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 final examination.
Problem 1:
In a CD player, light from a laser diode enters a plastic disc and reflects from a layer of shiny aluminum. Before it enters the plastic, the laser light has a wavelength of 780 nanometers and a frequency of 385,000,000,000,000 Hz. Once this light is inside the plastic, its wavelength
You are standing at the shore of a calm lake, trying to look beneath its surface. All you see is a strong reflection of the sky overhead. But when you put on polarizing sunglasses, the glare almost disappears and you can see fish swimming under the surface of the water. If you turn your head sideways, so that the sunglass lenses are rotated 90° from before, the glare will
Which of the following fields push on a stationary electron?
Electricity produced in a generating plant passes through a large step-up transformer. This step-up transformer produces the high voltages needed to send electric power long distances across the countryside. Which of the following is transferred from the transformer's primary coil to its secondary coil while the transformer is operating?
Ultraviolet light causes chemical damage to your skin while infrared light doesn't because
Wet paper is more transparent than dry paper because
You place a marshmallow between two graham crackers and squeeze, crushing the marshmallow into a thin pancake. The act of crushing the marshmallow
Clouds are white, a fact which indicates that the water molecules in clouds
An engineer at the company you're working for has just reported finding an unusual electromagnetic wave. This wave consists only of an electric field, with no magnetic field accompanying it. You are certain that the engineer is mistaken because
For a nuclear weapon to explode normally, the fissionable material in its core must be assembled very quickly. For technical reasons, a nuclear bomb developed by a terrorist group would probably not achieve such rapid assembly. As a result of its slow assembly, such a bomb would
A satellite dish is actually a mirror telescope for microwaves, with its receiving antenna located at the real image of the distant satellite. But unlike the smooth, shiny mirror of a light telescope, a typical satellite dish is made of dull metallic mesh. Microwaves reflect almost perfectly from this mesh because
A glass fiber can act as a pipe for light-light that enters the fiber at its end follows the fiber almost indefinitely without escaping through the fiber's surface. The light follows the fiber because
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
The Arcoroc company makes glass cups and goblets out of tempered glass. These cups are surprisingly difficult to break but when they do break, they instantly crumble into tiny chips. Like all tempered glass, these cups resist breaking because
When you put a normal incandescent light bulb in a microwave oven and turn the oven on,
The fallout from a nuclear explosion and the waste from a nuclear reactor contain radioactive isotopes. For example, iodine-131 (a fission byproduct) is radioactive while iodine-127 (normal iodine) is not. An atom of iodine-131 differs from an atom of iodine-127 because the iodine-131 atoms has more
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 denting the elastic cushion behind it, the net force on your head is
The electrons in an atom don't all travel in the lowest energy orbital-the orbital closest to the atom's nucleus. That's because
One advantage of using an enormous mirror to collect light in a telescope is that the real image is relatively bright. But there is also a benefit to letting the waves of starlight pass through the largest possible apertures. Sending a light wave through a small aperture truncates the wave severely and causes it to diffract. Because of this diffraction, light from a white star
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 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
When a free neutron collides with a uranium-235 nucleus and causes that nucleus to fission, the collision fragments contain a total of 2.5 free neutrons, on average, including the original free neutron. The critical mass for uranium-235 is about 50 kilograms. Suppose that another radioactive material, darthmaulium-999, undergoes fission when struck by a neutron and the collision fragments contain a total of 0.5 neutrons, on average, including the original free neutron. What would be the critical mass of darthmaulium-999?
