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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.

Like most materials, iron expands as you heat it. But as iron’s temperature exceeds 723 °C, the iron suddenly shrinks slightly because

(A) it changes from one crystal form (ferrite) to another (austenite).

(B) its thermal energy increases with increasing temperature up to 723 °C and decreases with increasing temperature above that temperature.

(D) it loses its vibrational elasticity at that temperature.

(A) microwaves do not contain enough energy to melt ice.

(B) frozen foods reflect microwaves while thawed foods do not.

(D) microwaves are much hotter than ice so that little heat flows between the microwaves and the ice.

You are taking a photograph of friends on the other side of the room. The lens of your camera has a focal length of 50 mm (50 millimeters). To produce a sharp, focused photograph, the camera lens should be

(A) a little less than 50 mm away from the film.

(B) a little more than 50 mm away from the film.

(D) 50 mm away from the film.

Ordinary incandescent light bulbs convert more than 50% of the electric power they consume into electromagnetic waves. Nonetheless, they are very inefficient at providing illumination. Those statements do not contradict one another because most of the electromagnetic waves the bulb produces

(A) are trapped inside the glass envelope and never escape to provide illumination.

(B) are not photons and therefore do not provide the energy needed for vision.

(D) are not visible light.

The discharge in a fluorescent lamp won’t operate well unless it continuously produces positively charged mercury ions. These ions help negatively charged electrons travel through the tube. Which of the following changes would prevent the lamp from turning mercury atoms into mercury ions and spoil the discharge?

(A) Substantially reducing the voltage drop through the lamp.

(B) Substantially reducing the current passes through the lamp.

(D) Substantially reducing the diameter of the lamp.

To make ordinary glass sheeting, the manufacturer pours molten glass onto the surface of liquid tin. The glass is then cooled fairly quickly below its melting temperature and it soon stiffens into a rigid solid. One consequence of cooling the glass too slowly and letting it remain just below its melting temperature for a long time would be that the glass would

(A) develop a milky white character rather than being perfectly clear.

(B) never fully harden and would not be rigid, even at room temperature.

(D) curl as it hardened and wouldn’t be useable for windows.

Your heirloom porcelain teacup has beautiful gold decorations on it, including a ring of gold that encircles the lip of the cup. If you heat the cup in a microwave oven, the gold ring will turn black in a few seconds because the microwaves will

(A) magnetize the gold ring and cause it to attract black iron particles in the paint.

(B) interfere destructively inside the gold ring and shift its color from gold to black.

(D) push currents back and forth through the gold ring, causing it to overheat and burn.

The faster a radio station shifts the charge on its antenna from positive to negative and back again, the

(A) higher the frequency of its radio wave and the shorter the wavelength of that wave.

(B) lower the frequency of its radio wave and the shorter the wavelength of that wave.

(D) higher the frequency of its radio wave and the longer the wavelength of that wave.

A television tube’s electron beam is steered horizontally and vertically so that it hits a particular point inside the tube’s screen. This steering is done using

(A) magnetic fields.

(B) strawberry fields.

(D) electric fields.

A needle-sharp lightning rod protects the top of a tall tower. This rod is connected by a wire to the earth far below. When a positively charged cloud passes over the tower, the lightning rod

(A) becomes negatively charged and begins to emit negative charges onto passing air particles which help to neutralize the cloud.

(B) becomes positively charged and repels any lightning strikes so that they hit far away from the tower.

(D) remains electrically neutral until lightning strikes it, at which time it suddenly becomes positively charged.

The recording head of a tape recorder is a ring with a narrow gap in it. As the tape passes this gap, sound information is recorded on the tape. The ring is made from

(A) a hard magnetic material that is magnetized during the process of building the tape recorder.

(B) a poor electric conductor that becomes a better conductor when it is exposed to light.

(D) a good electric conductor that becomes magnetic when a current flows through it.

