from flowing backward, the current does flow backward through the circuit. The light bulb doesn't care which way current flows through it and lights up normally.

The north pole of a permanent magnet is clinging to the front surface of your steel refrigerator, so the refrigerator clearly has a south pole at its surface. If you flip the permanent magnet over, so that its south pole faces the refrigerator, the refrigerator will

(A) keep a south pole at its surface and attract the permanent magnet.
(B) place a north pole at its surface and attract the permanent magnet.
(C) place a north pole at its surface and repel the permanent magnet.
(D) keep a south pole at its surface and repel the permanent magnet.
Answer: (B) place a north pole at its surface and attract the permanent magnet.
Why: The permanent magnet magnetically polarizes the steel surface of the refrigerator. If you put the north pole of a permanent magnet near the steel, the steel's domains shift to point a south pole toward that magnet. If you put the south pole of the permanent magnet near the steel, the domains will shift again to point a north pole toward the magnet. In any case, the permanent magnet is attracted to the magnetically polarized steel.

Which of the following fields push on a stationary electron?

(A) electric and gravitational fields but not magnetic fields.
(B) gravitational fields but not electric or magnetic fields.
(C) electric, magnetic, and gravitational fields.
(D) magnetic and gravitational fields but not electric fields.
Answer: (A) electric and gravitational fields but not magnetic fields.
Why: An electric field always pushes on an electric charge, but a magnetic field does not push on a stationary electric charge. Gravity pushes on everything.

Most commercial airliners have static dissipaters on their wingtips (see Figure #5 on the back page). These sharp metal spikes extend from the end of each metal wing and point toward the rear of the plane. Suppose a plane had just flown through a negatively charged cloud and acquired a large negative charge. It's now flying through neutral air. Which of the following should you expect to happen near the static dissipaters?

(A) Any charge on the dissipaters will flow toward the wings, leaving the dissipaters neutral.
(B) A corona discharge at the dissipater tips will spray negative charge into the air.
(C) A corona discharge at the dissipater tips will spray positive charge into the air.
(D) Nothing, because the air is neutral.
Answer: (B) A corona discharge at the dissipater tips will spray negative charge into the air.
Why: The negative charge on the airplane will push a substantial amount of negative charge onto the tip of a dissipater. There the closely packed negative charges will repel one another so strongly that they will push each other into the air as a corona discharge.

When you drop a strong magnet through the center of a copper pipe, the magnet

(A) descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet.
(B) descends slowly because it is attracted to the magnetic copper metal.
(C) descends rapidly because its motion causes currents to flow in the pipe and those currents attract the magnet.
(D) falls at the usual rate because copper metal is nonmagnetic.
Answer: (A) descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet.
Why: The falling magnet experiences a magnetic drag force--its motion causes electric currents to flow in the copper pipe and, according to Lenz's law, these currents exert repulsive magnetic forces on the falling magnet. The magnet has trouble falling through the pipe and descends slowly.

You are watching children play a game of tug-o-war with an old plastic clothesline. The two teams are pulling at opposite ends of the cord and each team is trying to drag the other team into a mud puddle that lies between them. After a few minutes without progress, the team on the right suddenly pulls hard toward the right. The team on the left has anticipated this threat and is able to keep their end of the rope from moving at all. The right end of the rope stretches toward the right and the rope breaks. It took energy to breaking the rope and that energy was provided by

(A) the team on the right.
(B) both teams.
(C) neither team. It was instead provided by chemical potential energy in the rope itself.
(D) the team on the left.
Answer: (A) the team on the right.
Why: Since the team on the left didn't move, they neither did work nor had work done on them. The team on the right exerted a rightward force on the rope and the rope end moved right--therefore they did (positive) work on the rope. It was this work that broke the rope.

Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.

Problem 1:

Ensuring fair play at the Olympics has never been easy, both on the field and off. With your tremendous understanding of physics, you've received many lucrative offers to help the scoundrels, but you've held fast to the traditions of the UVa honor system and refused them all. Instead, you have become the world's foremost authority on sneaky Olympic tricks and have foiled dozens of evil plots. Here are a few of your most famous cases:

(A) The Tomanian cycling team once placed a series of strong magnets beneath the bicycle track and slowed the progress of any athlete riding an aluminum alloy bicycle. You discovered the magnets and fingered the Tomanian team because they were the only people riding bicycles made entirely of plastics. Why did the magnets slow the aluminum bicycles but not the plastic ones?
Answer: Only the aluminum bicycles can conduct currents and become magnetic when they move past the track's magnets.
Why: Conducting objects that move past strong magnets experience electric fields and have currents induced in them. These currents render the objects magnetic and they experience magnetic drag forces that slow their motions.

(B) The Freedonian track team once tried to lower their times in the 5000 meter run by installing air jets all the way around the track. These jets were aimed so that they would always blow each runner forward with a force of 100 newtons. Minutes before the race, you spoiled their plans by reversing half the jets. As the result of your efforts, the runners traveled exactly the same distance with air jets pushing them forward at 100 newtons as they did with air jets pushing them backward at 100 newtons. Overall, how much work did these rearranged air jets do on each runner?
Answer: Zero work.
Why: Work is force times distance. Since the runners are pushed forward for half the distance and backward for half the distance, the work done on them when pushed forward is exactly cancelled by the negative work done on them when pushed backward.

