Sample Midterm Exam

Physics 106N - How Things Work - Spring, 1995

Sample Final Examination

(Extracted from the Final Exam for Spring, 1994)

Actual Exam will be Given Saturday, May 6, 1995, from 9:00 AM to 12:00 Noon

PART I: MULTIPLE CHOICE QUESTIONS

Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on this examination.

Problem 1:

You take two boxes of cookies with you on your second vacation to the moon. One of the boxes is labeled in kilograms, a unit of mass, and the other box is labeled in pounds, a unit of force. Before leaving, you compare each box against standard objects and find that they are both accurately labeled. When you arrive on the moon, you find that the

(A) labels on both boxes are still accurate.

(B) label on the pound box is still accurate but the label on the kilogram box is not correct.

(D) label on the kilogram box is still accurate but the label on the pound box is not correct.

Problem 2:

As a nucleus of 235U, the light isotope of uranium, falls apart, potential energy contained in the nucleus is converted to kinetic energy in the fragments. Before the nucleus falls apart, that potential energy is stored in the

(A) repulsive electrostatic force between protons in the nucleus.

(B) attractive nuclear force between particles in the nucleus.

(D) repulsive nuclear force between particles in the nucleus.

Problem 3:

You are watching two children play on a seesaw. They are equal in weight so that the seesaw balances nicely. The child on your right pushes down with her feet and the seesaw begins to rotate. When her feet leave the ground, the seesaw becomes perfectly balanced and experiences zero net torque. At that moment,

(A) the seesaw comes to a stop with its board at some angle, but not necessarily horizontal.

(B) the seesaw comes to a stop with its board oriented horizontally.

(D) the seesaw undergoes angular acceleration in the counter-clockwise sense.

Problem 4:

You are taking an outdoor portrait photograph of a friend and want the distant background to appear out of focus. You have a complicated camera with changeable lenses and lots of adjustable controls. To produce the portrait, you focus the lens on your friend but make sure that

(A) you move the camera rapidly to one side as you release the shutter.

(B) you are using a very long focal length lens.

(D) you are using a very short exposure time.

Problem 5:

One subatomic particle that is NOT contained in a typical iron atom is

(A) a proton.

(B) a neutron.

(D) a muon.

Problem 6:

A typical laser oscillator has two mirrors, one at each end of the laser medium. These two mirrors

(A) prevent light from escaping from those ends so that it is only emitted through the front face of the laser oscillator.

(B) prevent radio waves and microwaves from entering the laser amplifier, where they would overheat the laser medium.

(C) prevent heat from escaping from those ends so that the laser oscillator remains hot enough to emit brilliant laser light.

(D) keep a few photons bouncing back and forth through the laser medium so that they can be duplicated over and over again.

Problem 7:

When you talk to a friend by telephone, the two telephones are connected in a parallel circuit. The telephone company sends a constant current through the two telephones. Because of this arrangement, each time the current passing through your telephone increases,

(A) the current passing through your friend's telephone remains constant, but the voltage drop across that telephone decreases.

(B) the current passing through your friend's telephone decreases.

(D) the current passing through your friend's telephone remains constant, but the voltage drop across that telephone increases.

Problem 8:

The two medical imaging techniques, CT (Computed Tomography) and MRI (Magnetic Resonance Imaging), are complimentary because

(A) both techniques image bone but CT indicates bone as a light region of the image while MRI shows bone as a dark region.

(B) CT studies heavy atoms in bone while MRI studies hydrogen atoms in tissue.

(C) both techniques image bone but MRI indicates bone as a light region of the image while CT shows bone as a dark region.

(D) MRI studies heavy atoms in bone while CT studies hydrogen atoms in tissue.

Problem 9:

The speaker in a typical telephone, like most speakers, uses the forces between two magnets to move a thin membrane toward or away from your ear. The speaker is able to change the force between the magnets from attractive to repulsive at will because

(A) any temperature change in the magnets affects the force between them. As the heating coils heat or cool the magnets, they repel or attract respectively.

(B) one of the speaker's magnets is actually an electromagnet that reverses its magnetic poles whenever the electrical current through it reverses.

(C) any temperature change in the magnets affects the force between them. As the heating coils heat or cool the magnets, they attract or repel respectively.

