Physics 106N - How Things Work - Spring, 1997
Final Examination
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:
You throw a softball straight up. Neglecting any effects of air resistance,
the ball experiences the following force or forces after it leaves your
hand and before it hits the ground:
(A) its weight and a gradually decreasing upward force that appears just
after the ball leaves your hand and that diminishes to zero when the ball
reaches its peak height.
(B) its weight and a gradually decreasing upward force that appears just
after the ball leaves your hand.
(C) a gradually decreasing upward force that appears just after the ball
leaves your hand and that diminishes to zero when the ball reaches its
peak height, and a gradually increasing downward force of gravity that
appears just after the ball reaches its peak and that increases as it descends.
(D) its weight.
Answer: (D) its weight.
Problem 2:
When you bounce a tennis ball off a concrete wall, the ball
(A) retains essentially all of its energy and momentum.
(B) retains essentially all of its energy but transfers a great deal of
momentum to the wall.
(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: (B) retains essentially all of its energy but transfers a great
deal of momentum to the wall.
Problem 3:
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) allow the charges to flow through the photoconductor so that they wouldn't
accumulate on its surface.
(B) release magnetic poles that would cancel the electric charges.
(C) cause the magnetic poles to move, generating electricity that would
neutralize the electric charges.
(D) expand the coherence of the waves and prevent the charges from forming
a real image on the surface.
Answer: (A) allow the charges to flow through the photoconductor so
that they wouldn't accumulate on its surface.
Problem 4:
You toss a tennis ball directly upward and watch it rise to its peak height.
At the moment that it reaches that peak height, its velocity is
(A) zero and its acceleration is zero.
(B) downward and its acceleration is downward.
(C) downward and its acceleration is zero.
(D) zero and its acceleration is downward.
Answer: (D) zero and its acceleration is downward.
Problem 5:
When making glass, it's important to cool the liquid glass through its
melting temperature and below fairly quickly in order to
(A) keep the glass from turning dark.
(B) keep the glass from becoming too thick to work with.
(C) prevent crystals from nucleating and growing.
(D) prevent stresses from developing in the glass.
Answer: (C) prevent crystals from nucleating and growing.
Problem 6:
There is a metal screen on the front window of a microwave oven. This screen
is essential because it
(A) prevents electric charge from accumulating on the window.
(B) protects the window from splatters that may occur when food inside
the oven boils.
(C) reflects microwaves and keeps them inside the oven.
(D) prevents the glass from breaking in response to the rapid heating of
its inner surface and the thermal stresses that result.
Answer: (C) reflects microwaves and keeps them inside the oven.
Problem 7:
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 eyepiece with one having a
(A) larger aperture than before.
(B) smaller aperture than before.
(C) shorter focal length than before.
(D) longer focal length than before.
Answer: (C) shorter focal length than before.
Problem 8:
When you record music on a blank audio tape, the tape appears unchanged.
However, something has happened to the tape:
(A) north magnetic poles have been deposited on its surface.
(B) south magnetic poles have been deposited on its surface.
(C) its surface has been magnetized alternately back and forth.
(D) an electric current has been induced in the tape, making it magnetic.
Answer: (C) its surface has been magnetized alternately back and forth.
Problem 9:
Which of the following objects is experiencing the largest net force toward
the east?
(A) a bullet that is traveling eastward shortly after being fired from
a gun.
(B) a westward-bound car that is stopping at a light.
(C) a cannonball that is traveling eastward shortly after being fired from
a cannon.
(D) a stationary building that you are pushing toward the east as hard
as you can.
Answer: (B) a westward-bound car that is stopping at a light.
Problem 10:
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
(A) an electromagnetic wave must have a magnetic field that changes with
time to produce its electric field.
(B) electromagnetic waves always contain both electric charges and magnetic
poles, and magnetic poles are accompanied by magnetic fields.
(C) electromagnetic waves contain moving electric charges and charges produce
magnetic fields when they move.
(D) while waves consisting only of electric fields are common and travels
indefinitely through space, they are known as "electric waves," not "electromagnetic
waves."
