Final Exam

Physics 106 - How Things Work - Spring, 2000

Final Examination

Given Friday, May 12, 2000, from 9:00 AM to 12:00 Noon

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 this examination.

Problem 1:

Light travels long distances through the glass core of an optical fiber. The glass in this core is exceptionally clear so that very little light is absorbed during its travel. When a short pulse of white light passes through a very long fiber,

(A) the white light emerges from the fiber with all of its wavelengths together.

(B) it emerges from the fiber as a pulse of infrared light.

(D) red light emerges from the fiber first.

Problem 2:

The plastic covering of a fake leather couch bends and stretches but can’t be pulled apart easily. The plastic resists pulling apart because

(A) its crystals are very small and it can’t undergo slip.

(B) its magnetic poles are aligned and they attract one another.

(D) its temperature drops as you stretch it and it freezes into a stiff material before it separates.

Problem 3:

Lead absorbs X rays because each of its atoms has a large

(A) number of protons to attract X rays.

(B) nucleus with many nucleons in it.

(D) number of neutrons to attract X rays.

Problem 4:

A pair of squirrels is building a nest. One squirrel carries a mouthful of lightweight leaves up one tree, across some branches at the top and back down another tree to where the nest is. A second squirrel caries a mouthful of heavy sticks straight up the tree with the nest. Both squirrels start from the ground and end up at the same nest.

(A) Neither squirrel does work on their building materials; the materials do work on them.

(B) The first squirrel does more work on the leaves than the second squirrel does on the sticks.

(D) The second squirrel does more work on the sticks than the first squirrel does on the leaves.

Problem 5:

Your neighbor's house loses power during a windstorm. To help out, you run a long extension cord from your house to your neighbor’s house. Your neighbor’s appliances don't run properly when plugged into this cord because too much power is wasted in its thin wires. As current goes from you house to your neighbors, its

(A) current and voltage both gradually decrease along the length of the cord

(B) voltage gradually decreases along the length of the cord.

(D) voltage gradually decreases along the length of the cord and current gradually increases to partly compensate.

Problem 6:

As a figure skater leaps into the air with her arms extended, she pushes off of the ice in a twisting motion to start herself spinning. When she is in the air she is initially spinning slowly, but when she pulls her arms in near her body she spins rapidly because her moment of inertia

(A) becomes larger, but her angular momentum stays the same.

(B) becomes smaller and her angular momentum increases.

(D) stays the same, but her angular momentum becomes smaller.

Problem 7:

Which one of the following items emits radio waves?

(A) A stationary, uniformly charged metal flagpole that is vertical.

(B) A uniformly charged plastic stick that is spinning end-over-end in the air.

(D) A stationary loop of wire that has a steady electric current flowing in it.

Problem 8:

You jump off of a diving board into a swimming pool. As you fall toward the water, your velocity

(A) remains constant because your acceleration is zero.

(B) remains constant because your acceleration is constant.

(D) increases because your acceleration increases.

Problem 9:

The amount of information a DVD can hold is limited by physics. Which of the following changes would lead to the biggest increase in a DVD’s capacity?

(A) Spin the disc much faster during reading and writing.

(B) Use a gold reflective layer rather than an aluminum layer.

(D) Make the plastic disc thinner while keeping it equally stiff.

Problem 10:

When a xerographic copier first applies electric charges to the surface of its photoconductor, it does so in the dark. The reason for applying these charges in the dark is that light exposure would

(A) release magnetic poles that would cancel the electric charges.

(B) allow the charges to flow through the photoconductor so that they wouldn’t accumulate on its surface.

(D) expand the coherence of the waves and prevent the charges from forming a real image on the surface.

Problem 11:

A downhill skier goes off a jump and remains in the air for more than a second. While the skier is in the air, what force is pushing the skier forward?

(A) Momentum.

(B) Gravity.

(D) Nothing.

