Given Wednesday, March 8, 1995, from 1:00 PM to 1:50 PM
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 the midterm examination.
Problem 1:
You are riding on a swing at the local playground. As you swing back and forth, you begin to think about your speed and kinetic energy (this is obviously a fictional story). These two quantities clearly change between the top of each swing (when you are reversing directions) and the bottom of each swing (when you are passing directly beneath the supporting beam). You wonder when each of these two quantities is at its maximum value. Actually, your speed is at its maximum
(A) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.
(B) at the bottom of a swing and your kinetic energy is at its maximum at the top of a swing.
(C) at the top of a swing and your kinetic energy is at its maximum at the bottom of a swing.
(D) at the top of a swing and your kinetic energy is at its maximum at the top of a swing.
Problem 2:
You have a pair of fabulous stereo speaker cabinets. Each cabinet contains 5 individual speakers. Unfortunately, one of the individual speakers is broken, so you buy an identical replacement and install it yourself. In the process, you reverse the two electrical connections to the speaker so that current flows backward through the speaker coil. The speaker cone moves the wrong way and makes the entire stereo sound odd. The speaker cone moves the wrong way because
(A) the charge on the speaker coil becomes positive rather than negative so that it is pushed in the wrong direction by the speaker's permanent magnet.
(B) the current flowing backward through the speaker coil magnetizes it backward so that it is pushed in the wrong direction by the speaker's permanent magnet.
(C) the charge on the speaker coil becomes negative rather than positive so that it is pushed in the wrong direction by the speaker's permanent magnet.
(D) the current flowing backward through the speaker coil polarizes its electrical charges backward so that they are attracted when they should be repelled and repelled when they should be attracted.
Problem 3:
The aluminum rotor of an AC induction motor has no electrical connections and is not a permanent magnet. Nonetheless, it becomes magnetic during the motor's operation and is dragged around in a circle by the magnetic field around it. The rotor becomes magnetic because it is
(A) full of magnetic north poles that migrate to the surface when the rotor is spinning.
(B) made of aluminum, an intrinsically magnetic metal. When you bring a magnet up to a piece of aluminum, the aluminum develops a north and south pole and is attracted to the magnet.
(C) subject to vibrations which create an electrical dipole in it and give it a magnetic polarization.
(D) exposed to a changing/moving magnetic field that causes currents to flow in it.
Problem 4:
Your pet rabbit has chewed the cord to your desk lamp and has created a short circuit; an electrical connection from one wire to the other inside the cord. When you plug the lamp into the electrical socket,
(A) current will flow alternately through the bulb and through the short circuit, so that the bulb will blink on and off rapidly.
(B) current will bypass the bulb and the bulb will not light up.
(C) excessive current will pass through the bulb and the bulb will glow very brightly.
(D) the current will begin to flow backward through the bulb so that it glows at half its normal brightness.
Problem 5:
The small negative ion generators that are sold in appliance stores as a way to "improve your health" create negative ions in the air by transferring negative charge from a metal surface to passing air molecules. They generally use a number of sharp metal whiskers rather than a smooth metal ball to transfer the charge to the air because negative charges
(A) placed on sharp points repel one another more strongly than those placed on smooth surfaces.
(B) the whiskers can touch one another to form complete circuits and charge only flows when there is a complete circuit present.
(C) cannot accumulate on a metal ball without repelling one another.
(D) flow best through wires and the metal whiskers are essentially wires.
Problem 6:
A company that makes electric doorbells has been making electromagnets by wrapping copper wire around steel bolts. When current passes through the coil of copper wire, the steel bolt becomes magnetic. The electromagnet then attracts a clapper that rings the bell. But a new employee has used copper bolts as cores for some electromagnets. These copper-bolt electromagnets don't become very magnetic when current passes through their coils because
(A) copper is a better conductor of electrical current than steel so that the copper bolt short-circuits the magnetic field.
(B) both the bolt and the coil around the bolt are made of copper so that no magnetic dipole can be created.
(C) copper is not a metal and only metals can become magnetic.
(D) unlike the atoms in steel, the atoms in copper are not intrinsically magnetic.
Problem 7:
You are doing exercises at the gym. When you lift a weight over your head, you push upward on it both as you lift it and as you lower it. However, when you work out with a particular exercise machine, you push upward as you lift its bar but must pull downward to lower that bar. When you use that exercise machine,
(A) you do work on the bar as you raise it but it does work on you as you lower it.
(B) its bar does work on you as you raise it but you do work on it as you lower it.
(C) you do work on the bar as you raise it and as you lower it.
(D) its bar does work on you as you raise it and as you lower it.
Problem 8:
There is a large bar magnet built into the table in front of you. The north pole of that magnet is exposed and points upward. You have a small bar magnet in your hand. As you move the small magnet toward the table, its south pole is attracted toward the north pole of the table's magnet and the two poles stick together tightly. However, the north pole of the small magnet is repelled by the north pole of the table's magnet. The reason why this repulsion doesn't push the small magnet away from the table is that
(A) the north pole of the small magnet is weaker than its south pole.
