Given Wednesday, March 6, 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:
If you push against a heavy bookshelf but it remains in place, it experiences static friction with the floor. If the bookshelf begins to slide as you push on it, it experiences sliding friction with the floor. Friction extracts energy from the bookshelf
(A) only when it remains in place.
(B) only when you lift the bookshelf off the floor, not when you move it horizontally.
(C) both when it remains in place and when it's sliding across the floor.
(D) only when it's sliding across the floor.
Answer: (D) only when it's sliding across the floor.
Problem 2:
You're heading northward in your car. The light turns red in front of you, so you step on the brake. As you do,
(A) your car's velocity is southward.
(B) your car accelerates northward.
(C) your car accelerates southward.
(D) the net force on your car is zero.
Answer: (C) your car accelerates southward.
Problem 3:
A typical lawnmower has an electromagnetic device called a magneto that produces a brief pulse of 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) the magnetic field from the magneto attracts charge out of your radio's antenna so that it becomes electrically neutral.
(B) when the charge accelerates during each ignition pulse, it emits radio waves.
(C) the presence of electric charge in the magneto prevents current from flowing up and down your portable radio's antenna.
(D) the alternating current used in the lawnmower's ignition system is incompatible with your portable radio, which requires direct current for its operation.
Answer: (B) when the charge accelerates during each ignition pulse, it emits radio waves.
Problem 4:
The main memory of your computer, also called dynamic memory, stores each individual bit as the
(A) direction of current flow through a tiny electric circuit.
(B) amount of magnetism in a tiny iron bar.
(C) presence or absence of electric charge on a tiny capacitor.
(D) presence or absence of current in a tiny electric circuit.
Answer: (C) presence or absence of electric charge on a tiny capacitor.
Problem 5:
You have just pulled your clothes from the dryer and find that a sock is clinging to your jeans with static electricity. You hold the jeans in one hand and the sock in the other and pull the two apart. As jeans and sock move apart, the forces between them become weaker because the
(A) electric charge on each garment diminishes as they move apart.
(B) electric charge on each garment increases as they move apart.
(C) forces between electric charges become weaker with increasing distance.
(D) electric current passing through each garment diminishes as they move apart.
Answer: (C) forces between electric charges become weaker with increasing distance.
Problem 6:
A "D" battery is labeled as providing 1.5 volts. This voltage characterizes the
(A) amount of current that the battery supplies when it's installed in an electronic device.
(B) amount of electric charge that can be created by the battery.
(C) energy that the battery provides to each coulomb of electric charge it handles.
(D) acceleration that positive charges experience as they leave its positive terminal.
Answer: (C) energy that the battery provides to each coulomb of electric charge it handles.
Problem 7:
Your friend has a gold ring that forms a continuous loop around her finger. If you hold the north pole of a permanent magnet near that ring and then suddenly pull the magnet away, an electric current will flow around the ring because the
(A) magnet's north pole attracts positive charge and pulls that charge around the ring.
(B) magnet's north pole repels positive charge and pushes that charge around the ring.
(C) decreasing magnetic field will produce an electric field in the ring and this electric field will push charge around it.
(D) magnet's electric field decreases as you pull it away from the ring and this change will cause current to flow around the ring.
Answer: (C) decreasing magnetic field will produce an electric field in the ring and this electric field will push charge around it.
Problem 8:
When you bring the pole of a permanent magnet close to a piece of aluminum, the aluminum repels it briefly. However, this repulsion disappears after a second or two because
(A) the electric current that makes the aluminum magnetic loses energy and comes to a stop.
(B) the magnetic domains in the aluminum flip so that they begin to attract the permanent magnet.
(C) the magnetic domains in the aluminum return to pointing in random directions.
(D) the magnetization of the aluminum reverses directions 120 times per second, so that there is no average attraction or repulsion.
Answer: (A) the electric current that makes the aluminum magnetic loses energy and comes to a stop.
Problem 9:
A xerographic copier makes sure that the two sides of its photoconductor are oppositely charged before it exposes that photoconductor to light from the original document. If the photoconductor were not electrically charged before the exposure to light from the document,
(A) charge would flow through it backward during the exposure and its two sides would end up charged the wrong way.
(B) no charge would flow through it during the exposure.
(C) only the regions that were not exposed to light would become charged and the copier would produce a negative image on the paper.
(D) charge would flow through it back and forth, as an AC current, and it would overheat.
Answer: (B) no charge would flow through it during the exposure.
Problem 10:
When an ice skater begins to spin on the point of one skate, her arms are outstretched. She then pulls in her arms and spins much more rapidly because
(A) her angular momentum can't change but her moment of inertia decreases.
(B) her mass decreases so the force of her momentum causes her to accelerate.
(C) her angular velocity can't change so her angular acceleration must increase.
