Physics 106 - Midterm Exam

Physics 106 - How Things Work - Spring, 2001

Midterm Examination

Given Friday, March 2, 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 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

(A) the team on the right.

(B) the team on the left.

(D) both teams.

Problem 2:

While an insulator is nearly perfect at preventing the flow of electric current, a semiconductor will permit some current to flow, even in the dark. This current flow is a consequence of thermal energy. In the semiconductor, thermal energy shifts electrons between level so that

(A) both the valence band and the conduction band become full.

(B) the valence band becomes full, while the conduction band becomes empty.

(D) the valence band has a few empty levels and the conduction band has a few filled levels.

Problem 3:

You were heading forward in your car before coming to a complete stop at a red light. The careless driver of the car behind you fails to stop and his car crashes into your car from behind. You suddenly find your head shifted deep into the elastic cushion of your seat's headrest. During the period when your head is deep in the cushion, the net force on your head is

(A) backward and its acceleration is backward.

(B) forward and its acceleration is forward.

(D) zero and it is not accelerating.

Problem 4:

When you bounce a rubber ball off a concrete wall, the ball rebounds at nearly its original speed. During the bounce, the ball

(A) transfers a great deal of momentum and energy to the wall.

(B) retains essentially all of its momentum but transfers a great deal of energy to the wall.

(D) retains essentially all of its energy but transfers a great deal of momentum to the wall.

Problem 5:

A woman leaps off the high diving board during a swimming competition and does a series of flips and spins in the air. As she plunges toward the water, her

(A) angular velocity is constant.

(B) angular momentum is constant.

(D) velocity is constant.

Problem 6:

Suppose that you remove the batteries from an ordinary flashlight, turn them all around, and reinsert them into the flashlight. The batteries and the flashlight’s wires all make good contacts, so that there is no unexpected break in the flashlight’s circuit. If you now turn the flashlight on, it will

(A) glow about twice as bright as usual, but the bulb will burn out soon.

(B) not emit any light.

(D) glow about half as bright as usual.

Problem 7:

You let a permanent bar magnet align freely with an extremely strongly magnetic field. You then rotate the permanent magnet 180° so that it appears to be pointing the wrong way. If you now let the permanent magnet turn freely it will

(A) rotate 180° because it has been demagnetized and is no longer magnetic.

(B) rotate 180° because permanent magnets can’t be remagnetized.

(D) not rotate because it has been demagnetized and is no longer magnetic.

Problem 8:

You have two boxes. In Box A, there is only an electric field that points northward. In Box B, there is only a magnetic field that points westward. You hold a positive charge motionless in each box and then let it go. (Ignore gravity)

(A) The charge in Box A accelerates northward, while the charge in Box B accelerates westward.

(B) The charge in Box A remains motionless, while the charge in Box B accelerates westward.

(D) The charge in Box A accelerates southward, while the charge in Box B accelerates eastward.

Problem 9:

If you hold a stationary positive charge in your left hand and a stationary north magnetic pole in your right hand, the positive charge would exert

(A) a leftward force on the north magnetic pole.

(B) zero force on the north magnetic pole.

(D) an upward force on the north magnetic pole.

Problem 10:

A needle-sharp lightning rod protects the top of a tall tower. This rod is connected by a wire to the earth far below. When a positively charged cloud passes over the tower, the lightning rod

(A) becomes negatively charged and begins to emit negative charges onto passing air particles which help to neutralize the cloud.

(B) becomes negatively charged and repels any lightning strikes so that they hit far away from the tower.

(D) becomes positively charged and repels any lightning strikes so that they hit far away from the tower.

Problem 11:

A positively charged dust particle will stick to an electrically neutral wall because the dust particle will

(A) charge the wall by causing negative charge to be created at the surface of the wall so that the wall and dust attract.

(B) induce an electric current in the wall, making the wall magnetic so that it attracts the dust particle magnetically.

(C) charge the wall by transferring part of its positive charge to the wall so that the wall and dust attract one another.

(D) polarize the wall, shifting the wall’s negative charges toward the dust and the wall’s positive charges away from the dust.

Problem 12:

Moment of inertia is the measure of an object’s rotational inertia. If you wanted to figure out which of two round, symmetric objects has the largest moment of inertia, you should

(A) spin each object in your hand and then drop it to the floor. The one that falls fastest while spinning has the larger moment of inertia.

(B) lift each object and see which one takes the most force to lift upward at constant velocity.