(B) the two north poles are relatively far apart and the forces between magnetic poles decrease quickly as the distance between them increases.
(C) the attractive forces between opposite magnetic poles are inherently stronger than the repulsive forces between like magnetic poles.
(D) the south pole of the small magnet shields its north pole so that the magnetic forces do not affect the north pole as much. (The north pole lies behind the south pole.)
Problem 9:
An electrical insulator can't carry an electrical current because
(A) it contains no atoms.
(B) its electrons can't respond to an electric field.
(C) its electrons are positively charged rather than negatively charged, as they are in an electrical conductor.
(D) it contains no electrons.
Problem 10:
The blades of a fan do work on the air in blowing it across the room. An electric motor keeps those fan blades turning. If you remove the fan blades from the motor, the motor will
(A) keep turning but consume the same amount of electrical power.
(B) keep turning but consume more electrical power.
(C) stop turning.
(D) keep turning but consume less electrical power.
Problem 11:
A typical lawnmower has an electromagnetic device called a magneto that produces a brief pulse of very high voltage electric charge. This charge runs through a short wire to the spark plug, where it jumps across a gap to produce the spark that ignites the gasoline. Unfortunately, the lawnmower interferes with the reception on your portable radio because
(A) when the charge accelerates during each ignition pulse, it emits radio waves.
(B) the presence of electric charge in the magneto prevents current from flowing up and down your portable radio's antenna.
(C) the alternating current used in the lawnmower's ignition system is incompatible with your portable radio, which requires direct current for its operation.
(D) the magnetic field from the magneto attracts charge out of your radio's antenna so that it becomes electrically neutral.
Problem 12:
You are watching a child is flying a kite at the park. The kite is hovering motionless in the sky, about 100 m above the ground. The wind is blowing smoothly toward the east. The net force on the kite is
(A) in the upward direction.
(B) toward the west.
(C) toward the east.
(D) zero.
Problem 13:
The back of your UVA ID card has a strip of magnetic tape on it. Like a music tape, this strip stores information as a pattern of magnetized patches. The machine that reads this information is essentially a tape player. When someone uses the reader to read your ID card, they pull the card quickly through the reader. It's important that the card move through the reader because the playback head can only respond to moving or changing magnetic fields. That is because moving or changing magnetic fields
(A) generate light in photocells; making it possible to detect the pattern of magnetization on the strip.
(B) can change the weight of a small steel ring so that it accelerates up or down.
(C) produce temperature fluctuations that can easily be detected with a bimetallic strip thermometer.
(D) produce electric fields that can cause currents to flow in a coil of wire.
Problem 14:
Even when you are driving at a constant 60 miles-per-hour along a straight, level road, your car's engine must be running. As the engine turns the car's wheels, friction between the ground and the tires exerts a forward force on the car. The car needs this forward force from the ground because
(A) air drag (air resistance) exerts a backward force on the car.
(B) an object that is moving requires a net force to keep it moving. In the absence of any net force, objects are motionless.
(C) an object's velocity points in the direction of the net force on that object.
(D) the car has a velocity and is thus accelerating. In order to accelerate, the car must be experiencing a net force.
Problem 15:
The principal advantage of sending electrical power across the country on very high voltage transmission lines is that
(A) the electrical power lost in the wires is greatly reduced.
(B) they carry less energy per charge than low voltage transmission lines.
(C) they carry much more current than low voltage transmission lines.
(D) these transmission lines are less likely to get in the way than low voltage transmission lines (which are much closer to the ground).
Problem 16:
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) angular momentum.
(B) angular velocity.
(C) momentum.
(D) velocity.
Problem 17:
The aurora borealis or "northern lights" is produced when electrically charged particles emitted by the sun spiral downward toward the earth's north pole. They are guided in that direction by the earth's magnetic field because
(A) positively charged particles are attracted toward north magnetic poles.
(B) moving charged particles experience forces when they pass through a magnetic field.
(C) positively charged particles are repelled by both north and south magnetic poles.
(D) positively charged particles are attracted toward south magnetic poles.
Problem 18:
A quick trip down a plastic playground slide has caused a child's thin white-blond hair to begin standing straight up so that she looks like a dandelion puff. During the trip down the slide, sliding friction has
(A) polarized her body so that she has a north pole near her head and a south pole near her feet. Her hair is following the lines of magnetic force that extend between the two poles.
(B) aligned the magnetic poles inside her body so that they all point in one direction. Her hair is levitated magnetically.
(C) turned her kinetic energy into heat and given her hair a blow-dried look as a result.
(D) transferred electric charge to her body and hair so that the like charges on her hair are repelling one another strongly.
Problem 19:
A battery
(A) creates negative charge.
(B) creates positive charge.
(C) pumps positive charge from its positive terminal to its negative terminal.