(D) magnets are also attracted or repelled by static electric charge so that the changing electric charge on the stationary magnet in the speaker either attracts or repels the moving magnet on the membrane.

Problem 10:

You have been running track races in smooth-soled shoes. During each start, you have been wasting 100 joules of energy as internal energy because of friction between your shoes and the track. To help this situation, you purchase a pair of spiked shoes. Now when you start a race, the frictional force your feet experience from the track is increased by a factor of 5 and the shoes do not slide across the track at all. During each start, the amount of energy you now waste as internal energy because of friction between your spiked shoes and the track is

(A) 500 joules.

(B) 4 joules.

(D) 20 joules.

Problem 11:

When a photoelectric cell is exposed to light, that light

(A) oscillates the photoinductor and induces current in the circuit attached to the photoelectric cell.

(B) produces separated electric charge on the two sides of a p-n junction.

(C) causes the photoelectric cell to accelerate and the force of this acceleration gives momentum to the electrons flowing in the circuit attached to the photoelectric cell.

(D) produces heat which accelerates current from one wire to the other.

Problem 12:

If you hold a strong permanent magnet inside a metal ring and then quickly remove that magnet, you create an electric field in the metal ring and an electrical current flows through the metal ring. If you repeat this procedure with a glass ring, no current flows in the glass ring because

(A) the glass ring creates its own magnetic field in the opposite direction, opposing the field of the permanent magnet and preventing any current flow.

(B) the glass ring is made of a non-magnetic material. No magnetic fields can be present inside the ring so removing the magnet has no effect on the ring.

(D) although removing the magnet creates an electric field in the glass ring, the glass ring has no mobile electric charges to carry an electrical current.

Problem 13:

Perhaps the most important components in audio amplifiers (and many other electronic devices) are MOSFETs. These devices behave as

(A) resistors whose resistances can be controlled electronically.

(B) capacitors whose capacitances can be controlled electronically.

(D) inductors whose inductances can be controlled electronically.

Problem 14:

A demonstration of superconductivity, shown in class, involves a small permanent magnet that is lowered onto a superconducting surface. The magnet never touches the superconductor but instead remains suspended above its surface. This suspension occurs because

(A) the superconductor becomes electrically polarized by the approaching magnet and exerts a repulsive force on the magnet when the two are very close together.

(B) the superconductor becomes electrically charged by the approaching magnet and exerts a repulsive force on the magnet when the two are very close together.

(D) the magnet induces a current in the superconductor as they approach one another and the magnetic field from this current repels the magnet.

Problem 15: (Deleted)

Problem 16:

The best modern magnetic tapes, so called "Metal-Particle Tapes", are based on tiny elongated iron particles. Magnetic recording tapes are not based on tiny elongated aluminum particles because aluminum metal is

(A) intrinsically non-magnetic, having no microscopic magnetic order.

(B) not strong enough to form and maintain the elongated shapes needed to hold the metal particles flat on the surface of the tape.

(D) too shiny and will reflect the magnetic waves that are used to record information on the magnetic tape.

Problem 17:

Water containing soap or detergent molecules cleans your laundry better than water alone because these molecules

(A) attack oil and fat molecules, reducing them to carbon dioxide and water.

(B) surround oil and fat droplets and allow water to carry them away.

(D) convert oil and fat molecules into salts, which dissolves well in water.

Problem 18:

The electrical power to a neighborhood is usually delivered by wires with a voltage difference of about 5000 volts. Normal household electrical outlets have a voltage difference of about 120 volts. Electrical power is delivered to the neighborhood at the high voltage because that arrangement reduces the

(A) number of cycles per second that are needed to carry the power to the neighborhood.

(B) amount of energy carried by each electron traveling through the wires to the neighborhood.

(D) amount of current that must flow through the wires to the neighborhood.

Problem 19:

An acorn falls 20 meters from the high branch of an oak tree to the ground. This fall takes about 2 seconds. After it has fallen for only 1 second, the acorn is still falling and remains

(A) considerably more than 10 meters above the ground.

(B) 10 meters above the ground.

(D) considerably less than 10 meters above the ground.