(D) spin each object in your hand and then drop it to the floor. The one that falls slowest while spinning has the larger moment of inertia.

Problem 13:

A permanent magnet sticks motionlessly to the steel front of your refrigerator because the permanent magnet

(A) shifts the electrons and protons in the steel slightly apart, so that a magnetic polarization occurs. The protons have north magnetic poles and the electrons have south magnetic poles.

(B) causes currents to flow in the steel, making the steel magnetic so that it attracts the poles of the permanent magnet.

(D) attracts isolated magnetic poles (also known as monopoles) in the steel toward the surface of the steel, where they bind strongly to the poles of the permanent magnet.

Problem 14:

The secondary coil of a transformer normally winds in one direction around and around the magnetic core. Instead of going all the way around the core, it would be easier for the wire to go half way around the core and then turn around and head back toward its starting point. It could do this many times, creating a zigzag pattern that never actually goes all the way around the core. Such a secondary wire would produce

(A) only half as large a voltage rise as a normal secondary coil.

(B) only a quarter as large a voltage rise as a normal secondary coil.

(D) zero net voltage rise in any current passing through it.

Problem 15:

The lamp on your desk has a short circuit. A fragment of metal is connecting the two wires at the point where the bulb screws into the socket. Because of this short circuit, the bulb glows

(A) normally, but the current in the lamp’s power cord is smaller than usual.

(B) dimly or not at all, but the current in the lamp’s power cord is larger than usual.

(D) more brightly than normal and is likely to burn out.

Problem 16:

The cord of your desk lamp has two wires inside it. It needs two wires because

(A) the power company provides both electric and magnetic power. One wire carries electric power and the other wire carries magnetic power.

(B) one wire carries current to the lamp and the other wire carries current back to the power company.

(D) north and south magnetic poles cannot be separated from one another. The north and south poles must travel together on the two wires.

Problem 17:

If you want to send lots of electric power to a distant consumer and not waste very much of that power, you should use

(A) a very small current of very high voltage charges.

(B) a very small current of very low voltage charges.

(D) a very large current of very low voltage charges.

Problem 18:

Suppose you have two identical-looking sticks, each with exactly the same total mass. In one stick, most of the mass is located at its ends. In the other stick, most of the mass is located at the middle. If you grip both sticks at the middle and exert equal torques on them with your wrists,

(A) the sticks will undergo equal accelerations because they have equal masses.

(B) the stick with its mass near its ends will undergo more angular acceleration.

(D) the sticks will undergo equal accelerations because they have equal moments of inertia.

Problem 19:

You jump off of a diving board into a swimming pool. As you fall toward the water, your velocity

(A) increases, but your acceleration remains constant

(B) increases because your acceleration increases

(D) remains constant because your acceleration is constant

Problem 20:

When it moves slowly on a calm lake, a canoe will coast almost forever at constant velocity. Suppose that two canoes are both coasting northward at constant velocity (no one is paddling) when the rear canoe bumps into the canoe in front of it. There is a loud “thump” sound. Despite the collision, the total

(A) velocity of the two canoes remains unchanged.

(B) kinetic energy of the two canoes remains unchanged.

(D) thermal energy of the two canoes remains unchanged.

Problem 21:

A battery

(A) creates negative charge.

(B) creates positive charge.

(D) pumps positive charge from its negative terminal to its positive terminal.

Problem 22:

A toy top spins for a long time on its point. If you made the point sharper, the top would spin longer because

(A) the force the top exerts on the floor becomes smaller.

(B) the parts of the sharper point travel less far against the force of sliding friction.

(D) the support force that the floor exerts on the top becomes larger.

Problem 23:

Copper is a nonmagnetic metal. When you move the north pole of a bar magnet toward a sheet of copper,

(A) current flows through the copper and repels the approaching north pole.

(B) nothing happens to the copper.

(D) current flows through the copper and attracts the approaching north pole.

Problem 24:

There are no permanent magnets made out of pure aluminum metal because aluminum

(A) does not have enough mass to overcome the inertia of permanent magnetism.

(B) is a soft magnetic material and quickly demagnetizes when you remove it from any external magnetic fields.

(D) has no internal magnetic structure at all.

Problem 25:

A current loses energy as it flows through a wire. Because of this loss of energy, the current experiences a

(A) voltage rise as it passes through the wire and a magnetic field therefore pushes the current forward.

