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:

Electric power passes through a nearby power transformer on its way to your home. The current that enters your home actually receives its power when it passes through the secondary coil of that transformer. The coil has a carefully chosen number of turns in it because doubling the number of turns would

(A) halve the voltage of the current entering your home.
(B) double the frequency (cycles-per-second) of the alternating current entering your home.
(C) double the voltage of the current entering your home.
(D) halve the frequency (cycles-per-second) of the alternating current entering your home.
Answer: (C) double the voltage of the current entering your home.
Why: The current passing through the secondary coil of a transformer picks up more energy per unit of charge with each trip around the secondary coil. That's because there is an electric field pushing the charge around the coil and doing work on that charge. If you let the current pass around the coil twice as many times, by doubling the number of turns in the coil, then the current will pick up twice as much energy per unit of charge. With twice as much energy per charge, the current has twice its original voltage.

A basketball player bounces a ball off the floor repeatedly as he dribbles it down the court. Each time the ball hits the floor, it rebounds almost to its original height. From that bouncing behavior, you know that during a collision with the floor,

(A) the ball retains most of its energy but transfers a large amount of momentum to the floor.
(B) the ball transfers a large amount of energy and momentum to the floor.
(C) the ball retains most of its momentum but transfers a large amount of energy to the floor.
(D) the ball retains most of its energy and its momentum.
Answer: (A) the ball retains most of its energy but transfers a large amount of momentum to the floor.
Why: To return to almost its original height, the ball needs almost its original energy. That energy went from gravitational potential energy, to kinetic energy (during the descent), to elastic potential energy (during the actual bounce), to kinetic energy (during the rebound), to gravitational potential energy (during the rise). But the ball's momentum changed dramatically because it started with a downward momentum and ended with an upward momentum. It gave all its downward momentum to the floor and went even further--it acquired an upward momentum by transferring still more downward momentum to the floor.

The surface of an audio recording tape must

(A) have an electric field around it.
(B) be positively charged.
(C) contain many tiny permanent magnets.
(D) conduct electricity.
Answer: (C) contain many tiny permanent magnets.
Why: The sound information on an audio recording tape is represented by the magnetization of the tape's surface. This surface consists of countless tiny particles, each of which is highly magnetic but which can be magnetized in one of two directions. These recording particles are miniature permanent magnets.

A typical bar magnet has an "N" stamped on its north pole end and an "S" stamped on its south pole end. With the right tools, you might be able to change this bar magnet in one or more of the following ways: (1) remove its magnetic poles altogether, (2) reverse its magnetic poles so that it has a north pole at the end stamped "S" and a south pole at the end stamped "N", (3) convert its north pole into a south pole so that it has south poles at both ends. Which of those three options is physically possible?

(A) Only (1) and (2).
(B) None of them is possible.
(C) Only (2) and (3).
(D) All three are possible.
Answer: (A) Only (1) and (2).
Why: While it is possible to demagnetize a permanent magnet, by randomizing the magnetic orientations of its microscopic magnetic domains, or remagnetize a permanent magnet backwards, by exposing it to a strong, reversed magnetic field, you can't get give it a net magnetic pole. Putting south pole at both ends of the magnet would leave it with a net south magnetic pole and that's not possible. No free magnetic pole has ever been found in nature, so north and south poles always appear in equal pairs.

In which one of the following situations are you doing (positive) work on a bowling ball?

(A) When you lower it downward at constant velocity from a high shelf to the floor.
(B) When you hold it motionless above your head.
(C) When you slide it horizontally across the floor at constant velocity.
(D) When you carry it horizontally at constant velocity.
Answer: (C) When you slide it horizontally across the floor at constant velocity.
Why: The only case in which you do work is the one in which you exert a force on the ball and the ball moves in the direction of that force. When the ball is motionless, as in (B) or moving at right angles to the force you exert on it, as in (C), you do no work. When the ball moves in the direction opposite to the direction of the force you exert on it, as in (A), you do negative work on it. But when you slide the ball across the floor and struggle against the force of sliding friction, you push the ball in the same direction it moves and thus do (positive) work on it.