Problem 20:

You are moving two files cabinets. One cabinet is empty and is resting directly on the floor. The other cabinet is full of documents but is resting on a dolly with wheels. You push each cabinet forward 5 meters by exerting a forward force of 100 newtons on it. At the end of this pushing,

(A) the rolling cabinet and the sliding cabinet each have about 500 joules of kinetic energy.

(B) the rolling cabinet and the sliding cabinet each have much less than 500 joules of kinetic energy.

(C) the sliding cabinet has about 500 joules of kinetic energy while the rolling cabinet has much less than 500 joules of kinetic energy.

(D) the rolling cabinet has about 500 joules of kinetic energy while the sliding cabinet has much less than 500 joules of kinetic energy.

Problem 21:

The laser in a compact disc player produces light with a wavelength of 780 nanometers in air (503 nanometers in plastic). This light is focused onto the metal layer of a compact disc, where it reflects differently from flat regions and from ridges. The ridges in the metal layer of a compact disc are about 500 nanometers wide because

(A) that is the width of the diamond stylus used to play back the compact disc.

(B) the technique used to mass-produce the discs cannot duplicate features that are smaller than 500 nanometers in any dimension.

(D) that is the size of each metal atom in the layer.

Problem 22:

Of the following types of electromagnetic waves, the type with the highest frequency is

(A) visible light.

(B) ultraviolet light.

(D) infrared light.

Problem 23:

When you move a strong permanent magnet rapidly toward the right, just above a stationary, horizontal aluminum surface, that magnet will experience

(A) a downward force attracting it to the aluminum and a rightward force trying to speed it up.

(B) an upward force repelling it from the aluminum and a leftward force trying to slow it down.

(D) a downward force attracting it to the aluminum and a leftward force trying to slow it down.

Problem 24:

When chemicals polymerize to make plastic, small molecules

(A) bind together chemically to form giant molecules.

(B) settle to the bottom of the liquid in which they are suspended and produce a thick paste. This paste then dries to form a stiff solid.

(D) first melt and then refreeze to form a stiff solid.

Problem 25:

Electrostatic air cleaners use very thin wires to transfer electric charge to dust particles primarily because very thin wires

(A) heat up more quickly and easily than thick wires.

(B) break easily, require frequent repair, and are a boon to the manufacturer's service department .

(D) bring the electrically charged particles so close together that their repulsion pushes some of them onto the air and dust.

Problem 26:

You get on your bicycle and begin pedaling. You travel forward faster and faster and soon reach full speed. As you are picking up speed, friction between the ground and the bicycle wheels is exerting

(A) zero force on the bicycle.

(B) an upward force on the bicycle.

(D) a backward force on the bicycle.

Problem 27:

During a particular brown out, the voltage of the electrical power provided to your home by the power company is reduced by 5%. The voltage drop across your reading lamp is reduced by 5% so the current passing through it is also reduced by 5%. Overall, the power consumed by the lamp

(A) drops by about 25%.

(B) drops by about 10%.

(D) remains the same but the lamp becomes dimmer.

Problem 28:

Cotton fabric normally looks white but becomes relatively transparent when wet. Water allows the fabric to transmit more light because

(A) the surface between water and a clear cotton fiber reflects less light than the surface between air and a cotton fiber.

(B) water dissolves the white pigment in cotton, leaving only the clear cotton itself.

(D) water makes the cotton fibers shrink so that you can see through the enlarged holes between fibers.

Problem 29:

The photoconductor used in a xerographic copier

(A) emits greenish-white light when a current flows through it.

(B) does not conduct electrical current in the dark.

(D) emits white light when a current flows through it.

Problem 30:

The three primary colors of light, the colors that are visible as tiny spots when you look very closely at a white region of a color television screen, are

(A) red, yellow, and green.

(B) yellow, cyan, and magenta.

(D) red, yellow, and blue.

Problem 31:

When you look at a distant bird through a pair of binoculars, you are using two simple telescopes, one for each eye. Each telescope consists of two converging lenses and some prisms. The prisms fold the optical path, shortening the binoculars and making the bird appear upright through the eyepieces. Two lenses are needed because the first lens

(A) creates a virtual image and the second lens acts as magnet, pulling that virtual image closer to your eye.