Problem 12:

You have a flashlight that requires 4 “D” batteries but you only have 3 of them. You find a piece of metal that has the same shape as a “D” battery, wrap its side with tape so that it won’t cause any short circuits, and load the flashlight. All the connections are made properly and there is no short circuit. You’ve simply formed a circuit with a chain of 3 batteries instead of the chain of 4 that the flashlight was designed for. You turn on the flashlight and

(A) its bulb fails to light at all.

(B) its bulb glows, but rather dimly.

(D) its bulb glows at full brightness, but the batteries run out relatively quickly.

Problem 13:

The mercury atoms in a fluorescent lamp emit ultraviolet light. If the lamp used barium atoms instead of mercury atoms, the barium atoms would emit green light. In that case, the lamp wouldn’t be able to produce white light because its phosphors wouldn’t be able to emit

(A) red light.

(B) green light.

(D) yellow light.

Problem 14:

Car tires can't be melted for recycling because

(A) the polymer molecules in rubber are just too long to reptate (move along their length).

(B) the polymers in tires have been crystallized, which causes them to burn before they melt.

(D) the crosslinked polymers are kept from reptating (moving along their length) as temperature is increased.

Problem 15:

A wax sphere can act as a converging lens for microwaves and will focus a beam of microwaves to a narrow spot. Evidently, when microwaves enter wax from air, they

(A) speed up.

(B) undergo total internal reflection.

(D) slow down.

Problem 16:

The sky is blue because the tiny particles in air

(A) scatter long wavelength light more efficiently than short wavelength light.

(B) resonantly absorb red light and then emit lots of it out into space.

(D) scatter short wavelength light more efficiently than long wavelength light.

Problem 17:

An AM radio station transmits its signal from a tall antenna because

(A) only a tall antenna can produce the large amplitude signals required for amplitude-modulated transmissions.

(B) a tall antenna will transmit the signal farther.

(D) the band of frequencies set aside for AM transmissions have short wavelengths.

Problem 18:

If you "cook" a CD or compact disc briefly in a microwave oven,

(A) it becomes permanently magnetized.

(B) it vibrates back and forth violently in response to the fluctuating electric field.

(D) it becomes hot enough to soften and sparks leap about its surface.

Problem 19:

If you rub a balloon in your hair it will become electrically charged. When you put this charged balloon against an insulating wall it can stick because the wall becomes

(A) charged.

(B) electrically polarized.

(D) magnetically polarized.

Problem 20:

A charged particle moving to the left can be accelerated by

(A) neither an electric field that points up nor a magnetic field that points up.

(B) an electric field that points up, but not a magnetic field that points up.

(D) an electric field that points up or a magnetic field that points up.

Problem 21:

Laser light exhibits much stronger interference effects than light from an incandescent lamp because laser light is

(A) much narrow than incandescent light

(B) a single electromagnetic wave while incandescent light is not.

(D) emitted by atoms while incandescent light is not.

Problem 22:

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 kinetic energy.

(B) The upward momentum is transferred to the earth.

(D) The upward momentum is converted into gravitational potential energy.

Problem 23:

You are showing some friends slides from your latest vacation, using a slide projector. The slide is placed behind a lens and its image is projected onto the wall. More friends show up and you want to make the image larger so that everyone can see. To do this the projector must be

(A) kept in the same position, but its lens must be moved farther from the slide.

(B) kept in the same position, but its lens must be moved closer to the slide.

(D) moved closer to the wall and its lens moved farther from the slide.

Problem 24:

One of the problems with tubes that emit X rays is that their targets can overheat and melt. Carbon has the highest melting temperature of any element, yet it’s not used as a target in X-ray tubes because

(A) its nuclei have too little mass and charge, and its atoms have too few electrons.

(B) it is an insulator and can’t be electrically charged.

(D) it is black and absorbs X rays.

Problem 25:

You walk to class at a steady pace along a flat, horizontal path. Your backpack weighs 100 newtons and your trip is 500 meters long. How much work do you do on your backpack?