A thief can sometimes break a steel padlock by chilling it with liquid nitrogen and then striking it with a hammer. Padlock steel is normally very tough, meaning that you must do lots of work on it before it breaks. But cooling steel can spoil its toughness because at very low temperatures,

(A) the steel crystallizes and is then unable to undergo slip or plastic deformation.

(B) steel cannot conduct electricity and insulators are brittle.

(D) dislocations can't move through steel to assist slip and permit plastic deformation.

When you first turn on a high-pressure mercury vapor lamp, as I did in class one Friday, it glows with dim blue-purple light. Its starting spectrum contains only a few specific wavelengths of light. But after a minute or two of warming up, the lamp is bright white and the specific wavelengths in its starting spectrum have spread out into broad ranges of wavelength. This broadening of its specific emission wavelengths into ranges of wavelengths is the result of

(A) the mercury reaching the temperature at which its blackbody spectrum is equivalent to that of the sun.

(B) the lamp’s phosphors being activated by exposure to hot halogen gas.

(D) the lamp’s electrodes reaching the point at which they can sustain a full discharge, rather than a starting discharge.

Some clear plastics slow light more than others. The lens of a disposable camera is made from a plastic that slows light a moderate amount. If the manufacturer were to replace this plastic with a plastic that slows light considerably more, the real image would form

(A) too far from the lens and would contain less total light.

(B) too far from the lens, but would contain more total light.

(D) too close to the lens and would contain less total light.

The channel of an n-channel MOSFET is formed from p-type semiconductor—semiconductor that has chemical impurities that leave its valence band partially empty. The presence of positive charge on the MOSFET’s nearby gate surface causes this channel to

(A) remain p-type, but it acquires a net positive charge.

(B) become n-type, although it remains electrically neutral.

(D) remain p-type and its valence band to become even emptier.

A mobile is a work of art that’s constructed from sticks, strings, and objects. The objects hang on strings from the long, straight sticks and these sticks hang from one another in complicated arrangements. You are building a mobile for your office and want to be sure that it is stable—that it doesn’t tip wildly to one side and collapse. To be sure that it is stable in its desired arrangement, you should make sure that this arrangement

(A) has no momentum when it is not moving.

(B) has as little kinetic energy as possible.

(D) has the lowest possible total potential energy.

You are using a tape recorder to capture the sounds of the great outdoors for a new multimedia wildlife publication. Today, you are recording the sounds of Antarctica on a windless afternoon—total silence. After the recording tape has passed under the recording head and had information corresponding to total silence recorded on its surface, the magnetic particles on that tape are

(A) magnetized alternately one way and then the other, with a separation between the two patches of approximately 0.001 millimeter.

(B) magnetized alternately one way and then the other, with a separation between the two patches of approximately 0.1 millimeter.

(D) magnetized alternately one way and then the other, with a separation between the two patches of approximately 1 millimeter.

The X rays used to image your bones are chosen so that they are strongly absorbed by calcium atoms in bone, but only weakly absorbed by the simpler atoms in tissue. However, sometimes it’s important to use X rays that pass easily through calcium in order to find objects made of iron or other more complex atoms. Compared to ordinary imaging X rays, those that see right through bone must

(A) travel at faster speeds.

(B) have lower frequencies.

(D) have longer wavelengths.

A tank circuit consists only of an inductor and a capacitor, wired together so that each end of the inductor is connected to an end of the capacitor. The tank circuit’s energy shifts back and forth between two forms: energy in the magnetic field and energy in the electric field. The magnetic field energy reaches its maximum at the moment

(A) the voltage difference between the two plates of the capacitor is at its maximum.

(B) the current passing through the inductor reaches its maximum.

(D) the separated charge in the capacitor is at its maximum.

A radio station uses a single vertical antenna to transmit its radio waves. These waves are

(A) horizontally polarized and contain only electric fields.

(B) vertically polarized and contain both electric and magnetic fields.