(B) creates a real image and the second lens acts as a magnifying glass, allowing you to inspect that real image more closely.

(D) creates a virtual image and the second lens acts as a magnifying glass, allowing you to inspect that virtual image more closely.

Problem 32:

An FM radio station has a vertical antenna. Your little pocket FM radio has an antenna, too, but you can twist it around in whatever direction you like. You will probably get the best reception when the antenna of your radio is oriented

(A) horizontally, pointing away from the station's antenna.

(B) horizontally, pointing at right angles to the line between you and the station's antenna.

(D) horizontally, pointing toward the station's antenna.

Problem 33:

In the explosion of a nuclear weapon, many uranium nuclei fragment into smaller nuclei. These nuclei soon acquire the proper number of electrons and start to behave like normal atoms. Many of them participate in the chemistry of our everyday lives. However, these atoms are dangerous because

(A) they are still hot from the nuclear explosion.

(B) their nuclei are not entirely stable and tend to fall apart.

(D) they are chemically very different from normal atoms and many of them are poisonous.

Problem 34: (Deleted)

Problem 35:

If you were to hold a strong permanent magnet near the front face of a color television tube, the picture would become very distorted, both in shape and color. This distortion would occur because the magnetic field from your magnet would

(A) deflect the electron beams so that they would miss their intended destinations.

(B) exert forces on the phosphor particles inside the picture tube so that the velocities of their accelerations would increase their masses.

(D) attract the magnetic paint particles inside the picture tube and distort their positions.

Problem 36:

Water in the gas tank of a car sits at the bottom of the tank, where it can get into the fuel line and freeze solid during cold weather. If water dissolved in gasoline, this sort of problem would not occur. Water does not mix with gasoline because

(A) water is less dense than gasoline and materials of different densities cannot dissolve in one another.

(B) water molecules are non-polar and are much more strongly attracted to other non-polar molecules than to the polar gasoline molecules.

(D) water molecules are polar and are much more strongly attracted to other polar molecules than to the non-polar gasoline molecules.

Problem 37:

There are no electrical connections to the aluminum rotor of an induction motor yet it experiences a torque when the motor is turned on. The rotor feels this torque and undergoes angular acceleration because

(A) of gravity and the distance between the rotor's center of mass and center of gravity.

(B) of dynamic friction between the rotor and the stationary frame of the motor.

(D) electrical currents still flow through it and make it magnetic.

Problem 38:

Two children of equal size and weight run along a horizontal platform side by side and dive into a swimming pool at exactly the same instant. The only difference between the two children's dives is that one child jumps upward off the platform while the other child simply runs forward off the end. You watch them fall and notice that

(A) the two children reach the water at exactly the same instant.

(B) the child who jumped upward reaches the water first.

(D) the child who jumped upward does not feel the pull of gravity and never returns.

Problem 39:

The microscope structure of quartz glass closely resembles that of

(A) gelatin (unflavored and unsweetened).

(B) crystalline quartz (quartz crystals or quartz sand).

(D) molten quartz (liquid quartz).

Problem 40:

If the manufacturer accidentally forgot to put the white coating on the inside of the tube in a fluorescent lamp, the tube would still operate but you would see

(A) very bright reddish light coming from the tube.

(B) very bright greenish light coming from the tube.

(D) a very thin, very bright line of light in the middle of the tube.

Problem 41:

On a sunny day, you can use a magnifying glass to burn wood by focusing the sun's light to a small spot on the wood. Light from the sun reaches the magnifying glass, bends as it passes through the lens, and forms a small round, brilliant spot of light on the wood. The spot of light is round is because

(A) a focus means that all light passing through the lens is brought together to a single point in space. A single point always looks round.

(B) it is a real image of the round sun itself. The magnifying glass behaves like a camera lens.

(D) particles of light are spherical and, when brought together at a focus, form a circular spot.

Problem 42:

You are pulling a toy wagon filled with bricks and it is accelerating forward. If you are exerting a forward force of 30 newtons on the wagon's handle, the wagon's handle is exerting a backward force of

(A) 30 newtons on your hand.

(B) somewhat less than 30 newtons on your hand.

(D) zero newtons on your hand.

Problem 43:

Common steel is relatively soft and is not suitable for use in cutting tools. To make much harder tool steel for use in knives, drills, and other cutting instruments, they

(A) add oxygen to the steel.