(A) 0 joules (0 newton-meters).

(B) 1/5 joule (1/5 newton-meters).

(D) 5 joules (5 newton-meters).

Problem 26:

Spring steel is brittle because

(A) it contains a mixture of small iron crystals and small iron carbide crystals.

(B) it contains a mixture of large iron crystals and large iron carbide crystals.

(D) it is held together by very strong ionic bonds.

Problem 27:

When an electron makes a radiative transition in an atom, it emits

(A) a neutron.

(B) an electromagnetic wave.

(D) a gamma ray.

Problem 28:

A makeup mirror is concave—its center is bent away from your face. When you stand close to the mirror, you see an enlarged and upright virtual image of your face located far behind the mirror. However, when you back away far enough, you see an inverted real image of your face hovering between the mirror and you. If you hold a small piece of paper in the place where you see this real image,

(A) a rainbow pattern of light will appear on the paper, but it will contain only those wavelengths of light that are reflected by your face.

(B) the inverted real image of your face will appear as a pattern of light on the paper.

(D) an inverted real image of the mirror will appear on the paper.

Problem 29:

If you quickly poke at a starch-and-water solution, it feels hard. If you push slowly on the same solution your finger will sink into it and become coated with the sticky starch paste. The starch solution feels hard when you poke at it because

(A) the starch crystals don't have time to slip and rearrange.

(B) the heat from your body does not have time to flow into the starch solution.

(D) the long polymer molecules don't have time to move around your finger.

Problem 30:

It’s important not to overheat good steel tools because that can cause them to lose their hardness. This thermal softening occurs because at high temperatures

(A) steel crystallizes into aluminum.

(B) steel becomes a non-crystalline (or amorphous) metal.

(D) carbon atoms can move and shift within the steel.

Problem 31:

A skydiver leaps out of an airplane and begins to fall. After a few seconds, she reaches a constant downward speed of about 300 kilometers-per-hour (190 miles-per-hour). The net force she is experiencing at that point is

(A) zero, because the upward drag force exerted on her by the air is equal to her weight.

(B) upward, because she is in free fall and the only force she experiences is an upward centrifugal force.

(D) downward, because the only force acting on her is gravity. She is in free fall.

Problem 32:

You are driving eastward on the highway and the sun is setting behind you. You are following a pickup truck with a large flat rear window. There is a blinding glair from the sun’s reflection off of the window. Sunlight is reflected from this window because

(A) the glass of the rear window is tempered.

(B) sunlight is polarized.

(D) light travels slower in glass than in air.

Problem 33:

When two pieces of semiconductor touch to form a p-n junction, a depletion region forms around that junction. In the depletion region, all the valence levels are filled and all the conduction levels are empty. The depletion region is

(A) electrically conducting.

(B) filled only with spin-up electrons.

(D) magnetic.

Problem 34:

In a nuclear weapon, a chain reaction occurs when each fission

(A) produces an energy equal to the mass times the speed of light squared.

(B) produces at least one more fission.

(D) produces at least one neutron.

Problem 35:

A softball weighs about twice as much as a baseball. Suppose that a softball and a baseball roll off a horizontal table at the same speed and soon hit the horizontal floor. In that case, the

(A) lighter baseball will land about half as far from the edge of the table as will the heavier softball.

(B) heavier softball will land much, much closer to the edge of the table than will the lighter baseball.

(D) two balls will land at about the same distance from the edge of the table.

Problem 36:

Ordinary iodine atoms (iodine-127) and radioactive iodine atoms (iodine-131) are chemically indistinguishable because both have

(A) virtually identical masses.

(B) virtually identical arrangements of electrons.

(D) the same gamma rays in their nuclei.

Problem 37:

An inexpensive microwave oven may heat food unevenly because

(A) the microwaves bouncing around inside the oven can interfere, producing regions where the electric field oscillations are large and regions where the oscillations are small.