(D) vertically polarized and contain only magnetic fields.

You are at the county fair and are throwing objects at a stack of heavy milk bottles. If you can push the bottles off the far end of the table, you’ll win a giant stuffed animal. The objects you can throw are (1) a beanbag that doesn’t bounce or stick, (2) a blob of putty that doesn’t bounce but sticks to anything it hits, and (3) a rubber ball that bounces well and doesn’t stick. Assuming that you give each of these three objects the same momentum when you throw them, which object is most likely to push the bottles off the table?

(A) Since you throw them with equal momenta, they are equally good at pushing the bottles off the table.

(B) The blob of putty (item 2).

(D) The beanbag (item 1).

You have three batteries: a 9.0-volt battery and two 1.5-volt batteries. Using clips and wires, you can link these batteries together in various chains. The possible voltage rises that you can obtain with these batteries are

(A) 1.5 volts, 3.0 volts, 9.0 volts, and 10.5 volts.

(B) 1.5 volts, 3.0 volts, 9.0 volts, 10.5 volts, and 12.0 volts.

(D) 1.5 volts, 3.0 volts, and 9.0 volts.

You are working out on a trampoline, doing flips in the air. As you travel through the air on one of these flips, which of the following physical quantities is constant?

(A) Your angular momentum.

(B) Your angular velocity.

The glass used in color television screens is designed to absorb X rays because color picture tubes can produce some X rays. Those X rays are created when high-energy electrons

(A) bend slowly in the presence of the tube’s electric fields.

(B) travel long distances through the vacuum tube without hitting anything.

(D) collide with heavy atoms in the shadow mask and screen.

A nuclear weapon must assemble its nuclear fuel into a supercritical mass in the blink of an eye or the weapon won’t explode properly. If the assembly is too slow, the chain reaction will start before assembly is complete. Such an early start for the chain reaction will cause

(A) most of the fuel’s nuclei to fission before the fuel reaches maximum density, greatly decreasing the size of the shockwave and subsequent explosion.

(B) the fissions to release too many neutrons. Since neutrons do not help a chain reaction, producing them in large quantities spoils the chain reaction.

(C) the fissions to create unusually small nuclear fragments such as hydrogen and helium nuclei. The hydrogen nuclei will then burn to create an undesirably large fireball.

(D) the nuclear fuel to overheat and fly apart before many of its nuclei have fissioned.

A transformer can sometimes transfer power from one electric circuit to another. If the first electric circuit is carrying a steady direct current (DC), the transformer’s primary coil will

(A) not produce a magnetic field because only a changing current can produce a magnetic field.

(B) produce an electric field but no magnetic field because electric charges are not magnetic and cannot produce magnetic fields.

(C) produce a magnetic field, but there will be no electric field to push current through the secondary coil because only a changing magnetic field can produce an electric field.

(D) produce a magnetic field, which will produce an electric field that pushes current through the secondary coil.

The area around the p-n junction of a semiconductor diode is known as the depletion region. It is electrically insulating because electrons from the

(A) conduction band of the p-type semiconductor have crossed the junction to fill empty levels in the valence band of the n-type semiconductor.

(B) depletion region have flowed outward to the edges of the diode as the result of electrostatic repulsion.

(C) conduction band of the n-type semiconductor have crossed the junction to fill empty levels in the valence band of the p-type semiconductor.

(D) depletion region have flowed outward to the edges of the diode as the result of electrostatic attraction.

Lightning strikes near your neighbor’s car and puts a large amount of negative charge on it. Because the car is insulated by its tires, this charge remains on the car indefinitely. You take out a charge detector and begin looking for where this charge is located. You find that it is

(A) spread uniformly through the car’s volume.

(B) only on the outside surface of the car’s metal shell.

(D) on both the inside and outside surfaces of the car’s metal shell.

A diamond drill cuts through glass easily, making it easy to run wires through ordinary windows. But such a drill can’t be used on a tempered glass window because cutting through the surface of the tempered window

(A) will cause the window to shred itself into tiny fragments.