Answer: (A) an electromagnetic wave must have a magnetic field that
changes with time to produce its electric field.
Problem 11:
An FM radio station instructs your radio receiver how to move its speaker
cone by varying the
(A) amount of electric charge it pushes up and down its antenna.
(B) amplitude of the radio wave it produces.
(C) strength of the electric and magnetic fields in the radio wave it produces.
(D) frequency of the radio wave it produces.
Answer: (D) frequency of the radio wave it produces.
Problem 12:
The image that you see on the front surface of a typical television is
created when
(A) electric currents flowing through the inside surface of the screen
heat small regions of phosphor hot enough to emit light.
(B) electromagnetic waves from the television's transmitter are focused
on the phosphors inside the front surface of the picture tube and cause
those phosphors to glow.
(C) a laser beam scans rapidly across the inside of the front surface of
the picture tube.
(D) rapidly moving electrons from the back of the picture tube collide
with phosphors inside its front surface.
Answer: (D) rapidly moving electrons from the back of the picture tube
collide with phosphors inside its front surface.
Problem 13:
You are pushing a file cabinet across the floor in a straight line at a
steady speed. Which of the following statements about the forces acting
on the file cabinet is correct?
(A) The magnitude of the force that you're exerting on the file cabinet
must be equal to the magnitude of the force that friction is exerting on
it.
(B) If you were to exert twice as much force on the file cabinet, it would
slide across the floor at twice its original speed.
(C) The magnitude of the force that you're exerting on the file cabinet
must be more than the magnitude of the force that friction is exerting
on it.
(D) The amount of forward force you are exerting on the file cabinet must
be more than its weight.
Answer: (A) The magnitude of the force that you're exerting on the file
cabinet must be equal to the magnitude of the force that friction is exerting
on it.
Problem 14:
A capacitor is an electronic device that contains two metal surfaces that
are separated by a thin layer of insulation. Suppose that you have a large
capacitor that has no overall net charge. If you wanted to see whether
the capacitor is storing electrostatic potential energy, you could
(A) hold the capacitor in your hand and see if your hair stands up.
(B) hold a magnet near the capacitor and see of that magnet is attracted
toward the capacitor.
(C) provide a conducting path from one metal surface to the other and see
if an electric current flows through that path.
(D) hold a compass near the stationary capacitor and see if the compass
needle deflects.
Answer: (C) provide a conducting path from one metal surface to the
other and see if an electric current flows through that path.
Problem 15:
Vegetable oil and Pyrex glass have almost exactly the same indices of refraction-the
speed of light is the same in both materials. If you submerge a clear Pyrex
dish in vegetable oil,
(A) the Pyrex dish will be virtually invisible because no light will reflect
from its surfaces.
(B) you will see a pattern of beautiful rainbow colors in the glass as
dispersion separates white light.
(C) the Pyrex dish will act as a lens and you will see highly distorted
images of objects beyond the dish.
(D) the surfaces of the Pyrex will appear mirror-like-reflecting light
perfectly as the result of total internal reflection.
Answer: (A) the Pyrex dish will be virtually invisible because no light
will reflect from its surfaces.
Problem 16:
While small satellite dishes are cheaper and easier to install than large
satellite dishes, the larger dishes
(A) don't require such high-tech materials and can be made with conventional
metals rather than special graphite composites.
(B) don't collect more microwaves but are better able to distinguish between
the microwaves from two nearby satellites.
(C) collect more microwaves but are not more capable of distinguishing
between the microwaves from two nearby satellites.
(D) collect more microwaves and are better able to distinguish between
the microwaves from two nearby satellites.
Answer: (D) collect more microwaves and are better able to distinguish
between the microwaves from two nearby satellites.
Problem 17:
There is a simple explanation for why bluer "cool white" fluorescent lamps
are slightly more energy efficient than redder "warm white" lamps. It's
because
(A) a phosphor wastes more energy when converting an ultraviolet photon
into red light than into blue light.
(B) bluer lamps use higher currents and smaller voltages than redder lamps.
(C) bluer lamps operate at higher temperatures than redder lamps and produce
less invisible infrared light as a result.