(B) add more carbon to the steel.

(D) remove carbon from the steel.

Problem 44: (Deleted)

Problem 45:

A gymnast doing a double back flip leaps off the floor with her arms and legs extended and then pulls herself into a very compact position. In her compact shape, she rotates very rapidly and completes two full rotations before opening back up to land on the floor. During the time that she is not touching the floor, the one aspect of her motion that is constant is her

(A) velocity.

(B) momentum.

(D) angular momentum.

Problem 46:

You used to cook popcorn on the stove, in a pot containing a small amount of corn oil. Now you cook popcorn in the microwave oven. Adding corn oil to the paper bag with the popcorn in it will not help popcorn cook in a microwave oven because

(A) corn oil conducts electricity, reflecting the microwaves and preventing them from reaching the corn kernels.

(B) the microwaves directly heat the water molecules inside the corn kernels, bypassing the corn oil.

(D) the corn kernels already contain enough corn oil. Heat flows from the microwaves to the corn oil and pops the corn.

Problem 47:

The reason why 238U, the heavy isotope of uranium, cannot be used by itself in a nuclear reactor is that 238U

(A) has more electrons than 235U, the light isotope of uranium, and wastes most of its energy heating those electrons.

(B) does not contain very much nuclear energy and thus cannot create much heat.

(D) rarely undergoes fission when struck by a neutron.

Problem 48:

Brighteners are added to laundry detergent to increase the amount of blue light emitted by the clothes so as to prevent white garments from looking yellowish as they age. The brighteners work by

(A) absorbing yellow light and reemitting blue light.

(B) absorbing ultraviolet light and reemitting blue light.

(D) removing red dyes from the fabrics.

Problem 49:

You are working in a factory for the summer. Lifting a barrel of salt directly upward 1 meter from the parking lot to the loading dock requires 2000 joules of energy. If you create a ramp with a 5 meter long board, you can roll the barrel slowly and steadily up the ramp with an uphill force of

(A) 10,000 newtons.

(B) 50,000 newtons.

(D) 2,000 newtons.

Problem 50:

The sky is blue because

(A) the air contains atoms and molecules that absorb red, yellow, and green light, leaving only the blue light unaffected.

(B) tiny particles in the air scatter long wavelength light more effectively than they scatter short wavelength light.

(D) it contains atoms and molecules that emit blue light when they are exposed to cosmic rays.

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 found a job in the fitness section of a department store. You are showing a treadmill to a couple interested in getting exercise indoors during the winter. The treadmill you are showing them has a fabric belt that turns endlessly between two rollers. An electric motor turns the rollers at a very steady pace and you remain in one place on the machine if you walk at just the right speed.

(A) With the belt exactly horizontal, you step onto the machine. You are soon walking steadily forward but, since the belt is turning toward you, you are not moving anywhere at all. The net force on you is zero. How much work are you and the machine doing on one another?

(B) You now tilt the machine so that the belt is uphill and continue walking steadily forward but without going anywhere. You tell the couple that much more effort is required now than when the belt was horizontal. But their know-it-all friend tells them that, since you are not going anywhere, you are not actually doing any work on the belt. Use the relationship between force, distance, and work, to show them that you are indeed doing work on the belt as you walk.

(C) When you increase the machine's tilt, the amount of work you do with each step increases. What changes about the force or distance to increase the work you do?

(D) The only thing connected to the rollers is the electric motor. Somehow, all of the work that you are doing on the belt is going into the motor and the motor converts it into electricity. How can a motor create electricity rather than consume it?

Problem 2:

The inexpensive disposable cameras that are sold in grocery stores contain quite a number of technologically interesting components. Despite their apparent simplicity, these cameras are carefully designed, manufactured, and assembled.

(A) The camera lens is normally made from a clear plastic with a refractive index of 1.55. If someone accidentally made the lens from a clear plastic with a refractive index of 1.45, the photographs would be blurry. Why?

(B) The camera lens is normally made from a plastic with very little dispersion. If someone accidentally made the lens from a plastic with a lot of dispersion, each bright point of light in the original scene would appear with rainbows of color around it in the photograph. Why?