(C) One of the Lilliputian high jumpers decided to give himself an advantage by putting huge negative charges on both himself and on the landing mat under the bar. You caught him in time to remove the negative charge from the mat. The jumper experienced no repulsion from the mat and failed to clear the bar. Instead, the plastic bar stuck to him and the audience laughed as he fought to get it off. Why did the uncharged plastic bar stick to the negatively charged jumper?
Answer: The negative charge polarizes the plastic bar and the two attract
Why: The proximity of the jumper's negative charge shifts the charges inside the plastic bar. The positive charges in the bar move slightly toward the jumper and the negative charges in the bar move slightly away from him. Since the positive charges are closer to the jumper, the attraction they experience is stronger than the repulsion experienced by the negative charges on the bar.

(D) In a misguided attempt to win the 100 meter dash, a runner from the Duchy of Grand Fenwick coated the bottoms of her shoes with Superslideä , the "ultimate in frictionless lubrication." Her shoes experienced exactly zero friction. When the gun went off, the runner found herself unable to move forward and remained at the starting point with her legs churning furiously. What force or other physical effect kept her from moving forward?
Answer: Inertia.
Why: Without friction, the runner experienced no horizontal forces. Her inertia kept her motionless.

You are riding on a huge roller coaster with the tallest, steepest first hill in the world. To make the roller coaster even more exciting, its designers have used high technology to eliminate air resistance and friction, so that the coaster follows the laws of physics without producing any thermal energy. The first hill can be divided into three parts: top, middle, and bottom (see Figure #6 on the back page). While the top portion of the first hill slopes gradually downward, the middle portion of the hill dives almost straight down. The bottom portion of the hill is less steeply sloped in the downward direction, becoming more and more gradual so that it eventually levels out completely.

(A) Along which portion of the first hill does the roller coaster have its greatest speed?
Answer: Along the bottom portion.
Why: There is nothing to extract energy from the roller coaster, no friction or air resistance, so it retains all of its energy as it plunges down the hill. It converts its gravitational potential energy into kinetic energy as it descends and has its highest kinetic energy near the bottom of the hill. Since kinetic energy is proportional to the square of speed, the roller coaster's speed is also highest near the bottom of the hill.

(B) Along which portion of the first hill does the roller coaster have its greatest downward acceleration?
Answer: Along the middle portion.
Why: The downward acceleration of an object on a ramp increases as the ramp becomes steeper. The middle portion of the track is the steepest, so it is also the portion during which the roller coaster's downward acceleration is greatest.

(C) Along which portion of the first hill, if any, does the roller coaster have its greatest upward acceleration?
Answer: Along the bottom portion.
Why: Along the bottom portion of the track, the roller coaster is changing its direction of travel. It is turning upward from a downward plunge to a horizontal motion. This turning of its velocity involves an upward acceleration. This is the only upward acceleration that occurs on the first hill--the acceleration is downward along the top and middle portions of the track.

(D) Compare the roller coaster's total energy on the top portion of the track with its total energy on the bottom portion of the track.
Answer: They are equal.
Why: Energy is conserved and the roller coaster has no way of transferring energy to anything. There is no friction or air resistance, and the track doesn't move so the roller coaster can't do work on it. All of the energy the roller coaster had near the top of the hill (mostly in the form of gravitational potential energy) is still there near the bottom of the hill (mostly in the form of kinetic energy).

In the days before cars had electronic ignition systems, the spark that ignited gasoline in an engine came from a second coil of wire wrapped around an electromagnet. A steady DC current provided by the car's battery would flow through the electromagnet until it was time to ignite the gasoline (see Figure #1 on the back page).

(A) While the current in the electromagnet is steady direct current, what field(s) surround the electromagnet (if any)?
Answer: A magnetic field
Why: The steady current is magnetic and produces a magnetic field around the electromagnet.

(B) Even though some power is flowing to the electromagnet in the steady direct current, no power is transferred to charges in the second coil of wire. Why not?
Answer: There is no electric field in the second coil of wire to push the charges forward and give them energy.
To push forward on the charges in the second coil of wire and give them energy, there has to be an electric field. Since the magnetic field of the electromagnet is steady, it creates no electric field so nothing gives the charges in the second coil energy.

(C) But when a mechanical device separates the "points" and abruptly stops current from flowing through the electromagnet, a current suddenly appears in the second coil of wire. This current acquires a huge voltage as it moves forward through this coil and it leaps as a spark between the tips of the sparkplug. This spark ignites the gasoline. What field pushes the charges forward through the coil of wire and gives them energy?
Answer: An electric field
Why: The changing magnetic field creates an electric field and this electric field pushes charges forward in the second coil of wire.

(D) Why must the current in the electromagnet change in order for the process described in (C) to occur?
Answer: Only when the current in the electromagnet changes will the magnetic field change and create an electric field
Why: When the current tries to stop flowing through the electromagnet, the magnetic field begins to disappear. This changing magnetic field creates an electric field.