(D) bluer lamps use higher voltages and smaller currents than redder lamps.
Answer: (A) a phosphor wastes more energy when converting an ultraviolet
photon into red light than into blue light.
Problem 18:
Which of the following materials is most appropriate for making an elastic
coil spring?
(A) 18-8 (austenitic) stainless steel.
(B) Pure iron.
(C) Iron carbide.
(D) Tempered carbon steel.
Answer: (D) Tempered carbon steel.
Problem 19:
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 completely motionless.
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) the team on the left.
(B) both teams.
(C) neither team. It was instead provided by chemical potential energy
in the rope itself.
(D) the team on the right.
Answer: (D) the team on the right.
Problem 20:
You place an aluminum soda can on a hard floor and step on it with your
foot, crushing it flat. The act of crushing the can
(A) decreases the can's total energy.
(B) has an undeterminable effect on the can's total energy.
(C) leaves the can's total energy unchanged.
(D) increases the can's total energy.
Answer: (D) increases the can's total energy.
Problem 21:
Some fiber optic communication systems use laser amplification to boost
the intensity of light pulses passing through an optical fiber. A short
segment of laser amplifier fiber is spliced into the main fiber so that
the light pulses must pass through the laser amplifier fiber on their way
to their destination. The light emerging from the laser amplifier fiber
is much brighter than it was when it entered. One interesting aspect of
the light emerging from the laser amplifier fiber is that it
(A) has the same color as the light entering the amplifier fiber.
(B) is slightly redder than the light entering the amplifier fiber-its
wavelength is longer.
(C) is slightly bluer than the light entering the amplifier fiber-its wavelength
is shorter.
(D) travels faster than the light entering the amplifier fiber.
Answer: (A) has the same color as the light entering the amplifier fiber.
Problem 22:
You have just put fresh batteries in your flashlight and it's working nicely.
Now suppose that you remove all of the batteries and reinsert them backward.
Despite the fact that their positive and negative terminals are reversed,
the batteries connect properly to one another and to the flashlight, so
that there are no connection problems in the flashlight. As the result
of this battery reversal, the flashlight will
(A) work in reverse, lighting up when you put its switch in the "off" position
and not lighting up when you put its switch in the "on" position.
(B) remain lit regardless of the position of its on-off switch.
(C) not light up at all, regardless of the position of its on-off switch.
(D) work normally, lighting when you switch it "on" and turning off when
you switch it "off."
Answer: (D) work normally, lighting when you switch it "on" and turning
off when you switch it "off."
Problem 23:
Your friends have talked you into jumping off the high diving board. You
are quite nervous about hurting yourself when you hit the water, so you
decide to make sure that your speed is a small as possible when you reach
the water's surface. Of the following ways to leave the diving board, which
one will give you the smallest speed when you reach the water?
(A) Running as quickly as possible off the board.
(B) Hooking your toes around the end of the board and pulling yourself
downward as you walk off the board.
(C) Walking as slowly as possible off the board.
(D) Jumping upward as you walk off the board.
Answer: (C) Walking as slowly as possible off the board.
Problem 24:
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) electric charges in the glass attract the light and prevent it from
accelerating outward, away from the center of the fiber.
(B) the fiber contains many tiny diverging lenses that focus the light
back into the fiber over and over again.
(C) whenever it tries to leave the glass at a shallow angle, it's perfectly
reflected.
(D) the fiber contains many tiny converging lenses that focus the light
back into the fiber over and over again.
Answer: (C) whenever it tries to leave the glass at a shallow angle,
it's perfectly reflected.
Problem 25:
Many sophisticated cameras have zoom lenses. When you select the telephoto
setting, the objects in front of you appear much closer. In this setting,
(A) the focal length of the lens is shorter than normal.
(B) the aperture of the lens is larger than normal.
(C) the focal length of the lens is longer than normal.
(D) the aperture of the lens is smaller than normal.
Answer: (C) the focal length of the lens is longer than normal.
Problem 26:
When a beam of charged particles that's flowing through space enters a
stationary magnetic field, the beam's speed
(A) decreases.
(B) increases.