(B) low power microwaves are only absorbed in the densest parts of the food.

(D) the metal mesh on the front of cheap microwave ovens is not as efficient at keeping the microwaves in.

Problem 38:

In a xerographic copier, charge is sprinkled uniformly over the surface of a photoconductor. The charge is then removed in some places by illuminating the photoconductor with a pattern of light. The charge is removed when light

(A) causes the photoconductor to become insulating.

(B) causes the photoconductor to become conducting.

(D) ionizes atoms.

Problem 39:

When an X-ray photon is absorbed in your body it can damage cells because

(A) its oscillating electric field accelerates atoms and heats them.

(B) X-ray photons carry enough energy to ionize atoms.

(D) cells act as almost perfect antennas, which absorb the X rays.

Problem 40:

The oscillating electric field inside a microwave oven heats food by

(A) accelerating charged water molecules back and forth and causing them to bump into each other.

(B) twisting polar water molecules and causing them to rub against each other.

(D) breaking molecular bonds and releasing chemical potential energy as thermal energy.

Problem 41:

The side window of a car is made of tempered glass that crumbles into thousands of tiny pieces when it breaks. In addition to exhibiting this “dicing fracture,” tempered glass is also very difficult to break because

(A) its surface is compressed by the glass beneath the surface.

(B) it is made of relatively small quartz crystals.

(D) it is made of relatively large quartz crystals.

Problem 42:

Light emitted from a neon lamp appears red because

(A) the charges in the neon gas discharge are only accelerated with enough energy to produce red photons.

(B) the inside of the lamp is coated with phosphors that emit red light.

(D) the lamp runs at a cooler temperature than an incandescent light bulb and cooler objects emit redder light.

Problem 43:

The purpose of the graphite (carbon) in the Chernobyl nuclear reactor was to

(A) lubricate the control rods, so that they would experience less sliding friction.

(B) slow down fission neutrons.

(D) prevent the cooling water from boiling.

Problem 44:

Charged particles travel from the sun to the earth in what is known as “the solar wind.” When these particles encounter the earth’s magnetic field, they spiral in toward the earth’s magnetic poles because

(A) negative charges are attracted to north magnetic poles while positive charges are attracted to south magnetic poles.

(B) electric charges are attracted to the nearest magnetic pole, regardless of whether that pole is north or south.

(D) magnetic fields push on moving charges.

Problem 45:

A 60-watt, 120 volt light bulb is designed to reduce the voltage of a 1/2 amp current by 120 volts. If you screw this bulb into a fixture that is powered by very long thin wires, it will glow more dimly than intended because the voltage that the current through the bulb loses will be

(A) 120 V and the current through the bulb will be 1/2 A.

(B) 120 V, but the current through the bulb will be less than 1/2 A.

(D) less than 120 V, but the current will still be 1/2 A.

Problem 46:

A television contains a small step-up transformer that produces the very high voltages needed to accelerate electrons toward the front of the picture tube. When low voltage alternating current passes through the primary coil of this transformer, power is extracted from that current and passed to an alternating current flowing through the secondary coil of the transformer. Each charge passing through the secondary coil receives a relatively large amount of energy because the secondary coil

(A) is wound in the opposite direction from the primary coil. Charges passing through the two coils push apart as they circle the transformer's magnetic core so that charges in the secondary coil accelerate to very high speeds.

(B) uses thinner wire than is used in the primary coil, so that a smaller current flows through the secondary coil and a charge picks up more energy each time it circles the transformer's magnetic core.

(C) has many more turns than the primary coil, so that the charges in the secondary coil circle the transformer's magnetic core many times.

(D) is made from a magnetic wire that repels charges and pushes them to very high energies as they pass through it.

Problem 47:

Someone hands you a capacitor and asks you to find out whether it has separated electric charge on its two plates. To do this successfully, you would determine if

(A) there is a difference between the voltages of the two plates.