(B) will allow the carbon in the glass to go back into solution and make the glass soft.

(D) is impossible because the molecules in tempered glass are harder than the drill.

The photoconducting surface of a xerographic copier must be made out of pure semiconductor, not semiconductor that has been chemically doped to form n-type or p-type semiconductor. Those doped semiconductors would not work in a copier because

(A) they have net electric charges, even in the dark.

(B) they conduct electricity, even in the dark.

(D) they are magnetic, even in the dark.

To minimize power loss in the process of sending electricity long distances, it is sent as a small current of very high voltage charges. This procedure makes sense because the energy wasted in a wire is proportional to

(A) the square of the current in that wire.

(B) the square of the voltage in that wire.

(D) the square root of the current in that wire.

Superglue and Crazy Glue are both cyanoacrylate glues. They harden on contact with moisture because the moisture

(A) causes the small individual molecules in the glue to chain together into giant polymer molecules.

(B) plasticizes the glue and shifts it from the liquid flow regime to the glassy regime.

(D) replaces the solvent molecules that dissolved the glue and cause it return to its normally solid form.

Reducing the sizes of wires that connect logical elements in a computer is crucial to reducing that computer’s power consumption. After all, each time a logical element switches its output wire from positive to negative, there is a net transfer of charge from the positive terminal of the computer’s battery to its negative terminal. This process consumes the battery because the battery

(A) must then use its chemical energy to pump the arriving charge back to its positive terminal.

(B) is magnetized by this arriving charge and must produce a current to repel the magnetic poles.

(D) accelerates in response to this arriving charge and converts its chemical potential energy into kinetic energy.

When a recorded magnetic tape travels past the playback head of a tape player, the recording head produces an electric current that is proportional to the air pressure fluctuations in the recorded sound. The energy in this current comes from

(A) thermal energy in the playback head.

(B) kinetic energy of the moving tape.

(D) gravitational energy in the tape itself.

You have a large, Frisbee-shaped magnet and you fling it rapidly across the surface of a huge aluminum table. The magnet will

(A) flip over and stick to the aluminum table.

(B) veer to the right because of its magnetic current and the right-hand rule.

(D) veer to the left because of its negative magnetic current and the right-hand rule.

Light remains trapped inside an optical fiber because it always encounters the surface of the fiber's central glass core at a grazing angle and experiences total internal reflection. The fiber is able to keep light inside its central glass core because the glass cladding around that core

(A) is photoconducting and becomes insulator-like when exposed to light.

(B) has a higher index of refraction (slower light speed) than the core itself.

(D) has a lower index of refraction (faster light speed) than the core itself.

A typical laser stops working properly when you reflect some of its outgoing light back into it because the reflected light

(A) has the wrong wavelength for the laser to work with and the laser shuts down.

(B) is traveling in the wrong direction through the laser and the laser can only handle light traveling through it in one direction.

(D) is amplified inside the laser, wasting the laser’s energy and perhaps even damaging it.

Copper is a nonmagnetic metal. When you move the north pole of a bar magnet toward a sheet of copper,

(A) current flows through the copper, but the copper does not become magnetic.

(B) current flows through the copper and attracts the approaching north pole.

(D) nothing happens to the copper.

Naturally occurring uranium isn’t suitable for a nuclear weapon because it contains two different isotopes of uranium and only the rare isotope is fissionable. These two isotopes of uranium are different because they contain different numbers of

(A) protons in their nuclei.

(B) electrons in their orbitals.

(D) orbitals that can hold electrons.

On earth, a typical astronaut weighs about 850 newtons. As he walks in space outside the earth-orbiting Space Shuttle, the astronaut weighs

(A) exactly 0 newtons and exerts no upward force on the earth.

(B) about 800 newtons and exerts an upward force on the earth that is also about 800 newtons.