A xerographic copier uses a special photoconductor surface that allows light from an original document to control the placement of black powder on white paper. The photoconductor only conducts electricity when it’s exposed to light because, in the dark,

(A) the photoconductor contains only negatively charged particles.
(B) the photoconductor contains only positively charged particles.
(C) electrons in the photoconductor completely fill its valence levels and can’t shift from one level to another in order to transport charge through the material.
(D) the photoconductor doesn’t contain any electrically charged particles.
Answer: (C) electrons in the photoconductor completely fill its valence levels and can’t shift from one level to another in order to transport charge through the material.
Why: The electrons in a semiconductor, like those in an insulator, are prevented from responding to electric fields because they have no nearby empty levels into which they can shift. The whole arrangement of electrons is rigid and immobile because of this constraint.

The plug of your desk lamp has two flat metal prongs (we are ignoring any cylindrical ground prong that may also be present). If you were to break off one of those flat prongs and plug the remaining prong into the outlet, the current that would then flow through the lamp would be

(A) zero.
(B) twice the current that flowed when both prongs were present and plugged in.
(C) the same as the current that flowed when both prongs were present and plugged in.
(D) half the current that flowed when both prongs were present and plugged in.
Answer: (A) zero.
Why: Without two wires connecting the lamp to the power company, there is no circuit. Charge flowing out the single remaining wire can't return to the power company so it accumulates in the lamp and repels further charge. So little charge flows that the lamp stays dark.

If you float an aluminum pie plate on the surface of a pond and move the north pole of a strong magnet in a clockwise circle just above that plate, the plate will

(A) begin turning counter-clockwise, as though it were being twisted away from the magnet.
(B) begin turning clockwise, as though it were being dragged along with the magnet.
(C) remain stationary.
(D) be lifted out of the water and will stick to the strong north pole above it.
Answer: (B) begin turning clockwise, as though it were being dragged along with the magnet.
Why: The moving magnet induces electric currents in the aluminum and these current are magnetic. They repel the moving magnet and the aluminum disk is pushed in the direction of the moving magnet. In effect, the magnet drags the disk around in a circle with it as the result of this continued repulsion.

You are watching an ice hockey game and a player has just hit the puck toward her opponents’ goal. The goalie is out of position and the puck will reach the goal without anyone being able to stop it. Once the puck has left the player’s stick and is traveling across the ice toward the goal, it experiences

(A) no horizontal force in the forward direction.
(B) a forward horizontal force that remains constant all the way to the goal.
(C) a forward horizontal force that diminishes gradually as the puck approaches the goal.
(D) a forward horizontal force until it reaches the midpoint of its trip to the goal and then a backward horizontal force for the remainder of its trip.
Answer: (A) no horizontal force in the forward direction.
Why: The puck continues forward because of its inertia alone. The only horizontal forces it experiences are slowing forces--sliding friction and air resistance--and these push it backward to slow it down.

You are throwing a ball straight up and then catching it as it returns to your hand. When the ball leaves your hand, its momentum is in the upward direction but when it returns to your hand, its momentum is in the downward direction. During its flight above your hand, what happens to the ball’s initial upward momentum?

(A) The upward momentum is converted into kinetic energy.
(B) The upward momentum is converted into thermal energy.
(C) The upward momentum is converted into gravitational potential energy.
(D) The upward momentum is transferred to the earth.
Answer: (D) The upward momentum is transferred to the earth.
Why: As the ball rises, the earth pulls down on it and it pulls up on the earth. This pulling gradually transfers the ball's upward momentum to the earth and the ball's upward velocity diminishes.

Commercial electric power is sent across country using high voltage transmission lines. If low voltage transmission lines were used instead, those low voltage lines would

(A) have to use direct current, which would not operate most equipment properly.
(B) not be magnetic enough to transform electricity efficiently into energy.
(C) have to placed closer to the ground, where they would be more hazardous.
(D) have to be made of thicker metal, which would be expensive.
Answer: (D) have to be made of thicker metal, which would be expensive.
Why: To carry the necessary power with lower voltages, the transmission lines would have to employ larger currents. But power loss in a wire increases in proportion to the square of the current in that wire. To prevent power loses from becoming too large, the wires would have become better conductors. That means they'd have to become much thicker and more expensive. Edison's power companies floundered because of this problem.