Tool steel is an extremely hard steel that is used in drill bits and allows them to pierce ordinary steel with ease. However, when you drill holes in steel, you should always lubricate the bit with oil so that sliding friction doesn't overheat it. If the bit does become too hot, it can lose some of its hardness. That's because overheating
If you plug an electric hairdryer into the wall outlet and turn it on, current will begin to flow through wires inside the wall. Some of the electric power carried by that current will be wasted as heat in those wires. If you plug a second identical hairdryer into the same outlet and turn it on, the amount of current flowing through each wire in the wall will double. The amount of power wasted in the wires of wall will
You are bowling at the local alley. After the bowling ball leaves your hand and begins to roll smoothly toward the pins,
One way to thicken a sauce that you are cooking is to add starch to it. The added starch prevents the sauce from flowing easily because starch molecules
A gymnast is bouncing on a trampoline, rising 2 meters above its surface on every bounce. After the rising gymnast has left the surface of the trampoline but is still heading upward,
When an x-ray photon is absorbed in a tumor, it may cause the tumor cell to die because the photon
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
Laser light exhibits much stronger interference effects than light from a flashlight. That's because
Your small pocket camera has a telephoto setting in which the converging lens sticks far out in front of the camera body. The lens still projects a real image on film near the back of the camera body but now the lens is much farther from the film. When you shift the camera into this telephoto setting, the lens itself
If you exert a force of 100 N on a heavy file cabinet as it accelerates forward, it will push back on you with
In which one of the following situations are you doing (positive) work on a sack of flour?
Imaging x-rays go easily through living tissue but not so easily through bone. That's primarily because the calcium atoms in bone have
Silly Putty is a plastic with many unusual properties. If you poke at it swiftly, it feels hard and elastic. But if you stretch it patiently, it flows like a thick liquid. This behavior occurs because
Compared to the radio waves used in normal radio broadcasts, the microwaves used for satellite transmissions have
In a typical laser, the right color of light bounces back and forth between two mirrors and is amplified by the excited atoms in the laser medium. If you were to send some of the right color of light through the laser medium from the side of the laser, the laser medium would
A reflecting telescope has the advantage that its main optic has only one element (a curved mirror) and that this element doesn't have to be made of transparent material. In contrast, the simplest high-quality refracting telescope needs two elements in its main optic (a converging lens) and both elements must be perfectly transparent. It needs these two transparent elements in its lens because a single element lens
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
When a person beside you takes a flash photograph in a darkened room, the air in front of the flash lights up even when you can't see the flashlamp itself. The air in front of the flash appears bright because the air molecules are
When you add household Borax to Elmer's glue, the mixture
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 uranium-235 nucleus undergoes fission during a nuclear explosion. If you were to collect all the fragments of that original uranium-235 nucleus, you would find that their total combined mass is
For an appliance to receive and consume electric power,
Three factory workers are loading office chairs into a truck. All the chairs are the same and they have wheels on their legs. They are located on the ground behind the truck. The first person lifts chairs straight up in order to load them into the truck. The second person rolls the chairs slowly up a very long, very gradual ramp to load them into the truck. The third person rolls the chairs slowly up a rather short, rather steep ramp to load them into the truck. Which person does the most work on each chair while transferring it from the ground to the back of the truck?
If you were to remove an audio cassette from a tape recorder and examine the tape it contains, you would find that this tape is coated with a layer of
An n-channel MOSFET transistor begins to conduct electric current when you place positive charge on its gate. To allow the transistor to continue to conduct electric current indefinitely, you must
You put a thick, smooth metal spoon in a cup of coffee and begin to reheat the coffee in a microwave oven. The metal spoon will
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
Thermal fission reactors use moderators to thermalize the neutrons emitted by nuclear fissions. Moderators are small nuclei such as hydrogen or carbon and they are effective because neutrons
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:
You have just joined a start-up company that's going to build satellites for the new solar-system-wide cellular telephone network. Once your satellites are in place, people will be able to use their cell phones anywhere in the solar system. ET will definitely be able to phone home. However, powering these remote, unmanned satellites will be tricky. One of your colleagues suggests using extension cords, but a quick call to the hardware store indicates that extension cords just don't come much longer than 100 feet. For obvious reasons, each satellite must have its own local power source.
(B) Efficiency and light intensity are critical to solar cells in space. If a solar cell were perfectly efficient, each photon of light would use its energy to push one electron through the diode and that electron would experience an increase in energy as the result of its trip through the solar cell. Doubling the light intensity on the solar cell would double the number of electrons being pushed through the cell but would not change the amount of energy each electron received during its trip. How does doubling the light intensity on the solar cell affect the current through the cell and the voltage rise across the cell?
Answer: The current doubles but the voltage rise across the cell remains unchanged.