(D) about 800 newtons but exerts no upward force on the earth.

(A) magnetized spots on a surface containing tiny permanent magnets.

(B) tiny magnetized surface regions of a soft magnetic material.

(D) tiny accumulations of electric charge on an insulating surface.

In a television picture tube, a beam of negatively charged electrons flows from an electron gun in the back of the tube to the phosphor coating inside the tube’s front screen. Since the beam accelerates forward as it flies through the empty tube, it is clear that the

(A) voltage of the screen is much higher (more positive) than the voltage of the electron gun.

(B) current reaching the screen is much larger than the current leaving the electron gun.

(D) current reaching the screen is much smaller than the current leaving the electron gun.

Adding laundry borax to a white glue such as Elmer’s creates crosslinks between the glue molecules. The result of this crosslinking is that the glue

(A) turns almost instantly into an elastic putty-like solid.

(B) loses its cloud-like white appearance and becomes blue, like the sky on a clear day.

(D) crystallizes gradually into a clear, brittle solid.

If you take light from a flashlight, separate it into two beams with mirrors and steer those mirrors so that the beams overlap on a distant wall, you’ll simply see two bright spots superimposed on one another. However, if you do the same thing with light from a laser and look carefully at the wall, you’ll see remarkable bright and dark bands in the overlapping spots. These bands are the result of interference effects. The interference effects are visible only with laser light because laser light is

(A) formed from both electric and magnetic fields, while light from a flashlight has only electric fields in it.

(B) one giant wave and pieces of that wave interfere strongly with one another.

(C) much more intense than light from a flashlight and interference is a quantum mechanical effect that only occurs at high light intensities.

(D) much more highly focused than light from a flashlight and interference is a quantum mechanical effect that only occurs in highly focused light.

Mercury waste is an environmental problem, so it would be nice to eliminate mercury from fluorescent lamps. One alternative to mercury would be nontoxic neon. But no matter what phosphors you used in a neon-based fluorescent lamp, it would not produce much green or blue light because

(A) the phosphors can only redirect the light from the neon discharge, they cannot alter its wavelengths.

(B) neon emits mostly blue light, but this light is consumed by the phosphors in the process of producing red light. Little blue and virtually no green would leave the lamp.

(C) neon emits mostly red photons and there is not enough energy in red photons for phosphors to convert them into green or blue photons.

(D) neon emits mostly ultraviolet photons, which we can’t see. Phosphors can convert ultraviolet photons into red photons, but not into green or blue ones.

CDs and DVDs store their information as ridges in a reflective layer embedded deep in a layer of clear plastic. The plastic layer helps to protect the layer and to reduce the importance of dirt or dust, but it also allows information to be packed more tightly on the reflective surface. This tighter packing is possible because when light from the laser enters the plastic, its

(A) frequency decreases while its wavelength remains unchanged.

(B) wavelength increases while its frequency remains unchanged.

(D) wavelength decreases while its frequency remains unchanged.

Soon after the water in a fountain leaves the nozzle and begins its flight up into the air, it breaks up into round balls of water. As one of these water balls travels upward it experiences

(A) an upward force that decreases gradually and reaches zero at the moment the ball reaches its peak height.

(B) an upward force that remains constant until the ball reaches its peak height and then becomes a downward force.

(C) an upward force that diminishes gradually and reaches zero at the moment the ball returns to the height at which it left the nozzle.

(D) no upward force.

When you look at a thin film of clear oil or gasoline on the surface of water, that film appears colored. Suppose that you are looking at such a film and it appears green. As the film spreads out and becomes thinner, its color changes and it next appears

(A) is a soft magnetic material and quickly demagnetizes when you remove it from any external magnetic fields.

(B) has no internal magnetic structure at all.

(D) does not have enough mass to overcome the inertia of permanent magnetism.

A resistor is carrying a current of 2 ampere and that current is experiencing a voltage drop of 3 volts as it passes through the resistor. If you double the current to 4 amperes, the voltage drop through the resistor will be

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.