(C) doesn't change.
(D) increases or decreases, depending on the direction of the magnetic
field.
Answer: (C) doesn't change.
Problem 27:
In a well-designed water-cooled thermal fission reactor, the water is acting
as a moderator for the neutrons. If some of this water is lost, the chain
reaction in the reactor core will slow down because neutrons in the reactor
core will
(A) remain positively charged and will not be able to cause uranium 235
nuclei to fission.
(B) not be magnetic enough to stick to uranium 235 nuclei and cause them
to fission.
(C) be traveling too fast and will be absorbed by uranium 238 nuclei.
(D) be traveling too slowly and will be unable to cause uranium 235 nuclei
to fission.
Answer: (C) be traveling too fast and will be absorbed by uranium 238
nuclei.
Problem 28:
The motor that turns the blades of a typical house fan is an induction
motor. The motor's rotor-its rotating component-is an aluminum object that
has no wires connected to it at all. A typical fan motor has two speeds,
high and low. The reason that the rotor turns faster in the high speed
setting is that
(A) the electromagnets around the rotor have both north and south poles
in the high speed setting while they have only north poles in the low speed
setting.
(B) the magnetic poles created by the electromagnets around the rotor circle
the rotor faster in the high speed setting.
(C) the electromagnets around the rotor have both north and south poles
in the high speed setting while they have only south poles in the low speed
setting.
(D) alternating current is used for the high speed setting while direct
current is used for the low speed setting.
Answer: (B) the magnetic poles created by the electromagnets around
the rotor circle the rotor faster in the high speed setting.
Problem 29:
The rubber in a car tire cannot be recycled because
(A) its polymer strands are cross-linked and it can't melt.
(B) rubber decomposes before it melts.
(C) tire rubber never hardens fully once it has been melted.
(D) the polymer strands in tire rubber are so long that they can't be disentangled
for recycling.
Answer: (A) its polymer strands are cross-linked and it can't melt.
Problem 30:
The sky is blue because
(A) tiny particles in the air are more efficient at scattering short wavelength
light than they are at scattering long wavelength light.
(B) most polluting gases and dust particles in the air are bluish in color
and lend their color to that of the sky.
(C) air molecules absorb red light more efficiently than they do blue light
because of their electron orbitals.
(D) air molecules absorb blue light more efficiently than they do red light
because of their electron orbitals.
Answer: (A) tiny particles in the air are more efficient at scattering
short wavelength light than they are at scattering long wavelength light.
Problem 31:
You are trying to move a heavy air conditioner up the front steps and into
a building. The air conditioner is resting on a cart with wheels. You can
either roll it up a steep ramp beside the steps themselves or up a long,
gradual handicap access ramp nearby. If you roll the air conditioner up
the handicap access ramp, the amount of work you will do on the air conditioner
will be
(A) more than if you had rolled it up the steeper ramp.
(B) the same as if you had rolled it up the steeper ramp.
(C) much less than if you had rolled it up the steeper ramp.
(D) slightly less than if you had rolled it up the steeper ramp.
Answer: (B) the same as if you had rolled it up the steeper ramp.
Problem 32:
The plastics used in most ropes are in the elastic regime of behavior.
When you pull on these ropes, they stretch somewhat but do not break. The
extent to which one of these ropes stretches is limited principally by
the
(A) elasticity of the bonds holding the atoms in the plastic's polymer
strands together.
(B) numbers of atoms in the backbones of the plastic's polymer strands.
(C) lengths of the plastic's polymer strands.
(D) entanglements of the plastic's polymer strands.
Answer: (D) entanglements of the plastic's polymer strands.
Problem 33:
If you "cook" a compact disc briefly in a microwave oven,
(A) nothing significant happens.
(B) it becomes permanently magnetized.
(C) 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.
Answer: (D) it becomes hot enough to soften and sparks leap about its
surface.
Problem 34:
The tubes in fluorescent lamps have a white powder on their inner surfaces.
This powder
(A) conducts electricity from one electrode to the other and emits white
light as current passes through it.