(D) her angular acceleration can't change so her angular velocity must increase.
Answer: (A) her angular momentum can't change but her moment of inertia decreases.
Problem 11:
Suppose that you have a capacitor in front of you, with a wire connecting its two plates. You remove the wire and begin to transfer positive charges from the left plate to the right plate, one charge every second. With each additional transfer, your job gets harder because the forces pushing the charge in the opposite direction get stronger and stronger. The increasing amount of work you must do on each charge as you transfer it is reflected in the
(A) increasing voltage rise across the capacitor.
(B) increasing net charge on the capacitor.
(C) increasing magnetic field inside the capacitor.
(D) increasing current passing through the capacitor.
Answer: (A) increasing voltage rise across the capacitor.
Problem 12:
Your student ID card has a strip of magnetic tape on its back. When that card is swept through a magnetic tape reader, it travels across a magnetic playback head. The sweeping motion is important because it
(A) allows the magnetic field from the playback head to induce currents in the magnetic tape and make that tape magnetic.
(B) establishes the magnetic repulsion that accelerates the playback head and causes vibrations in its detection circuit.
(C) creates the magnetic drag force on the card that is used to read information from the surface of the magnetic strip.
(D) produces the changing magnetic fields that are needed to push currents through the wires in the playback head.
Answer: (D) produces the changing magnetic fields that are needed to push currents through the wires in the playback head.
Problem 13:
To produce the high voltages it needs for its tube, a neon sign uses a transformer. AC current from the power company flows through that transformer's primary coil and a separate current flows through the transformer's secondary coil. To obtain a large voltage rise across its secondary coil, the transformer is built with
(A) many more turns in the primary coil than in the secondary coil.
(B) thicker wires in the secondary coil than in the primary coil.
(C) many more turns in the secondary coil than in the primary coil.
(D) thinner wires in the secondary coil than in the primary coil.
Answer: (C) many more turns in the secondary coil than in the primary coil.
Problem 14:
The lighted image that appears on the screen of a conventional television is created when
(A) a beam of electrons collides with a phosphor coating on the inside surface of the screen.
(B) the magnetic field from a coil of wire inside the picture tube induces currents in the phosphor coating on the inside of the screen.
(C) currents passing through the photoconductor coating inside the picture tube cause it to emit light through the phosphor screen.
(D) electric currents inside the picture tube heat its phosphors until they emit light.
Answer: (A) a beam of electrons collides with a phosphor coating on the inside surface of the screen.
Problem 15:
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 less electric power.
(B) stop turning.
(C) keep turning but consume the same amount of electric power.
(D) keep turning but consume more electric power.
Answer: (A) keep turning but consume less electric power.
Problem 16:
You are walking your dog on a leash. The dog sees a rabbit and begins to run after it. The two of you run across the park, with the dog in the lead and you following. The leash is tight. As the two of you run,
(A) you are transferring equal amounts of energy to one another.
(B) there is no transfer of energy between you and the dog.
(C) the dog transfers energy to you.
(D) you transfer energy to the dog.
Answer: (C) the dog transfers energy to you.
Problem 17:
A battery
(A) creates positive charge.
(B) pumps positive charge from its positive terminal to its negative terminal.
(C) creates negative charge.
(D) pumps positive charge from its negative terminal to its positive terminal.
Answer: (D) pumps positive charge from its negative terminal to its positive terminal.
Problem 18:
When you step off the high diving board,
(A) your mass goes to zero but your weight remains unchanged.
(B) both you mass and your weight go to zero.
(C) your weight goes to zero but your mass remains unchanged.
(D) your mass and weight don't change.
Answer: (D) your mass and weight don't change.
Problem 19:
Many mail order catalogs sell lamp dimmer disks. The dimmer disk contains a single diode through which current must pass on its way to or from 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 divides the current in half and only allows one half of that current to flow through the filament.
(B) 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.
(C) 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.
(D) the diode halves the voltage of the current passing through the light bulb so that the power it receives is also halved.
Answer: (C) 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.
Problem 20:
Which of the following emits radio waves:
(A) positive and negative charge that are separated from one another.
(B) positive charge that is accelerating.
(C) positive charge that is moving at a constant velocity.
(D) positive charge that is stationary.
Answer: (B) positive charge that is accelerating.
Problem 21:
You are pushing sideways on the edge of a merry-go-round with a steady force. You are thus exerting a steady torque on the merry-go-round. As a result, the merry-go-round's
(A) angular acceleration is decreasing.
(B) angular acceleration is increasing.
(C) angular velocity is constant.
(D) angular acceleration is constant.
Answer: (D) angular acceleration is constant.