(B) there is a current flowing on either one of the two plates.

(D) the capacitor emits infrared light.

Problem 48:

Nuclear weapons can’t be built from natural uranium because uranium-238

(A) absorbs fission neutrons without fissioning itself.

(B) emits too many neutrons when it fissions.

(D) is below the critical mass required for a chain reaction.

Problem 49:

You are observing a distant ship with a telescope but you can't quite read the ship's name. To increase the magnification of the telescope, you replace the current eyepiece with one that

(A) contains a less highly curved lens (a longer focal length).

(B) has a smaller aperture (smaller diameter).

(D) contains a more highly curved lens (a shorter focal length).

Problem 50:

In a nuclear reactor, which uses natural uranium, a moderator is used to slow the fast neutrons resulting from the fission of uranium-235 nuclei. These neutrons are slowed in order to

(A) keep the reactor from reaching critical mass.

(B) keep uranium-238 nuclei from absorbing them.

(D) reduce the radioactivity of the waste products.

PART II: SHORT ANSWER QUESTIONS

Please give a brief answer in the space provided. Words written outside of the allotted space will not be read during grading. Part II is worth 33% of the grade on this examination.

Problem 1:

Before boarding an airplane, you must go through a screening process to make sure that you don't carry on any dangerous objects. The two principal screening devices are a metal detector for your person and an X-ray machine for your carry-on bags.

(A) As you walk through the metal detector, you are exposed to the rapidly varying magnetic field of an AC-powered electromagnet. The current passing through the coil of this electromagnet reverses directions thousands of times each second. You have an aluminum pen in your pocket and it becomes magnetic while you are in the metal detector. What occurs inside the pen to make it become magnetic?

(B) Because of its magnetization, your pen causes currents to flow through a second coil of wire in the metal detector. The metal detector beeps to alert the operators that you are carrying a metal object. How does the pen cause currents to flow through that second coil?

(C) As your carry-on bag goes through the X-ray machine, it is exposed to a weak stream of X rays. These X rays are produced by a vacuum tube in which electrons collide with a tungsten metal surface and emit X rays. The tube uses a small current of high voltage electrons to produce these X rays. Why can't it use a large current of low voltage electrons to produce X rays?

Problem 2:

After taking pictures of the mountains far in the distance, you decide to take a close-up picture of a friend.

(A) To shift the camera’s focus to a closer object, which way should you move the camera’s lens relative to its film?

(B) When you hold the camera as close to your friend as you would like, you find that the camera can’t quite focus the real image on the film. Your friend is just a little closer than the camera can handle. However, you can get around this problem by increasing the camera’s depth of focus by playing with its aperture. How should you change the lens’s aperture to get a sharp picture of your too-close friend?

(C) If you try to take a photograph of an object that’s closer to the lens than that lens’s own focal length, you can’t get a real image to form on the film at all. Explain why no real image forms.

Problem 3:

Most of you have probably ridden on bumper cars at an amusement park. These cars use electrically powered motors.

(A) A metal rod sticks up, out of the back of each car and at the top of this rod is a flat piece of metal, which rubs against the metal ceiling. This is the wire through which the current enters from the ceiling to the motor of the car. A second wire connects the motor to the floor. Why is this second wire necessary?

(B) The motor has a large electromagnet in it. If the conductor on the ceiling briefly loses contact while a large current is flowing through the electromagnet, a spark will form between the two. Why is there a spark?

(C) Your friend's car is stopped directly in front of you. You smash into him from behind. After the collision, your car is stopped. Explain what happens to your friend’s car using the concept of conservation of momentum.

Problem 4:

You’re tired of sitting in restaurants, listening to people talk on their cell phones. To solve the problem, you build a small device that emits a strong radio wave at a frequency you can select. You wait for a talker to pull out a cell phone and then turn on your little transmitter.