(B) slows down the white light produced by the gas atoms inside the tube
so that less of that white light reflects as it passes through the walls
of the tube.
(C) scatters the white light produced by the gas atoms inside the tube
so that it is both vertically and horizontally polarized and doesn't cause
glare.
(D) absorbs ultraviolet light and uses the energy from that light to produce
white light.
Answer: (D) absorbs ultraviolet light and uses the energy from that
light to produce white light.
Problem 35:
Computers of the 1960's were often refrigerator-sized or larger. These
big computers had to operate more slowly than smaller modern computers
because
(A) the bigger, more massive computers vibrated more slowly than smaller,
less massive modern computers and they had slower clock rates as a result.
(B) the big computers were more massive than modern computers and accelerated
more slowly. This slow acceleration reduced their peak speeds.
(C) bigger computers have more inertia than smaller computers and respond
less rapidly to electric signals.
(D) it took so long for electric signals to travel between various points
within the big computers.
Answer: (D) it took so long for electric signals to travel between various
points within the big computers.
Problem 36:
A good camera lens contains optical components in which two pieces of glass
are glued directly together to form a single transparent object. These
two-element optical components are important because they
(A) bend light more efficiently than simple lenses.
(B) reflect less light than simple lenses.
(C) have larger depths of focus than simple lenses.
(D) avoid the color focusing problems of simple lenses.
Answer: (D) avoid the color focusing problems of simple lenses.
Problem 37:
When a narrow laser beam passes through a fine wire mesh before arriving
at the wall, it forms a complicated pattern of bright spots on the wall.
This pattern of spots would not occur if you sent a flashlight beam through
the mesh because light from the flashlight
(A) is not a single electromagnetic wave.
(B) can't be sent through a single opening of the mesh.
(C) is horizontally polarized, while laser light is vertically polarized.
(D) is vertically polarized, while laser light is horizontally polarized.
Answer: (A) is not a single electromagnetic wave.
Problem 38:
While low pressure sodium vapor lamps are the most energy efficient light
sources commonly available, producing a pure orange-yellow light with a
wavelength of 590 nanometers, high pressure sodium vapor lamps produce
a richer spectrum of light that is more pleasing to the eye. One effect
that contributes to this richer spectrum is
(A) radiation trapping-in which the 590 nanometer light has trouble escaping
from the lamp.
(B) interference-in which light traveling to your eye via several paths
within the lamp becomes brighter or darker, depending on how its electric
fields interact with one another.
(C) thermionic emission-in which electrons in the hot electrodes enter
the gas of the discharge.
(D) dispersion-in which the different colors of light travel at different
speeds within the lamp.
Answer: (A) radiation trapping-in which the 590 nanometer light has
trouble escaping from the lamp.
Problem 39:
When uranium undergoes fission, it releases an enormous amount of energy.
That energy was stored in the uranium nuclei as
(A) kinetic energy in the nucleons.
(B) electrostatic potential energy-in the electrostatic forces between
nucleons.
(C) gravitational potential energy-in the gravitational forces between
nucleons.
(D) nuclear potential energy-in the nuclear forces between nucleons.
Answer: (B) electrostatic potential energy-in the electrostatic forces
between nucleons.
Problem 40:
A fisherman casts his fishing rod by swinging its tip in an arc from behind
him to in front of him. The rod's handle barely moves as the tip moves,
so the rod undergoes rotational motion. The time it takes to complete the
cast is determined principally by the rod's
(A) moment of inertia and the force exerted on it by the fisherman.
(B) mass and the torque exerted on it by the fisherman.
(C) mass and the force exerted on it by the fisherman.
(D) moment of inertia and the torque exerted on it by the fisherman.
Answer: (D) moment of inertia and the torque exerted on it by the fisherman.
Problem 41:
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 remains constant all the way to 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) no horizontal force in the forward direction.
(D) a forward horizontal force that diminishes gradually as the ball approaches
home plate.
Answer: (C) no horizontal force in the forward direction.
