Midterm Examination

Physics 106N - How Things Work - Spring, 1997
Midterm Examination - Questions and Answers

Given Friday, February 28, from 1:00 PM to 1:50 PM

PART I: MULTIPLE CHOICE QUESTIONS

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:

An n-channel MOSFET consists of three pieces of semiconductor: two n-type pieces connected by a p-type piece. This n-channel MOSFET will allow current to flow through all three pieces when

Answer: (D) positive charge is put on its gate and the middle piece of semiconductor effectively becomes n-type.

Why: When you put positive charge on the gate, it attracts negative charge into the p-type material at the center of the MOSFET. These additional electrons fill the vacant valence levels and some electrons enter the conduction levels. Once the conduction levels in the central semiconductor have begun to fill, that material is effectively n-type semiconductor. The MOSFET is then composed entirely of n-type semiconductor and it conducts electricity.

Problem 2:

You are doing exercises at the gym. When you lift a free weight over your head, you push upward on it both as you lift it and as you lower it. However, when you work out with a special exercise machine, you push upward as you lift its bar but must pull downward to lower that bar. When you use that exercise machine,

Answer: (C) you do work on the bar both as you raise it and as you lower it.

Why: On the way up, you push the bar upward and it moves upward, so you do work on it. On the way down, you pull the bar downward and it moves downward, so you do work on it.

Problem 3:

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

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.

Problem 4:

You are standing on a plastic bench that insulates you from your surroundings. Both you and the helium balloon you are holding are electrically neutral. You now rub the balloon against your sweater, so that the balloon becomes negatively charged, and then let the balloon float away. You are left

Answer: (D) with a positive electric charge.

Why: You began electrically neutral, so when the balloon acquired its negative charge, it acquired it from you. Since you lost negative charge, you ended up positively charged. Overall, you and the balloon remain neutral, as you must since electric charge is a conserved quantity.

Problem 5:

If you push against a heavy bookshelf but it remains in place, it experiences static friction with the floor. If the bookshelf begins to slide as you push on it, it experiences sliding friction with the floor. Friction extracts energy from the bookshelf (A) only when the bookshelf remains in place.
(B) both when the bookshelf is sliding across the floor and when it remains in place.
(C) only when the bookshelf is sliding across the floor.
(D) only if the net force on the bookshelf is zero.
Answer: (C) only when the bookshelf is sliding across the floor.
Why: Only sliding friction does negative work on an object because static friction involves no motion and no distance. Sliding friction pushes the bookshelf in the direction opposite the bookshelf's motion and does negative work on the bookshelf. It extracts energy from the bookshelf. Problem 6:

A hummingbird is hovering motionless in front of a flower. The net force exerted on the bird is (A) zero.
(B) downward and slightly less than the bird’s weight.
(C) downward and equal to the bird’s weight.
(D) upward and equal in magnitude (amount) to the bird’s weight.
Answer: (A) zero.
Why: If the hummingbird isn't moving and is remaining that way, then both its velocity and its acceleration are zero. An object that isn't accelerating has zero net force on it. Problem 7:

When you bounce a tennis ball off a concrete wall, the ball (A) retains essentially all of its energy but transfers a great deal of momentum to the wall.
(B) retains essentially all of its energy and momentum.
(C) transfers a great deal of momentum and energy to the wall.
(D) retains essentially all of its momentum but transfers a great deal of energy to the wall.
Answer: (A) retains essentially all of its energy but transfers a great deal of momentum to the wall.
Why: The ball can't do work on the wall because the wall is rigid and won't move. So the ball retains essentially all of its energy. However, the ball transfers a great deal of energy to the wall--it transfers so much momentum that its direction of travel reverses. The ball initially has momentum toward the wall and, after the bounce, has momentum away from the wall. That's a huge change in its momentum. Problem 8:

A photoconductor can’t carry an electric current in the dark because all of its valence levels contain two electrons and moving a valence-level electron into one of the empty conduction levels requires too much energy. While the photoconductor would be able to carry current if its electrons could move from one valence level to another, such movement is impossible because (A) the velocity of an electron in the photoconductor is conserved and can’t change.
(B) no more than two electrons can be in each valence level; one spin-up and one spin-down.
(C) the momentum of an electron in the photoconductor is conserved and can’t change.
(D) the laws of motion prevent electrons from changing valence levels.
Answer: (B) no more than two electrons can be in each valence level; one spin-up and one spin-down.
Why: The electrons can shift between valence levels because all those levels are filled and can't take any more electrons. This state of "being filled" is a consequence of the Pauli exclusion principle, which forbids two indistinguishable electrons from being in the same level at the same time. Since the electrons can't shift between valence levels, they can't respond to electric fields and can't carry electric currents. Problem 9:

You are riding on a swing at the local playground. As you swing back and forth, you begin to think about your speed and kinetic energy (this is obviously a fictional story). These two quantities clearly change between the top of each swing (when you are reversing directions) and the bottom of each swing (when you are passing directly beneath the supporting beam). You wonder when each of these two quantities is at its maximum value. Actually, your speed is at its maximum (A) at the bottom of a swing and your kinetic energy is at its maximum at the top of a swing.
(B) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.
(C) at the top of a swing and your kinetic energy is at its maximum at the top of a swing.
(D) at the top of a swing and your kinetic energy is at its maximum at the bottom of a swing.
Answer: (B) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.
Why: At the bottom of the swing, all of your energy is in the form of kinetic energy rather than gravitational potential energy. You are also moving as fast as possible, because kinetic energy is proportional to the square of your speed. Problem 10:

The power supply for your answering machine is a small black cube that plugs directly into an electric outlet. The main component in this supply is a transformer and moderate current from the 120 volt power line flows through the primary coil of this transformer. If the transformer’s primary coil has 20 times as many turns of wire in it as the secondary coil has, then the secondary coil provides (A) a large voltage rise for the large amount of current that flows through it.
(B) a small voltage rise for the large amount of current that flows through it.
(C) a small voltage rise for the small amount of current that flows through it.
(D) a large voltage rise for the small amount of current that flows through it.
Answer: (B) a small voltage rise for the large amount of current that flows through it.
Why: A secondary coil with few turns in it gives the charges passing through it only small amounts of energy. Without a long distance over which to do work on the charges flowing in the coil, the transformer produces only a small rise in the voltage of those charges. However, the coil can give this small voltage rise to a large current without requiring too much power from the input circuit. Problem 11:

You are working in a pizza parlor and have learned how to toss and spin the dough to form large disks. You find that the larger each disk becomes as you spin it, the harder it is to stop the disk from spinning. While a small twist is all it takes to stop a ball of dough from spinning, by the time that same dough becomes a 16 inch disk a similar twist barely slows it down. This effect occurs because spreading the dough into a disk increases its (A) velocity.
(B) weight.
(C) mass.
(D) moment of inertia.
Answer: (D) moment of inertia.
Why: An object's resistance to angular acceleration, its rotational inertia, is measured by its moment of inertia. Increasing the distance between the pizza's mass and its center of rotation increases the pizza's momentum of inertia and makes it harder to start and stop spinning. Problem 12:

The huge steam-powered generators found in electric power plants produce electricity by (A) moving electric charges up and down inside capacitors.
(B) rubbing copper disks against sheets of glass.
(C) spinning iron cores inside of transformers.
(D) moving magnets past coils of wire.
Answer: (D) moving magnets past coils of wire.
Why: When a magnet moves, it produces an electric field. If there is conducting material around when that electric field appears, charges will begin to move in the conducting material. A generator uses a coil of conducting wire to carry current that is pushed along by the electric field of the moving magnet. Problem 13:

Your cat has chewed the cord to your desk lamp and has created a short circuit—an electric connection from one wire to the other inside the cord. When you plug the lamp into the electric outlet, (A) current will bypass the bulb and the bulb will not light up.
(B) current will flow alternately through the bulb and through the short circuit, so that the bulb will blink on and off rapidly.
(C) the current will begin to flow backward through the bulb so that it glows at half its normal brightness.
(D) excessive current will pass through the bulb and the bulb will glow very brightly.
Answer: (A) current will bypass the bulb and the bulb will not light up.
Why: The short circuit will provide a more effective path for the current heading toward the bulb through one wire and leaving through the other. Instead of flowing through the bulb's filament and lighting it up, most of the current will bypass the bulb by flowing through the short circuit. Problem 14:

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. The right end of the rope stretches toward the right and the rope breaks. Breaking the rope required energy and that energy was provided by (A) both teams.
(B) neither team. It was instead provided by chemical potential energy in the rope itself.
(C) the team on the right.
(D) the team on the left.
Answer: (C) the team on the right.
Why: Since the team on the left is motionless, it neither does work on the rope nor is work done on it by the rope. However, the team on the right pulls the rope to the right and the rope's end moves toward the right. Thus the team on the right does work on the rope and it is this energy transferred to the rope that breaks the rope. Problem 15:

The blades of a fan normally do work on the air as they blow it forward across the room. An electric motor keeps those fan blades turning. Suppose that a wind begins to blow air forward through the blades so fast that the blades stop doing work on the air and the air begins to do work on the blades. In that case, the motor will (A) stop having any electric charges and its rotor will turn freely.
(B) draw so much electric power that it will overheat and burn out.
(C) begin to generate electricity.
(D) stop having any magnetic poles and its rotor will turn freely.
Answer: (C) begin to generate electricity.
Why: If the motor stops doing work on the blades and instead has work done on it by the blades, then it will stop extracting power from the current passing through it and will instead begin to deliver power to the current passing through it. The motor becomes a generator. Problem 16:

The recording surface of a metal particle audio tape contains tiny particles of (A) copper.
(B) aluminum.
(C) brass.
(D) iron.
Answer: (D) iron.
Why: Of the metals listed, only iron is intrinsically magnetic--it is composed of atoms that retain their magnetism even as part of the solid. The other metals have no magnetic order at all as solids. Problem 17:

You are watching a baseball game and the pitcher has just thrown the ball toward the batter at home plate. Once the ball has left the pitcher’s hand and is heading forward toward home plate, it experiences (A) a forward horizontal force that diminishes gradually as the ball approaches home plate.
(B) a forward horizontal force until it reaches the midpoint of its trip to home plate and then a backward horizontal force for the remainder of its trip.
(C) a forward horizontal force that remains constant all the way to home plate.
(D) no horizontal force in the forward direction.
Answer: (D) no horizontal force in the forward direction.
Why: After it leaves the pitcher's hand, the ball coasts toward home plate because of its inertia alone. There is no forward force propelling it forward. Problem 18:

Dynamic memory in a computer stores bits as the presence or absence of separated charge on tiny capacitors. Making the insulating layers of those tiny capacitors as thin as possible reduces the energy needed to store separated charge on them—recording a bit—because (A) the separated charges have less momentum when they are stationary on opposite sides of a thin insulator than when they are stationary on opposite sides of a thick insulator.
(B) a thin insulator is a better conductor of electricity than a thick insulator.
(C) bringing those opposite charges closer together reduces their overall electrostatic potential energy.
(D) a thin insulator is less magnetic than a thick insulator.
Answer: (C) bringing those opposite charges closer together reduces their overall electrostatic potential energy.
Why: The closer opposite charges are to one another, the less potential energy they have. By making the insulator in a capacitor very thin, the energy stored in the separated charge is reduced. Problem 19:

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

Answer: (D) 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.

Problem 20:

A battery (A) creates positive charge.
(B) pumps positive charge from its positive terminal to its negative terminal.
(C) creates negative charge.
(D) pumps positive charge from its negative terminal to its positive terminal.
Answer: (D) pumps positive charge from its negative terminal to its positive terminal.
Why: A battery can't create charge; it can only move it from where it "wants" to be (the negative terminal of the battery) to where it doesn't want to be (the positive terminal of the battery). In the process, the battery does work on the charge and increases its energy and voltage. Problem 21:

You are going to a picnic at the top of a mountain with your friends and are carrying the picnic basket in your hand. If you were to walk directly up the steep side of the mountain, from the parking lot to the mountain top, you would have to do 20,000 joules of work on the basket. If you walked along the road instead, a gradual incline that would also take you from the parking lot to the mountain top but would require you to walk 4 times as far, the amount of work you would do on the basket would be (A) 80,000 joules.
(B) 5,000 joules.
(C) 40,000 joules.
(D) 20,000 joules.
Answer: (D) 20,000 joules.
Why: No matter how you raise the picnic basket from the parking lot to the mountain top, you'll have to do 20,000 joules of work. That is the gravitational potential energy that the basket gains in rising up to its new altitude. Problem 22:

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. While your head is pressed into the cushion, the net force on your head is (A) backward and its acceleration is forward.
(B) zero and it is not accelerating.
(C) forward and its acceleration is.
(D) backward and its acceleration is backward.
Answer: (C) forward and its acceleration is (forward).
Why: If your head is denting the cushion behind it, your head must be pushing backward on the cushion and that cushion, in turn, must be pushing forward on your head. That's Newton's third law. With only a forward force on your head, your head must be experiencing a forward net force and it must be accelerating in the direction of that force--forward. Problem 23:

The back of your UVA ID card has a strip of magnetic tape on it. Like an audio tape, this strip stores information as a pattern of magnetized patches. The machine that reads this information is essentially a tape player. When someone uses the reader to read your ID card, they pull the card quickly through the reader. It’s important that the card move through the reader because the playback head can only respond to moving or changing magnetic fields. That is because moving or changing magnetic fields (A) can change the weight of a small steel ring so that it accelerates up or down.
(B) generate light in photocells; making it possible to detect the pattern of magnetization on the strip.
(C) produce temperature fluctuations that can easily be detected with a bimetallic strip thermometer.
(D) produce electric fields that can cause currents to flow in a coil of wire.
Answer: (D) produce electric fields that can cause currents to flow in a coil of wire.
Why: Like a tape recorder's playback head, the reader needs moving magnetic fields from the magnetized card surface to create electric fields that propel charges through a coil of wire. The reader monitors these currents of electric charges and uses them to determine what information was recorded on the ID. Problem 24:

The principal advantage of sending electric power across country on very high voltage transmission lines is that (A) they carry less energy per charge than low voltage transmission lines.
(B) electric power lost in the wires is greatly reduced.
(C) These transmission lines are less likely to get in the way than low voltage transmission lines—which are much closer to the ground.
(D) they carry much more current than low voltage transmission lines.
Answer: (B) electric power lost in the wires is greatly reduced.
Why: The whole point of high voltage transmission is to convey the required power with as few charges per second as possible. Since power wasted in the wires depends on the square of the current in those wires, it's best to use a small current of very high voltage charges to carry the power through the wires. Problem 25:

A motor made entirely with permanent magnets—no electromagnets—would (A) turn continuously only in one direction. It would turn only briefly in the other direction.
(B) turn only briefly before coming to a stop.
(C) turn continuously in either direction, but its rate of rotation would not be adjustable.
(D) turn continuously only in one direction. It would be unable to turn in the other direction.
Answer: (B) turn only briefly before coming to a stop.
Why: The motor's rotor would quickly find its ideal situation--north poles as close to south poles as possible and south poles as close to north poles as possible--and nothing more would happen. The rotor would stop turn and the motor would just sit there. PART II: SHORT ANSWER QUESTIONS

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

Problem 1:

If you hold the north pole of a permanent magnet motionless near a stationary sheet of aluminum metal, the magnet won’t exert any forces on the aluminum. However, if you move that permanent magnet’s north pole steadily toward the aluminum, the aluminum will develop its own north pole and will repel the permanent magnet.

Answer: An electric current flowing through the aluminum.

Why: Aluminum is not an intrinsically magnetic metal--it only becomes magnetic when current flows through it. Moving a magnetic field into it exposes the aluminum to an electric field and that electric field causes mobile electric charges in the aluminum to move as a current. The aluminum then becomes magnetic.

Answer: You are doing work on the magnet.

Why: You are pushing the magnet toward the aluminum and the magnet is moving toward the aluminum. Since the force you exert on the magnet is in the same direction as its motion, you are doing work on the magnet.