You are shining the beam from your flashlight into a calm pool of water. You aim the beam forward and downward so that it arrives at the water’s surface at a 45° angle (half-way between horizontal and vertical).

(A) In which direction does the light beam travel once it is inside the water? Please mark the approximate direction on the figure.

(B) Do red and violet portions of the light travel the same direction in the water? If not, show approximately how their paths through the water differ by marking them on the figure.

(D) Light leaving the flashlight is unpolarized, meaning that it is an equal mixture of vertically and horizontally polarized light. (Note that because the light beam is tipped downward, the electric field of vertically polarized light is also tipped downward.) Is the light traveling through the water still an equal mixture of both polarizations? If not, which polarization is stronger in the water.

Some modern stovetops use a technique known as “induction heating” to heat cookware directly. The glass or ceramic surface of the stovetop stays cool and only the pot itself becomes hot. In principle, induction heating will work with any metallic pot, but for technical reasons that are beyond the scope of this problem, it works most efficiently with ferromagnetic metals.

(A) Just under the surface of an induction stovetop is a coil of wire. When the stove is turned on, a high-frequency alternating current passes through the coil. A metal pot placed just above the surface of the stovetop quickly becomes warm. This heating occurs because electric current flows through the pot. How does the presence of an alternating current in the stovetop’s coil cause current to flow in the pot?

(B)Why does current flow in the pot cause the pot to become hot?

(C) Glasses and ceramics contain electric charges, so why can’t this heating technique be used with a glass or ceramic pot?

(D) Producing high-frequency alternating current is relatively expensive. Why can’t this technique use a steady direct current in the stovetop’s coil?

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?

(D) Your favorite restaurant decides to block cell phones entirely by wrapping the entire dining room in metal mesh. The mesh has 1-millimeter holes in it and is almost invisible, yet it completely blocks the passage of cell phone radio waves. Why can’t the radio waves pass through the holes in the mesh?

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.

(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?

(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.

You have recently moved to Manhattan and are beginning to fix up the dilapidated bathroom in your small apartment.

(A) There is an old steel screw holding the towel rack on the wall. The metal in that screw is very soft and you damage its head as you unscrew it. After all, the steel in your screwdriver is much harder than the steel in the screw. Assuming that both the screw and the screwdriver are made of carbon steel, which contains a larger percentage of carbon and why does the carbon matter?

(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?

(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?

(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?

Your plans to become a rock star, brain surgeon, and test pilot are on hold because you forgot to pay a fine on an overdue library book. While the authorities sort things out, you have taken a job as a UN arms inspector, searching for weapons of mass destruction. You discover that several unscrupulous dictators have developed secret nuclear weapons programs but fortunately they’re all run by inept scientists and engineers.

(A) The first program you uncover is gearing up to build fission bombs out using uranium purchased over the web from a commercial chemical company. This uranium is known as “depleted uranium” because almost all the uranium-235 has been removed and it contains almost pure uranium-238. Why won’t this program be able to make a working fission bomb based on this commercial uranium?

(B) The second program is based on plutonium smuggled out of former Soviet republics. They have only 600 grams of plutonium overall, just a tenth the amount used in a typical US nuclear weapon, but they are willing to settle for an explosion with just a tenth the explosive yield. Why won’t they obtain any explosion at all?

(C) Participants in the third program have acquired almost 50 kilograms of plutonium. They plan to roll ten 5-kilogram balls of plutonium down the sides of a large bowl so that they meet at the middle and trigger the explosion. Why won’t this technique produce the large explosion they are expecting?

(D) The fourth and final program is quite ambitious. Unable to obtain any uranium or plutonium, they have been collecting the heavy isotopes of hydrogen: deuterium and tritium. Despite having great quantities of these hydrogen-bomb fuels, they won’t be able to make a working fusion bomb because