You stick a piece of adhesive tape on a glass window and then pull the tape off suddenly. The tape ends up with a negative electric charge. If you now hold the tape near the window again, the tape will be

(A) attracted by the window because the window will have a positive charge.
(B) repelled by the window because the window will have a negative charge.
(C) repelled by the window because the window will have a positive charge.
(D) attracted by the window because the window will have a negative charge.
Answer: (A) attracted by the window because the window will have a positive charge.
Why: Like most objects, the tape and window are nearly electrically neutral when left alone. But when you remove the tape from the window, there is a transfer of charge--the tape becomes negatively charged and the window becomes positively charged. Overall, the pair remains electrically neutral because they can't create or destroy charge. These two then attract one another because they are oppositely charged.

A construction crew is replacing the huge air conditioning unit on the roof of an office building. They know that if they push the unit off the roof and let it fall freely, it will have 10 million joules of kinetic energy by the time it reaches the ground. They choose instead to put the unit on a cart and let it roll freely down a ramp. The ramp has a 10 to 1 grade, meaning that it goes down 1 meter in height for every 10 meters you travel along its surface. By the time the air conditioning unit reaches the ground on this ramp, it will have

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 from moving at all. The right end of the rope stretches toward the right and the rope breaks. It took energy to breaking the rope and that energy was provided by

To pay your tuition this semester, you have decided to moonlight as a race car driver. It’s your first race and you’re heading full speed down a straight portion of the track. You place a small good luck charm in the middle of the dashboard and then begin turning the car toward the left. As you and your car make the left turn, the charm slides toward the right side of the dashboard. As the charm slides, the car is

Electricity produced in a generating plant passes through a large step-up transformer. This step-up transformer produces the high voltages needed to send electric power long distances across the countryside. Which of the following is transferred from the transformer’s primary coil to its secondary coil while the transformer is operating?

You are playing with a large spring, squeezing it and stretching it. If you begin with the spring at its normal length, in which case(s) are you doing work on it?

You remove the batteries from a working flashlight, turn both of them around as a pair, and reinsert them in the flashlight. They make good contact with the flashlight’s terminals at both ends, so that there is no mechanical problem preventing the flashlight from working. If you now switch on the flashlight, it will

You are working in a pizza parlor and have learned how to toss and spin the dough to form large disks. You find that the larger each disk becomes as you spin it, the harder it is to stop the disk from spinning. While a small twist is all it takes to stop a ball of dough from spinning, by the time that same dough becomes a 16 inch disk a similar twist barely slows it down. This effect occurs because spreading the dough into a disk increases its

An n-channel MOSFET consists of three pieces of semiconductor: two n-type pieces connected by a p-type piece. This n-channel MOSFET will allow current to flow through all three pieces when

A helicopter is hovering motionless above a disabled boat, while rescue workers use a rope to lift an injured sailor. While that sailor is being lifted upward, the net force on the motionless helicopter is

Which of the following can cause a stationary charged particle to accelerate?

You have a small DC motor that normally operates on 3 volt direct current, meaning that if you were to connect it to a 3 volt battery in a circuit, it would spin nicely. But instead of connecting the motor to a battery, you connect it to a 3 volt flashlight bulb in a circuit. Any current flowing through the motor must therefore flow through the bulb and back to the motor. When you spin the motor’s shaft rapidly with your fingers, the light bulb lights up. The motor is able to make the bulb light up because spinning its shaft

A semiconductor diode contains a p-n junction. One way to understand why electric current can only flow through such a diode in one direction is that

A battery in an operating flashlight

Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.

Problem 1:

You are establishing a new company to manufacture audio recording tape. You have just ordered clear plastic tape but you still have to buy the right coating material for that tape.

You are riding on a huge roller coaster with the tallest, steepest first hill in the world. To make the roller coaster even more exciting, its designers have used high technology to eliminate air resistance and friction, so that the coaster follows the laws of physics without producing any thermal energy. The first hill can be divided into three parts: top, middle, and bottom. While the top portion of the first hill slopes gradually downward, the middle portion of the hill dives almost straight down. The bottom portion of the hill is less steeply sloped in the downward direction, becoming more and more gradual so that it eventually levels out completely.

Modern audio amplifiers usually contain at least four important electronic components: resistors, capacitors, diodes, and MOSFETs. Let’s take a brief look at each of these devices.