(A) Your device has a short antenna and you carefully align that antenna parallel to the talker’s antenna. Why is this important in trying to affect the talker’s cell phone?

(B) You adjust the frequency of your transmitter until it matches that of the talker’s cell phone. Now it’s time to get the talker’s attention! The talker’s cell phone is using the amplitude modulation technique to communicate with the telephone company. What should you do to your transmitter to make noise in the talker’s ear?

(C) The police frown on your little trick, so you decide to use passive gadgets instead. You purchase several large metal sheets and find that if you position them just right around a talker, the radio wave that reaches the talker’s cell phone is surprisingly weak. You aren’t blocking the radio wave entirely, merely reflecting pieces of it toward the cell phone from different directions. The pieces of the radio wave are all reaching the cell phone, so why do they result in such poor reception?

Problem 5:

Many of the laboratories in this building make use of "dye lasers". In this type of laser, the laser medium is a solution of fluorescent dye molecules. These dye molecules behave similarly to the phosphor used in a fluorescent lamp.

(A) Energy is usually supplied to the dye molecules by light from another laser, the pump laser. If you are using dye molecules that emit yellow laser light at 590 nanometers, what wavelength should the pump laser have (please give a descriptive comparison with the laser wavelength)?

(B) In order to build a laser, a pair of mirrors must be placed on either side of the laser medium. Briefly explain how these mirrors allow a single, spontaneously emitted photon to be duplicated many times.

(C) Dye molecules contain bands of levels that allow them to emit a continuous range of colors of laser light across this band. In order to select the color that the laser emits, a prism (a wedge shaped piece of glass) can be used. What property of the prism allows it to separate the colors?

Problem 6:

On September 30, 1999, an accident occurred in a uranium processing plant in Tokaimura, Japan. About 35 pounds of somewhat enriched uranium (about 19% uranium-235 and 81% uranium-238) were dissolved in nitric acid in a tank surrounded by cooling water. A nuclear chain reaction occurred. For the purposes of this question, you can think of the uranium as being dissolved in water and surrounded by water.

(A) By itself, 35 pounds of this type of uranium would not constitute a critical mass. In fact, it would probably take hundreds of pounds or more to form a critical mass. Why is it so hard to form a critical mass of this type of uranium?

(B) Mixing the uranium with water made a huge difference. In a water environment, 35 pounds was enough to form a critical mass. What role did the water play and why did it lead to a critical mass?

(C) The chain reaction in the mixing tank emitted lots of neutrons, which then stuck to the nuclei of atoms throughout the building. Why are atoms with extra neutrons in their nuclei dangerous to people?

Problem 7:

You have just stepped out of a drenching spring rainstorm. You plug your hairdryer into the upper socket of a 120-volt electric outlet and begin to dry your hair. The hairdryer carries a current of 10 amperes, weighs 10 newtons, and produces air with a temperature of 70° C.

(A) How could you calculate how much electric power the hairdryer is using (you don't have to do this calculation; just describe how to calculate it)?

(B) As your hair dries out, it gradually becomes stiffer. Your hair is a polymer that dissolves lots of water. Why does getting your hair wet make it softer?

(C) You are brushing your hair with a cheap metal comb. The comb gets tangled in your hair and you pull hard, permanently bending several of the comb's teeth. What happened to the arrangement of atoms in the teeth as the teeth bent?

Problem 8:

An induction stove uses an electromagnet to heat a pan rather than burning gas or a hot element.

(A) Alternating current is supplied to an electromagnet, which is beneath a glass or ceramic surface. When a metal pan is place above this electromagnet, current will flow in the pan. Explain how the alternating magnetic field can cause this current to flow.

(B) Why should the pan be made of a metal that conducts electricity poorly?

(C) Assume that the magnetic field produced by the stove alternates between pointing up and pointing down. The induced current in the pan produces a second magnetic field. What is the direction of the force on the pan resulting from these two fields?