(C) The camera's electronic flash uses a clear tube containing the rare and expensive gaseous element xenon. If someone accidentally put the less expensive gaseous element neon in the tube, the flash would emit reddish light. What characteristic of neon gas causes it to emit reddish light?

(D) Some disposable cameras are enclosed in waterproof plastic cases so that you can take them to the beach without fear of water damage. While it may seem like plastics are always waterproof, there are actually some water-soluble plastics. What must be different about the molecules in these plastics to make them dissolve relatively easily in water molecules?

Problem 3:

You have just moved into a new apartment. The people who lived there before you painted the living room fluorescent green and installed ultraviolet bulbs (black light) in the overhead light fixture. This look is not exactly your taste, so you decide to paint over it.

(A) Before you paint, you throw a housewarming party. With only the ultraviolet lights on, the room appears dimly lit. However, the walls glow a brilliant green color. Where is the green light coming from?

(B) The party's over. You choose an off-white latex interior paint and begin to cover up the green walls. The paint is rated as have excellent hiding power and it does indeed make the green disappear with a single coat. How do the white pigment particles in the paint prevent light from passing through the coating, reaching the green layer, and returning to your eyes?

(D) (Deleted)

Problem 4:

A blender is a common kitchen appliance. It consists of a glass or plastic pitcher with a rotating blade at the bottom. The pitcher sits in a base containing an electric motor. When you push the on button, the motor spins very rapidly and turns the blade. The spinning blade stirs and liquefies the contents of the pitcher.

(A) If you put an ice cube into the pitcher and push the on button, the blade spins and chops the ice cube into small fragments. The bottom of the pitcher is smooth and the ice cube is slippery, so no outside forces keep the ice cube from moving and staying ahead of the spinning blade. Still the ice cube stays put and the blade slices through it. What holds the ice cube in place?

(B) The blender plugs into an electrical outlet. When you push the on button, the blender mixes. At a particular moment during its operation, which way is electrical current flowing through each of the two wires in the blender's power cord?

(C) The blender contains a universal motor and can actually run on either AC or DC electrical power. If you reverse the blender's plug, so that the two prongs trade places in the outlet, will the motor and blade continue turning in the same direction or will they now turn in the opposite direction?

(D) You leave a beverage mixing in the blender while you answer the telephone. When you return, you find that the beverage has become quite warm. How did the spinning blade heat up the liquid?

Problem 5:

Impressed by your mastery of video games, the local nuclear power plant has offered you a part-time job controlling their reactor. The graphics are lousy and the controls aren't very responsive, but the pay is good so you take the job.

(A) You are supposed to move the control rods up and down to maintain a steady rate of nuclear decays in the reactor core. You find that when you insert the control rods too far into the core, the rate of nuclear decays diminishes. Since natural radioactive decay is completely random and cannot be controlled by outside influences, how do the control rods affect the rate at which 235U nuclei decay in a reactor?

(B) Water flows through the reactor core at an enormous rate. Why must the water keep moving?

(C) The power plant produces electricity for weeks without needing any fuel shipments and without releasing anything into the environment. What is being consumed to produce this electrical energy?

(D) After four weeks, you become so bored with work that you decide to see what will happen when you pull the control rods completely out of the reactor core. The sound and light effects that follow are totally awesome but you don't get a replay. What has probably happened to the reactor core?

Problem 6:

Many businesses cannot afford to have their computers crash when the power fails. They use battery-backup power supplies to create something resembling normal 120 volt alternating current out of the direct current supplied by 12 volt batteries. The backup power supply first converts the 12 volt direct current into 12 volt alternating current. It then uses a transformer to increase the voltage to 120 volts, as required by the computer.

(A) Why can't the transformer simply convert 12 volt direct current into 120 volt direct current?

(B) An electronic switching system converts the 12 volt direct current from the batteries into 12 volt alternating current. This alternating current flows through one coil of the transformer. What is "alternating" about the current through the transformer?

(C) How does power move from one side of the transformer (the primary circuit) to the other side (the secondary circuit)?

(D) Energy is conserved. If there is an average current of about 20 amperes in the 12 volt primary circuit of the power supply's transformer, what is the average current in the 120 volt secondary circuit of that transformer?