Why: Since each photon moves an electron, the more photons arrive the more electrons move. The number of electrons moving through the cell is proportional to the current passing through the cell. But since each electron receives the usual amount of energy, the voltage rise (energy per charge) stays the same.
(C) Satellites far from the sun can't rely on solar cells. Instead, they must use nuclear fuels. Even though 1 kilogram of plutonium-239 is much less than the critical mass needed for a nuclear weapon, it releases a considerable amount of heat, which your satellites use to generate electricity. Why does this small quantity of plutonium-239 release heat?
Answer: Plutonium-239 is radioactive and when its nucleus falls apart, energy stored in that nucleus is released as thermal energy.
Why: Whenever a nucleus falls apart spontaneously (radioactivity), the fragments have more kinetic energy than the original nucleus had. This kinetic energy in the fragments quickly becomes thermal energy in a lump of plutonium.
(D) As your company assembles the small nuclear power units for some of its more remote satellites, the technicians have to be careful not to put the plutonium-239 units too close together. When two 1 kilogram units are placed side-by-side, they release more heat than the two units did when they were far apart. Why does the heat release increase when they're near each other?
Answer: Neutrons emitted by one unit of plutonium induce fissions in any other nearby units of plutonium.
Why: Some of the fissions in any lump of plutonium-239 are induced fissions--fissions caused by the impact of a neutron on a plutonium-239 nucleus. The more neutrons passing through a lump of plutonium-239, the more likely that there will be induced fissions. By putting lumps of plutonium-239 near one another, you are encouraging neutrons from one lump to induce fissions in the other lumps.
A small radio-controlled airplane is flying around overhead. The pilot standing to your left is controlling it with a hand-held radio transmitter.
(B) It seems only natural to point the transmitter's antenna directly toward the distant airplane, but each time the owner does this, she temporarily loses control of the plane. Why doesn't the airplane receive a strong radio wave from the transmitter when the transmitter's antenna is pointed directly toward the airplane?
Answer: There is essentially no radio wave emitted parallel to the length of an antenna.
Why: Just as it's difficult to see a charge moving back and forth when its headed exactly toward you and away from you, it's hard to detect the radio wave emitted by such a charge. Antennas radiate mostly in the plane perpendicular to their antenna. That's why most terrestrial antennas are vertical--they tend to radiate best in horizontal directions, where the people are.
(C) To avoid interfering with the first pilot, a second radio-controlled airplane pilot selects a slightly higher radio frequency for his transmitter and airplane. Surprisingly enough, if you multiply the frequency of the second transmitter's radio wave by its wavelength, you get the same value as you get when you multiply the frequency of the first transmitter's radio wave by its wavelength. What is that special value?
Answer: The speed of light (optional detail: actually slightly less because the air slows the electromagnetic wave down).
Why: If you multiply the wavelength of a wave times its frequency, you obtain its speed.
(D) The transmitter uses the AM technique to send control information to the airplane. What is happening to the radio wave when the pilot activates one of the controls?
Answer: The amplitude (or strength) of the radio wave changes.
Why: The AM or amplitude modulation technique is one in which the strength or amplitude of the radio wave is varied in time to send sound or other information.
Eyes are very much like cameras: you eye has a lens that bends the light entering your pupil so that it forms a clear image on your retina. People who have trouble forming a clear image often get help from eyeglasses.
(B) Eyeglasses help the far-sighted person see nearby objects clearly. Which way should these eyeglasses bend the light that passes through them in order to be helpful to the far-sighted person?
Answer: The eyeglasses should converge (or bring together or bend together) the light passing through them.
Why: The eyeglasses must help the person's lens bring the light rays together on the retina because the person's eye lens is too weak (too flat, not curved enough). So the eyeglasses provide an additional converging lens.
(C) The far-sighted person can often get by without eyeglasses on a sunny day because that person's pupils are very small. Why does the small pupil diameter allow the person to see close objects more clearly than usual?
Answer: Light rays passing through a narrow lens are almost together even when the real image doesn't fall exactly on the retina (or the depth of focus increases as the lens diameter shrinks).