Answer: The electric currents in the aluminum slow to a stop.

Why: In aluminum, electric currents don't flow without losing energy. The current in the aluminum gradually gives up its energy and that energy becomes thermal energy in the aluminum. Without any current left, the aluminum returns to its nonmagnetic state.

Answer: Steel is intrinsically magnetic while aluminum is not.

Why: Steel has magnetic order at the atom scale--it is composed of tiny magnets. These magnets align with the approaching permanent magnet and attract it. Aluminum has no such magnetic order and doesn't attract the permanent magnet at all.

Problem 2:

You’ve just been involved in an accident southbound on Rt. 29 and are being questioned by a police officer. However, this officer was a physics major and expects your description of the accident to be well-stated in terms of physics concepts. Luckily, you are a student in Physics 106N and this task should be no problem. Right? (This isn’t the first part of the question so you don’t get any points yet.) (A) You explain to the officer that you were traveling south on 29 at 45 mph in the middle lane when you saw the light turn red and hit the brakes. In what direction were you accelerating as you slowed down?
Answer: Northward.
Why: You gave up your southward velocity by accelerating in the direction opposite your motion--northward.

(B) While you were sitting at the light, you were rear-ended by a drunk driver. The driver’s car slammed into your car from behind and pushed it 50 feet forward. The officer points out that the drunk’s car transferred energy to your car during this collision. How did the officer show that this energy transfer occurred?
Answer: The drunk's car pushed your car forward and your car moved forward, so the drunk's car did work on your car.
Why: Since the direction of the force on your car is in the same direction as your car's motion, the drunk's car did work on your car.

(C) The collision pushed your car across the intersection and into a brick wall. When the front of your car hit the brick wall, you were thrown forward toward the steering wheel. What caused you to go forward relative to the car?
Answer: Your inertia (or your momentum).
Why: There was no forward force on you propelling you forward--you were just doing what any free object does: moves at constant velocity. Your motion was governed by the concept of inertia.

(D) Luckily, your airbag inflated and saved your life. Why did hitting the airbag cause less injury than hitting the steering wheel?
Answer: The airbag slowed you to a stop with a smaller force (exerted over a longer time).
Why: While you would give up all your momentum to either the airbag or the steering wheel, the transfer of momentum to the airbag involves a much smaller force exerted over a much longer time. Problem 3:

You have removed the DC motor from a toy and are experimenting with it. The motor has two wires, one green and the other blue, through which you can send current in order to make its rotor spin. If you attach the blue wire to the positive terminal of a battery and the green wire to the negative terminal, the rotor spins clockwise 10 times each second. (A) If you attach only the blue wire to the positive terminal of the battery and leave the green wire unattached, how fast will the motor spin?
Answer: It won't spin at all.
Why: Without a complete circuit to carry current from the battery to the motor and then back to the battery, no current will flow and no power will be transferred from the battery to the motor. The motor won't turn.

(B) If you attach the green wire to the positive terminal of the battery and the blue wire to the negative terminal of the battery—the reverse of the original attachments—will the rotor spin and, if so, which way?
Answer: Yes, the motor will spin backwards (counter-clockwise).
Why: A DC motor is constructed out of some permanent magnets and some electromagnets. When you reverse the connections to the batteries, current still flows through the motor's electromagnets and the motor still turns, but now all the poles of the electromagnets are reversed. As a result of this reversal, all the forces and torques are also reversed and the motor spins backward.

(C) You remove the battery and attach the motor’s two wires to a light bulb. When you spin the rotor clockwise with your fingers, the light bulb lights up. Why?
Answer: The motor is acting as a generator.
Why: When you spin the motor's magnet inside its coils of wire, the moving magnetic field creates electric fields in the coils of wire and push electric charges through them. Currents flow through the coils and the light bulb, and the light bulb lights up.

(D) If you spin the rotor counter-clockwise with your fingers, what will happen to the light bulb?
Answer: The light bulb will still light up.
Why: When you spin the motor backward, it still generates electricity but now in the reverse direction. It doesn't matter which way you send current through a light bulb--it will light up either way.