Problem 42:
You are hosting a romantic candlelight dinner in your room, but you forgot
to buy the candles and all the stores are closed. Your desk lamp is too
white and too bright to provide the right ambience, so you modify it by
inserting a diode between one of the power wires and the bulb. As the result
of this change, the lamp emits a reddish glow that's roughly half a bright
as without the diode. Just for fun, you reverse the diode so that the end
that used to connect to the power wire is now connected to the bulb and
vice versa. As the result of this reversal, the lamp
(A) emits a deep reddish glow that roughly half as bright as before you
reversed the diode and a quarter as bright as without the diode at all.
(B) emits a brilliant bluish glow that's roughly twice as bright as without
the diode.
(C) still emits the same reddish glow that it emitted before you reversed
the diode.
(D) no longer glows at all because no current flows through the diode.
Answer: (C) still emits the same reddish glow that it emitted before
you reversed the diode.
Problem 43:
Thomas Edison's dominance in the electric power industry was struck a serious
blow when his competitors, Nikola Tesla and George Westinghouse, began
to use transformers with their alternating current (AC) electric power
systems. Edison was unable to use transformers in his direct current (DC)
electric power systems because
(A) Tesla and Westinghouse held the U.S. patent rights to transformers
and prevented Edison from using them.
(B) direct current electric power must flow directly from the generator
to a home, without passing through any intermediate devices.
(C) a transformer requires changing currents in its coils in order to transfer
power between those currents.
(D) DC electric power must flow through thicker wires than can be used
efficiently in transformers.
Answer: (C) a transformer requires changing currents in its coils in
order to transfer power between those currents.
Problem 44:
The atoms in window glass are bound together by covalent bonds. Each of
these bonds involves the
(A) transfer of an electron from one atom to the other.
(B) sharing of a pair of electrons between two atoms.
(C) attraction of the positively charged end of one atom to the negatively
charged end of another atom.
(D) general sharing of electrons between all of the atoms in the glass.
Answer: (B) sharing of a pair of electrons between two atoms.
Problem 45:
If you move the north pole of a permanent magnet toward the surface of
an aluminum pot, a current will flow through that pot and the pot will
become magnetic-repelling your permanent magnet. If you stop the permanent
magnet just before it touches the pot and then hold the permanent magnet
stationary, the repulsive force between the pot and the permanent magnet
will gradually disappear. The repulsive force disappears because the electric
current in the pot
(A) becomes non-magnetic once the permanent magnet stops moving.
(B) stops increasing and becomes steady once the permanent magnet stops
moving.
(C) gradually slows to a stop once the permanent magnet stops moving.
(D) becomes an alternating current once the permanent magnet stops moving
and the pot's magnetic poles then flip back and forth rapidly.
Answer: (C) gradually slows to a stop once the permanent magnet stops
moving.
Problem 46:
A typical commercial jet has a sharpened metal rod projecting backward
from the tip of each wing. These two rods prevent the airplane from accumulating
a large net electric charge during flight by
(A) reducing sliding friction between the airplane and the passing airstream.
(B) transferring any charge on the plane to the air behind the plane.
(C) maintaining a steady electric current between the airplane's wings.
(D) generating high voltages while passing quickly through the earth's
magnetic field.
Answer: (B) transferring any charge on the plane to the air behind the
plane.
Problem 47:
Even when the core of a nuclear reactor exceeds critical mass by a small
amount, it doesn't overheat immediately. The fission chain reactions build
slowly because
(A) it takes seconds or minutes for a neutron released by one fission to
reach another radioactive nucleus and cause it to fission.
(B) the reactor has an enormous thermal mass and it takes many minutes
for its temperature to change significantly, even with nuclear heating.
(C) the half-life of uranium is several minutes so it takes at least that
long for the reactor core to overheat.
(D) some fission neutrons aren't released until seconds or minutes after
the fissions that eventually produce them.
Answer: (D) some fission neutrons aren't released until seconds or minutes
after the fissions that eventually produce them.
Problem 48:
When white sunlight passes through a vase made of cut lead crystal, the
light that emerges is often separated into brilliant colors. This coloration
is due to the fact that
(A) red light moves faster in lead crystal than violet light does.
(B) lead crystal absorbs violet light more strongly than it absorbs red
light.