(D) pumps positive charge from its negative terminal to its positive terminal.
Problem 20:
If you were to build a motor using only permanent magnets, its rotor might spin briefly but it would soon come to a stop as friction turned its energy into heat. To keep the rotor turning, you must replace one of the magnets with an electromagnet and control that electromagnet so that it always does work on the rotor, increasing its energy. To do work on the rotor, the electromagnet should be adjusted so that it always
(A) repels each magnet that moves toward it and repels each magnet that moves away from it.
(B) attracts each magnet that moves toward it and attracts each magnet that moves away from it.
(C) attracts each magnet that moves toward it and repels each magnet that moves away from it.
(D) repels each magnet that moves toward it and attracts each magnet that moves away from it.
Problem 21:
The presence or absence of a few electric charges on the gate of a MOSFET transistor can dramatically change that transistor's
(A) temperature and the color of the light it emits.
(B) frequency and the number of radio waves that it emits each second.
(C) magnetic field and its ability to attract nearby magnetic poles.
(D) ability to conduct electrical current.
Problem 22:
Ball bearings permit a wheel to turn freely on an axle without creating any heat because they form a mechanical system that involves
(A) no friction of either type.
(B) no electricity.
(C) no sliding friction.
(D) no static friction.
Problem 23:
When a television set is displaying the image of a red wall, so that the entire screen appears red,
(A) the electrons inside the picture tube are only striking those screen phosphors that emit red light.
(B) the electrons inside have relatively high energies so that the screen phosphors glow with red heat.
(C) only the red lamp inside the picture tube is illuminated.
(D) the electrons inside have relatively low energies so that the screen phosphors glow with red heat.
Problem 24:
Many mail order catalogs sell lamp dimmer disks. These disks, which you insert in the sockets of incandescent lamps, promise to reduce your electrical bills while making the light bulbs last virtually forever. Unfortunately, these disks simply reduce the power delivered to the bulbs' filaments so that they operate below their rated temperature and waste energy producing infrared light. The dimmer disk actually contains a single diode that forms a series circuit with the light bulb. AC current from the power line must flow through the diode and then through the filament (or vice versa). Overall, this diode reduces the power delivered to the filament by about a factor of two because
(A) the diode only allows current to flow in one direction so that during one half of each cycle of the AC power line, no current flows through the light bulb.
(B) the diode divides the current in half and only allows one half of that current to flow through the filament.
(C) the diode halves the voltage of the current passing through the light bulb so that the power it receives is also halved.
(D) the diode behaves like a filament so that there are two filaments in a series, with each of them receiving half the power that a single filament would receive.
Problem 25:
The huge steam-powered generators found in electrical power plants produce electricity by
(A) moving electric charges up and down capacitors.
(B) turning iron cores inside of transformers.
(C) moving magnets past coils of wire.
(D) rubbing copper disks against sheets of glass.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.
Problem 1:
The battery in your friends' car is dead so you are using jumper cables to help them get started. You are going to use the fresh battery in your car to power the starter motor in their car. If you can send enough current through the starter motor, it will make their engine turn.
(A) You connect the first jumper cable from the positive terminal
of your car's battery to the positive terminal of the battery
in your friends' car. One of them then tries to start their car
but nothing happens. Why isn't the first cable helping?
(B) You connect the second jumper cable from the engine of your
car (which is electrically connected to the negative terminal
of its battery) to the engine of your friends' car (which is electrically
connected to the negative terminal of its battery). One of them
then tries to start their car and the engine begins to turn slowly.
Briefly describe the path through which electrical current is
flowing in this situation.
(C) Your jumper cables are inexpensive and light weight. They
get warm as your friend operates the starter motor in their car
and the starter motor turns slowly; too slowly to start the car.
Why are these cheap jumper cables so ineffective?
(D) A passing motorist loans you a pair of heavy duty jumper cables
and these cables start your friends' car immediately. You then
notice that your friends' head lights were on the whole time,
running in a parallel circuit with the starter motor. Why did
the head lights make it harder to start their car through your
cheap jumper cables?
Problem 2:
You're trying to open a wide-mouthed jar of pickles. You hold the jar tightly on one hand while twisting the lid with the other hand. But try as you might, you cannot get the lid to turn.
(A) What is the net torque on the lid?
(B) Your grip loosens and your hand begins to slip across the
lid. Your skin feels warm as a result. Why?
(C) You find a jar opening device that grips the lid tightly in
its jaws and has a long handle that extends outward from the side
of jar. By exerting a modest force on this handle, you can exert
a very large torque on the lid. Why does the long handle increase
the amount of torque you can exert?
(D) The jar still doesn't open so you throw it out the window
and into the woods behind your house. As it flies, you notice
that the jar continues to rotate at a steady rate about a fixed
axis, up until the moment it hits a tree. Why doesn't its rotation
slow down in flight, as it would have if you had spun the jar
on the kitchen counter?
Problem 3:
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?