Problem 22:
A flashlight is a simple circuit with only two batteries, a bulb, and a switch. When the flashlight is on, current flows through these objects, one after the next, over and over again. When you turn the switch off, the current soon stops flowing because
(A) the switch's magnetic field drops to zero and it stops inducing current in the circuit.
(B) the current becomes AC, which is incompatible with the DC batteries.
(C) charges accumulate on either side of the open switch and repel any additional like charges that approach them from the bulb or battery.
(D) the switch's magnetic field drops to zero and it can no longer attract positive and negative charges from the battery.
Answer: (C) charges accumulate on either side of the open switch and repel any additional like charges that approach them from the bulb or battery.
Problem 23:
If you plug an electric hairdryer into the wall outlet and turn it on, current will begin to flow through wires inside the wall. Some of the electric power carried by that current will be wasted as heat in those wires. If you plug a second identical hairdryer into the same outlet and turn it on, the amount of power wasted in the wires will
(A) remain almost the same because the voltage in the wires will change very little.
(B) increase by a factor of 2.
(C) increase by a factor of 4.
(D) remain almost the same because the current in the wires will change very little.
Answer: (C) increase by a factor of 4.
Problem 24:
A person in a wheelchair is being pushed up a ramp to the entrance of a building. The ramp's surface is 24 m long and rises to a height of 2 m.
The total weight of the person and wheelchair is 600 N. How much work is done on the person and wheelchair during the ascent?
(A) 7,200 J (7,200 N·m).
(B) 50 J (50 N·m).
(C) 1200 J (1200 N·m).
(D) 14,400 J (14,400 N·m).
Answer: (C) 1200 J (1200 N·m).
Problem 25:
The value of an MOSFET (transistor) is that it allows a very small amount of electric charge to
(A) to exert large forces on an uncharged wire.
(B) to exert large forces on an electrically charged wire.
(C) control the flow of a large current.
(D) to produce an enormous magnetic field.
Answer: (C) control the flow of a large current.
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:
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.
(A) If you reverse the connections to the battery, so that the blue wire is attached to the battery's negative terminal and the green wire is attached to the battery's positive terminal, will the rotor spin and, if so, which way? It will spin counter-clockwise.
(B) 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 becomes a generator, producing electric power.
(C) If you spin the rotor counter-clockwise with your fingers, what will happen to the light bulb? It will still light up.
Problem 2:
You are trying to post photographs in your kitchen, using a box of cute little magnets.
(A) You take one of the magnets and use it to hold a picture against the steel refrigerator. Why is the magnet attracted to the steel? The magnet aligns all of the magnetic domains in the steel. The steel presents an opposite pole to magnet and the two attract.
(B) You take a second magnet and try to hold a photo against the brass light switch cover but it doesn't stick. In fact, you feel a slight repulsion as the magnet approaches the brass. What is creating the repulsive magnetic pole in the brass? The moving magnet induces current in the brass and this current is magnetic.
(C) You try to stick the magnet to a glass window, but nothing happens, even as the magnet approaches the glass. Why can't the glass respond the way the brass did? The glass does not conduct electricity so no currents flow in it.
Problem 3:
You're coasting along a street on your new rollerblades. The street is level and, with the help of a slight tail wind, you're traveling in a straight line at a steady speed.
(A) Suppose that you weigh 800 N and that you are traveling forward at 3 m/s. The net force on you is zero.
(B) You come to a gradual rise in the road and begin to slow down. How does the steepness of the rise affect your acceleration as you coast up the hill? The steeper the rise, the faster you accelerate (backward).
(C) You finally slow to a stop at the top of the rise. What has become of the kinetic energy you had earlier? It has become gravitational potential energy.
Problem 4:
Your friends is having trouble receiving a particular radio station with a portable radio.
(A) The first thing you notice is that the radio's antenna is oriented horizontally. The antenna should be vertical because the station's antenna is vertical and produces a wave with a vertical electric field. This electric field can't push charge along a horizontal receiving antenna.
(B) You notice that the radio isn't quite adjusted to the right frequency. This misadjustment reduces the amount of electric charge moving up and down the receiving antenna because the receiving antenna's misadjusted tank circuit won't allow charge to slosh back and forth in it at the station's frequency.
(C) You move the radio around and find a new location with much better reception. In the original location, a reflected part of the radio wave was interfering destructively with the unreflected part. How were the electric fields of those two parts arranged relative to one another? These electric fields point in opposite directions (so that they cancel one another).
Problem 5:
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 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? Its inertia.
(B) The blender plugs into an electric outlet. When you push the on button, the blender mixes. At a particular moment during its operation, which way is electric current flowing through each of the two wires in the blender's power cord? Current flows toward the blender through one wire and away from the blender through the other wire.
(C) The blender contains a universal motor and can actually run on either AC or DC electric 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? They will turn in the same direction.