(C) red light experiences stronger Rayleigh scattering in lead crystal
than violet light does.
(D) lead crystal absorbs red light more strongly than it absorbs violet
light.
Answer: (A) red light moves faster in lead crystal than violet light
does.
Problem 49:
Fission chain reactions in uranium are generally sustained by neutrons,
each of which is released by one fission and may induce a subsequent fission.
Each fission also releases other particles, most of which include protons.
However, proton-containing fission fragments aren't as effective at causing
subsequent fissions because they
(A) aren't massive enough to cause fissions in uranium.
(B) are repelled as they try to approach other uranium nuclei.
(C) don't exert any forces on the uranium nuclei they encounter.
(D) aren't hot enough to make a uranium nucleus boil.
Answer: (B) are repelled as they try to approach other uranium nuclei.
Problem 50:
The tools used to cut metals in a machine shop are often made from extremely
hard steels. It's important to cool these tools with cutting fluids to
prevent them from overheating because raising the steel's temperature several
hundred degrees will cause the steel to
(A) become extremely brittle.
(B) begin melting.
(C) lose some of its hardness.
(D) become a glass.
Answer: (C) lose some of its hardness.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is worth 33%
of the grade on this examination.
Problem 1:
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? It
will not spin at all.
(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? The
rotor will spin counter-clockwise.
(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? The motor is acting as a generator (or the moving
magnetic rotor induces currents in motor's stationary coils of wire).
(D) If you spin the rotor counter-clockwise with your fingers, what will
happen to the light bulb? It lights up.
Problem 2:
You are outdoors on a sunny day, playing with a magnifying glass that is
5 centimeters in diameter, 0.5 centimeters thick, and has a focal length
of 10 centimeters.
(A) You decide to use this glass lens to burn a piece of wood. You hold
it above the wood so that a bright circle of sunlight forms of the wood's
surface. To make the circle's edges sharp and clear, how high above the
wood should you hold the lens? 10 centimeters.
(B) What would you have to do to the lens to make the sharp circle of sunlight
it forms appear the same size as before, but brighter? Increase the
diameter (or aperture) of the lens.
(C) What would you have to do to the lens to make the sharp circle of sunlight
it forms appear larger than before? Increase its focal length and
move it farther from the wood.
(D) You've tired of burning wood, so you begin using the lens as a magnifying
glass to inspect a leaf. What's the farthest from the leaf you can hold
the magnifying glass and still see a virtual image of the leaf? (Almost)
10 centimeters.
Problem 3:
A typical thermal fission nuclear reactor uses natural uranium as its fuel.
(A) Despite the fact that the reactor releases enormous amounts of energy
every day, its fuel assemblies look essentially unchanged. What is happening
inside these fuel assemblies that releases so much energy? The uranium
235 nuclei are fissioning (or breaking).
(B) The reactor surrounds each fuel assembly with water and this water
serves to carry away the thermal energy released while the reactor is running.
But the water also serves another purpose, because without it, the reactor
could not sustain its fission chain reactions. How does the water assist
the chain reactions? It slows (or moderates) the neutrons.
(C) Even if someone were to steal all the new uranium fuel rods from this
reactor, they would not be able to use those rods to make a nuclear weapon.
What prevents these rods from sustaining an explosive fission chain reaction?
The uranium 238 nuclei (in natural uranium) absorb (fast moving)
neutrons.
(D) After the reactor has operated for while, its fuel rods contain modest
amounts of plutonium, a potential fuel for nuclear weapons. Why is it so
much easier to separate plutonium from everything else in a fuel rod than
it is to separate the light isotope of uranium-uranium 235-from everything
else in the fuel rod? Plutonium atoms are chemically different from
the other atoms while the uranium 235 and uranium 238 are chemically identical
to one another.
Problem 4:
You are helping to design a laser light show at a mountainous national
park. The goal is to project the light onto the face of a huge cliff from
another mountain top several kilometers away.
(A) Lasers are relatively expensive, so one of your friends suggests using
a conventional spot light with an incandescent light source rather than
a laser. You point out that the light from an incandescent source can't
possibly be focused to as small a spot as the light from a laser. Why not?