Why: When a narrow lens is used, the light rays that can pass through it are almost together both before and after the true focus occurs. As a result, the exact location of that focus isn't so important and a reasonably clear image is available even when objects aren't quite in true focus.
(D) Some eyeglasses come with an antireflection coating. In addition to getting rid of annoying ghost images of things behind the wearer, these coated eyeglasses also brighten the images on the retina by about 8%, as compared to ordinary uncoated eyeglasses. Why are the images about 8% brighter with glasses that don't reflect any light?
Answer: As light from a scene passes through the lens to your eye, it loses 4% as a reflection from each eyeglass surface (or the two reflections from the lens surfaces throw away 8% of the light that would otherwise reach your eye.
Why: Reflection not only let you see yourself in the window, they prevent about 8% of the light from outside from entering your home. That missing 8% is reflected back out to the outside viewers and appears as a reflection of the outdoors.
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 . 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.
(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).
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.
(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?
Answer: The pen's changing magnetic field produces an electric field that pushes charges through the second coil.
Why: The magnetization of the pen changes with time, so the magnetic field is accompanied by an electric field. This electric field pushes current back and forth through the 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?
Answer: Each x-ray photon is produced by a single electron so that electron must have lots of energy (it must be a high voltage electron).
Why: x-ray photons carry lots of energy and are produced only by electrons with lots of energy.
(D) The x-ray photons that make it through your carry-on bag strike atoms in a detector and cause them to release electrons. These electrons are then amplified and used to form an image of the bag's contents. Why does using large atoms with many electrons make the detector more effective at absorbing x-rays and emitting electrons?
Answer: The electrons in small atoms can't handle the enormous energy of an x-ray photon (or the loosely bound electrons in a small atom cannot absorb the huge energy in an x-ray photon.)
Why: The photoelectric effect occurs when a photon of light knocks an electron out of an atom. This only occurs when the electron that leaves winds up with relatively little energy. If the outgoing electron would have to have a large kinetic energy, it's unlikely to occur. Since small atoms have weakly attached electrons, an x-ray photon would toss one of these electrons out of the atom and give it lots of kinetic energy. As a result, this sort of event is rare. But a large atom has more tightly attached electrons that would leave with less kinetic energy after absorbing an x-ray photon. The photoelectron effect isn't so rare in the larger atoms. That's why large atoms absorb x-rays far more often than small atoms do.
You have recently moved to Manhattan and are beginning to fix up the dilapidated bathroom in your small apartment.
(B) You want to replace one of the glass mirrors, so you go to the hardware store. The salesperson cuts the new mirror to size by scratching it with a diamond tool and then bending it. The glass surface tears neatly along the scratch. Why doesn't the glass split along a crystal facet (a perfectly ordered surface of atoms) the way a diamond does?
Answer: Glass has no crystalline structure (or no crystal facets).
Why: Glass is an amorphous solid, meaning that it has no long range order within its atoms. There are no smooth crystal planes in glass, so it can't break along such a plane. It just tears irratically.
(C) You use a razor blade to scrap away the old silicon rubber chalking that seals the edges of the bathtub. This rubber originally squirted out of a tube as a liquid but exposure to air caused it to vulcanize into a cross-linked elastic rubber, similar to the rubber in a car tire. Why can't you melt this rubber with hot air and then wipe it away?
Answer: It's one giant molecule (or vulcanized rubber can't melt).
Why: The cross-linking that occurs during vulcanization turns the silicon rubber into a single, giant molecule. It simply can't flow any more. Heating will only cause it to scorch or burn; it won't ever melt and flow.
(D) There is a long strip of copper metal hanging out from under the sink. You have nothing that can cut it, so you bend it back and forth repeatedly until it breaks off. Why does the strip get stiffer each time you bend it?
Answer: You are breaking up the crystals into little pieces and these pieces can't undergo slip as easily (or you are work-hardening the metal).
Why: Large copper crystals deform easily because rows of atoms can slide smoothly across one another. But when you break up the large crystals into smaller ones, the slip process becomes difficult to produce. The crystallites don't agree on which way to slide their rows of atoms so they don't work together well and the overall cluster of crystallites is difficult to deform.