Physics 106 Midterm Exam

Physics 106 - How Things Work - Spring, 2000

Midterm Examination

Given Friday, March 3, 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 walk to class at a steady pace along a flat, horizontal path. Your backpack weighs 100 newtons and your trip is 500 meters long. How much work do you do on your backpack?

(A) 0 joules (0 newton-meters).

(B) 1/5 joule (1/5 newton-meters).

(D) 5 joules (5 newton-meters).

Problem 2:

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

(A) A stationary, constant magnetic field.

(B) A stationary, constant electric field.

(D) A stationary wire containing a constant electric current.

Problem 3:

You install a diode in one of the two wires connecting a desk lamp to the electric outlet’s alternating current. Any current passing through the bulb’s filament must also pass through the diode. With the diode in place, the bulb will glow only about half as bright as usual because the diode

(A) turns half the current into voltage, which has no effect on the bulb.

(B) turns half the voltage into current, which has no effect on the bulb.

(D) only allows half as much current to flow at any given time.

Problem 4:

The split iron ring of a tape recorder’s playback head is made of a magnetically soft material so that

(A) it doesn’t scratch the tape.

(B) its ends can become electrically charged easily.

(D) a large current can flow through it easily.

Problem 5:

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

(A) your velocity remains constant because your acceleration is constant.

(B) your velocity remains constant because your acceleration is zero.

(D) your velocity increases because your acceleration increases.

Problem 6:

When you put positive charge on the gate of an n-channel MOSFET, it conducts current well. Its three pieces of semiconductor are then effectively

(A) p-type, n-type, and p-type semiconductor respectively.

(B) pure, n-type, and pure semiconductor respectively.

(D) all n-type semiconductor.

Problem 7:

When you drop the north pole of a permanent magnet onto an aluminum sheet, it falls slightly slower than normal. The magnet is being repelled by the aluminum because

(A) aluminum is positively charged and repels approaching magnetic poles.

(B) aluminum has a net north magnetic pole that repels any approaching north poles.

(D) current induced in the aluminum by the approaching north pole produces a repelling north pole on the aluminum’s surface.

Problem 8:

You are playing Frisbee with a friend when the Frisbee gets caught in a tree. You throw a rock at the Frisbee to dislodge it. When the rock hits the Frisbee, knocking it loose,

(A) both energy and momentum are transferred to the Frisbee.

(B) momentum is transferred to the Frisbee, but no energy is transferred.

(D) neither momentum nor energy are transferred to the Frisbee since both must be conserved.

Problem 9:

A photoconductor’s electrical properties go from insulating to conducting when it is exposed to light because,

(A) light allows extra electrons to move out of the valence band giving the semiconductor a positive charge.

(B) light allows extra electrons to move into the conduction band giving the semiconductor a negative charge.

(D) light lowers the energy of the conduction levels eliminating the energy gap.

Problem 10:

A step-down transformer transfers power from the 120 volt alternating current passing through its primary coil to the 12 volt alternating current passing through its secondary coil. If you interchange the primary and secondary coil, and send the 120 volt alternating current through the new primary coil,

(A) a 12 volt alternating current will pass through its new secondary coil.

(B) a 12 volt direct current will pass through its new secondary coil.

(D) a 1200 volt alternating current will pass through its new secondary coil.

Problem 11:

A 60 watt, 120 volt light bulb is designed to reduce the voltage of a 1/2 amp current by 120 volts. If you screw this bulb into a fixture that is powered by very long thin wires it will glow more dimly than intended because the voltage drop across the bulb will be

(A) less than 120 V and the current through the bulb will be less than 1/2 A.

(B) 120 V, but the current through the bulb will be less than 1/2 A.

(D) less than 120 V, but the current will still be 1/2 A.

Problem 12:

The surface of a photoconductor has been coated with electric charge. This charge will remain in place until you expose the surface to

(A) an electric field.

(B) a magnetic field.

(D) light.

Problem 13:

After running a plastic comb through you hair several times you hold it above a small scrap of paper. The paper jumps off the table and sticks to the comb because the paper becomes

(A) magnetic.

(B) electrically charged.

(D) electrically polarized.

Problem 14:

You charge a capacitor by connecting its terminals to opposite ends of a battery. After disconnecting the battery,

(A) there are both electric and magnetic fields between the conducting plates of the capacitor.