All of the photons from an incandescent light source are independent
and focus at different points (or incandescent light is incoherent and
can't focus as tightly as coherent light).
(B) Your group obtains several lasers for the show and you install them
on a sturdy table. Another friend notices that in each laser, the light
emerges from a partially reflecting mirror. Since that mirror clearly prevents
a large fraction of the laser's light from leaving the laser, your friend
suggests removing the mirror to make the laser light brighter. You respond
that removing the mirror would be a bad idea. What would happen to the
laser if you removed its mirror? The laser will stop emitting light
(because it needs light reflected from the mirror to amplify in order to
sustain its laser oscillation).
(C) You find that if you send the laser beam directly at the distant cliff,
the light spot that forms on the cliff is pretty large. To make that spot
smaller, you must enlarge the laser beam with a beam expander-an optical
device that resembles a telescope-before directing it toward the cliff.
Why does starting with a larger beam of light reduce the rate at which
that beam grows wider as it travels through space? Diffraction causes
a narrow beam of light to spread and a wider beam of light experiences
less diffraction-induced spreading than a narrow beam.
(D) On the night of the laser show, you notice that you can actually see
the laser beam traveling through the sky on its way to the distant cliff.
What effect is making it possible for you to see the laser beam? Rayleigh
scattering (of laser light by the atmosphere).
Problem 5:
You are very proud of your new 12,000 square-foot house. However, you are
not just the occupant-you are also the gardener. Unfortunately, you are
now so deeply in debt that you can only afford to buy a cheap, used rotary
lawnmower. Your choices are a walk-behind mower or a riding mower. You
look at the 12 acres of grass surrounding your mansion and go for the riding
mower. The only problem: its blade is missing. Since you have access to
a big pile of scrap metal, you decide to make a blade yourself. A blade
for this type of mower is little more than a sharpened metal bar that spins
rapidly about a central pivot.
(A) Your first choice in the scrap heap is a bar of chemically pure iron.
You fashion a nice blade from this metal but find that it can't keep an
edge. What is going on inside of the pure iron that allows it to become
dull so easily? The planes of atoms are slipping across one another
(or the iron crystals are experiencing slip).
(B) Your next choice is a bar of stainless steel. You are helped in identifying
it as stainless steel because a magnet doesn't stick to it. What difference
between normal steel and stainless steel accounts for the fact that normal
steel is magnetic while most stainless steel is not? The atoms in
the two metals have different crystal structures (and normal steel's ferrite
structure is magnetic while stainless steel's austenite structure is non-magnetic.
(C) The stainless steel blade can't keep an edge either, so you finally
try a bar of carbon steel. After shaping the blade, you heat its cutting
edge in burning charcoal so that it soaks up extra carbon. You then cool
the blade very rapidly in water. The result is a fine blade that stays
sharp almost indefinitely. Why did adding carbon to the cutting edge and
then cooling the red hot edge quickly make it so resistant to dulling?
This heat treatment places tiny particles of a hard mineral (cementite
or iron carbide) throughout the iron crystals and prevents slip from occuring.
(D) Your mower cuts the grass beautifully. As the spinning blade slices
through a tall weed, the weed hardly moves at all. What keeps the weed
in place while it is being cut by the moving blade? Inertia.
Problem 6:
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)? Multiply the current (passing through the hairdryer) times the
voltage (drop across the hairdryer).
(B) As your hair dries out, it gradually becomes stiffer. You hair is a
polymer that dissolves lots of water. Why does getting your hair wet make
it softer? The water (dissolves in your hair and) acts as a plasticizer
(or the water lubricates the hair molecules).
(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? The (planes of) atoms (in the metal crystals) slipped across
one another.
(D) You put down the bent metal comb and pick up another comb that's made
of a hard, glassy plastic. This time when you pull on the comb after it
becomes tangled in your hair, several of its teeth break off. Why doesn't
this plastic comb bend the way the metal comb did? The atoms and
molecules in the glassy plastic aren't mobile and can't slip across one
another, so that the molecules break apart (when overstressed).