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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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?

(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?

(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?

(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?

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?

(B) Along which portion of the first hill does the roller coaster have its greatest downward acceleration?

(C) Along which portion of the first hill, if any, does the roller coaster have its greatest upward acceleration?

(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.

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)?

(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?

(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?

(D) Why must the current in the electromagnet change in order for the process described in (C) to occur?