(B) voltage rise as it passes through the wire and an electric field therefore pushes the current forward.

(D) voltage drop as it passes through the wire and an electric field therefore pushes the current forward.

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 may have noticed that when you use a power-hungry appliance such as a hairdryer in the other socket of an electric outlet that already has a lamp plugged into it, the lamp dims slightly. Let’s examine why. To begin with, suppose that only a lamp is plugged into the outlet in your room and that the lamp is turned on.

(A) Current travels between the power lines outside and the electric outlet inside through moderately thick wires. Between the point at which this current leaves the power lines for your home and the point at which it returns to the power lines from your home, it experiences a voltage drop of 120 volts. More specifically, the current experiences voltage drops in each of the two wires that connect the outlet to the power lines (the “house wires”) and in the lamp itself. How are these three voltage drops: (x) power line to outlet, (y) outlet to power line, and (z) through the lamp, related to one another or to 120 volts?

(B) How does the current passing through the lamp compare to the currents passing through each of the two house wires?

(C) You plug a hairdryer into the other socket of the same electric outlet and turn it on. Current that arrives at the outlet through one house wire now passes through either of the two appliances (lamp or hairdryer) before returning to the power lines through the second house wire. With more opportunities to flow from one house wire to the other, the current passing through the circuit increases dramatically. How does this current increase in the house wires affect the voltage drop through each house wire?

(D) What effect does this change in the voltage drop through each house wire have on the voltage drop in the lamp, the current through the lamp, and power consumed by the lamp? (Just state the answers, don’t tell us why.)

Problem 2:

Ensuring fair play at the Olympics has never been easy, both on the field and off. With your tremendous understanding of physics, you've received many lucrative offers to help the scoundrels, but you've held fast to the traditions of the UVa honor system and refused them all. Instead, you have become the world's foremost authority on sneaky Olympic tricks and have foiled dozens of evil plots. Here are a few of your most famous cases:

(A) The Tomanian cycling team once placed a series of strong magnets beneath the bicycle track and slowed the progress of any athlete riding an aluminum alloy bicycle. You discovered the magnets and fingered the Tomanian team because they were the only people riding bicycles made entirely of plastics. Why did the magnets slow the aluminum bicycles but not the plastic ones?

(B) The Freedonian track team once tried to lower their times in the 5000-meter run by installing air jets all the way around the track. These jets were aimed so that they would always blow each runner forward with a force of 100 newtons. Minutes before the race, you spoiled their plans by turning half the jets around so that they blow each runner backward. As the result of your efforts, the runners traveled exactly the same distance with air jets pushing them forward at 100 newtons as they did with air jets pushing them backward at 100 newtons. Overall, how much work did the air jets do on each runner?

(C) One of the Lilliputian high jumpers decided to give himself an advantage by putting huge negative charges on both himself and on the landing mat under the bar. You caught him in time to remove the negative charge from the mat. The jumper experienced no repulsion from the mat and failed to clear the bar. Instead, the plastic bar stuck to him and the audience laughed as he fought to get it off. Why did the uncharged plastic bar stick to the negatively charged jumper?

(D) In a misguided attempt to win the 100 meter dash, a runner from the Duchy of Grand Fenwick coated the bottoms of her shoes with Superslide™, the "ultimate in frictionless lubrication." Her shoes experienced exactly zero friction. When the gun went off, the runner found herself unable to move forward and remained at the starting point with her legs churning furiously. What force or other physical effect kept her from moving forward?

Problem 3:

Some modern stovetops use a technique known as “induction heating” to heat cookware directly. The glass or ceramic surface of the stovetop stays cool and only the pot itself becomes hot. In principle, induction heating will work with any metallic pot, but for technical reasons that are beyond the scope of this problem, it works most efficiently with ferromagnetic metals.

(A) Just under the surface of an induction stovetop is a coil of wire. When the stove is turned on, a high-frequency alternating current passes through the coil. A metal pot placed just above the surface of the stovetop quickly becomes warm. This heating occurs because electric current flows through the pot. How does the presence of an alternating current in the stovetop’s coil cause current to flow in the pot?

(B) Why does current flow in the pot cause the pot to become hot?

(C) Glasses and ceramics contain electric charges, so why can’t this heating technique be used with a glass or ceramic pot?

(D) Producing high-frequency alternating current is relatively expensive. Why can’t this technique use a steady direct current in the stovetop’s coil?