(B) there is a magnetic field between the conducting plates of the capacitor.

(D) there is an electric field between the conducting plates of the capacitor.

Problem 15:

If your bicycle tires were made of solid rubber rather than being filled with air, it would be harder to start them spinning because they would have

(A) a larger angular momentum.

(B) a larger moment of inertia.

(D) a larger kinetic energy.

Problem 16:

The north pole of a permanent magnet is clinging to the front surface of your steel refrigerator, so the refrigerator clearly has a south pole at its surface. If you flip the permanent magnet over, so that its south pole faces the refrigerator, the refrigerator will

(A) keep a south pole at its surface and repel the permanent magnet.

(B) keep a south pole at its surface and attract the permanent magnet.

(D) place a north pole at its surface and attract the permanent magnet.

Problem 17:

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 increases as they move apart.

(B) electric current passing through each garment diminishes as they move apart.

(D) forces between electric charges become weaker with increasing distance.

Problem 18:

A person playing Frisbee golf on a windless day throws a Frisbee at a distant target. When the Frisbee is flying forward but has left the hands of the thrower, it experiences

(A) a forward force that diminishes steadily and reaches zero just as the Frisbee strikes the target.

(B) no forward force.

(C) a forward force that increases in strength for the first half of the flight and then decreases in strength for the second half of the flight.

(D) a constant forward force.

Problem 19:

You are carrying a spinning toy top around your dorm on the surface of a dining hall tray. It continues to spin almost indefinitely because nothing can exchange any

(A) force with it.

(B) angular momentum with it.

(D) momentum with it.

Problem 20:

Your new toaster has two separate toasting units, each of which consumes 600 watts of power when it's in use. When you operate one unit, a current of 5 amperes flows through the wiring in your home and the wires waste about 1 watt of power handling that current. If you operate both toasting units at once, your toaster consumes 1200 watts and the current flowing through the wiring in your home doubles to 10 amperes. How much power will the wires in your home waste now?

(A) about 1 watt.

(B) about 0.5 watts.

(D) about 4 watts.

Problem 21:

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) both teams.

(B) the team on the left.

(D) the team on the right.

Problem 22:

Which one of the following objects can erase the information stored on the magnetic strip of your student ID card?

(A) A large battery.

(B) A capacitor with lots of separated charge on its two plates.

(D) A coil of wire with a large current flowing through it.

Problem 23:

Around high tension power lines there are

(A) electric, but no magnetic fields.

(B) neither electric nor magnetic fields.

(D) magnetic, but no electric fields.

Problem 24:

A battery in an operating flashlight

(A) does work while transferring positive charge from its negative terminal to its positive terminal.

(B) does work while transferring positive charge from its positive terminal to its negative terminal.

(D) has work done on it when positive charge flows from its positive terminal to its negative terminal.

Problem 25:

One use of a resistor in an audio amplifier is to

(A) amplify currents.

(B) restrict the flow of current to one direction.

(D) store charge.

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:

Electrical power is supplied to your house as an alternating current because it is easy to change the voltage of an alternating current without wasting much energy.

(A) The same amount of power can be delivered to your house using a large voltage and a small current or a small voltage and a large current. Why is a large voltage and small current used to transfer power over long distances?

(B) Having high voltages in your house can be dangerous so the power company uses a step-down transformer to reduce the voltage of the power supplied to your house. Compare the number of turns in the primary coil in this transformer to the number of turns in the secondary coil.

(D) Some electronic devices require a direct rather than alternating current. Why is a diode useful for converting an alternating current into a direct current?

Problem 2:

You have a little lamp for reading books in bed. This lamp requires 6 volt electricity. You can power it either with four 1.5 volt batteries in a battery pack or with a little step-down transformer that plugs into the electric outlet and produces 6 volt alternating current.

(A) There are two wires connecting the battery pack to the lamp. Why isn’t one wire sufficient?

(B) If one of the four batteries is inserted backward, the lamp will glow dimly because

(C) When you use the transformer to reduce 120 volt alternating current to 6 volt alternating current, what fraction of the electric charges passing through the electric outlet also pass through the lamp?

(D) When you operate the lamp from the transformer, there are moments when the lamp receives no electric power. Why is that?

Problem 3:

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?