Physics 1060 - Spring 2020 - Midterm Exam 2 - Posted Version
Problem 1:
A hot ember in a campfire is glowing red. It's almost pure carbon (charcoal), so what determines the color of its reddish glow?
(A) Hot charcoal emits white light, but its darker surface filters that light so that it appears reddish.
(B) Charcoal is black, but like any black surface, it emits thermal radiation characteristic of its temperature -- red in this case.
(C) Hot charcoal emits blue light, but that blue light is absorbed and reemitted as red light.
(D) Charcoal is naturally reddish, so it emits primarily reddish light.
Problem 2:
When 120 volt AC power is connected to the primary coil of a particular transformer, the secondary coil of that transformer supplies 30 volt AC power to its secondary circuit. Suppose you modify the transformer's secondary coil, doubling the number of turns in that secondary coil. How does that change affect the coil and secondary circuit?
(A) Twice as much current now flows through the secondary coil. The coil now supplies 15 volt AC power to its secondary circuit.
(B) Each charge moving through the secondary coil now has twice as much work done on it by the transformer's magnetic field. The coil now supplies 60 volt AC power to its secondary circuit.
(C) Twice as much current now flows through the secondary coil. The coil still supplies 30 volt AC power to its secondary circuit.
(D) Each charge moving through the secondary coil now has twice as much work done on it by the transformer's electric field. The coil now supplies 60 volt AC power to its secondary circuit.
Problem 3:
A radio station emits a radio wave from an antenna that is located high in the air and far from any conducting surfaces. Why is the length of that transmitting antenna approximately 1/2 the radio wave's wavelength?
(A) The antenna needs to be long enough to accommodate the entire upward arrow of the radio wave's magnetic field.
(B) The antenna is then resonant at the radio station's frequency, making it easier for the station to move large amounts of charge up and down the antenna.
(C) The antenna needs to be long enough to accommodate the entire upward arrow of the radio wave's electric field.
(D) The antenna can then hold only a crest or a trough of the radio wave, ensuring that there is no destructive interference on the antenna.
Problem 4:
A microwave oven’s cooking chamber has 6 sides. What are those sides made of?
(A) 5 are metal and 1 is metal mesh.
(B) 5 are plastic and 1 is metal mesh.
(C) 5 are plastic and 1 is plastic mesh.
(D) 5 are metal and 1 is plastic mesh.
Problem 5:
When electric power is transmitted long distances, it is usually carried through the wires by a small current at a large voltage. Power is transmitted that way because
(A) it does not require the use of transformers.
(B) a smaller current wastes less power in the wires.
(C) it does not require the use of a complete circuit.
(D) a larger voltage wastes less power in the wires.
Problem 6:
A twist-tie is a long, thin, sharp metal wire wrapped in paper. If you place a twist-tie in a microwave oven and turn the oven on, what will happen to the twist-tie?
(A) The twist tie will remain cool, but electric charge will spray out of its sharp ends.
(B) An electric current will flow through it, heating it up, but no charge will leave the wire.
(C) An electric current will flow through it, heating it up, and electric charge will also spray out of its sharp ends.
(D) The twist tie will not be affected by the microwaves because it contains no water.
Problem 7:
When you connect an electromagnet to a source of alternating current, its magnetization reverses direction repeatedly. An electric charge near that AC electromagnet
(A) experiences zero force because it is not affected by magnetic fields.
(B) experiences a force because the AC electromagnet's changing magnetic field produces an electric field.
(C) experiences zero force because the AC electromagnet's magnetic field averages to zero.
(D) experiences a force directly from the AC electromagnet's magnetic field.
Problem 8:
One end of a AAA battery has a voltage of +0.75 volts and the other end has a voltage of –0.75 volts. Suppose you have a small positive charge located at 0 volts. What work is required to move that positive charge to each of end of the battery?
(A) Negative work is required to move the charge to the +0.75 volts end and positive work is required to move the charge to the –0.75 volts end.
(B) Positive work is required to move the charge to the +0.75 volts end and negative work is required to move the charge to the –0.75 volts end.
(C) Positive work is required to move the charge to the +0.75 volts end and positive work is required to move the charge to the –0.75 volts end.
(D) Negative work is required to move the charge to the +0.75 volts end and negative work is required to move the charge to the –0.75 volts end.
Problem 9:
When they are far away from any external magnetic influences, two blocks of pure iron exert no magnetic forces on one another. Why not?
(A) Each iron block contains many highly magnetic domains, but those domains are oriented in semi-random directions. The domains' magnetic fields cancel so each block appears non-magnetic.
(B) Each iron block is highly magnetic but its north pole is equally strong as its south pole. The attractive forces between opposite poles of the two blocks are therefore cancelled by the repulsive forces between like poles of the two blocks.
(C) Iron is a non-magnetic metal, meaning that it has no intrinsic magnetism, even at the atomic or molecular scale.
(D) Each iron block contains as many north poles as it contains south poles. The net magnetic pole of each block is therefore zero and the blocks are completely non-magnetic.
Problem 10:
You are a firefighter and the alarm sounds in your fire station. As you slide down the fire pole at constant velocity, your gravitational potential energy is decreasing. What is becoming of that energy?
(A) It is becoming kinetic energy.
(B) It is becoming downward force.
(C) It is becoming thermal energy.
(D) It is becoming downward momentum.
Problem 11:
Your electric hairdryer was designed to operate on 120 volt alternating current. You are traveling overseas and your friend connects it to the local electric power, which is 240 volt alternating current. What happens to your electric hairdryer?
(A) The current in the hairdryer's heating element is about twice as large as it should be. The heating element consumes about 4 times as much power as it should and overheats.
(B) The hairdryer's heating element consumes the correct amount of electric power and operates normally.
(C) The temperature of 240 volt alternating current is much higher than the hairdryer was designed to handle and it overheats.
(D) The correct amount of current flows through the electric hairdryer's heating element, but that current alternates too rapidly and overheats the heating element.
Problem 12:
When you're using your smartphone on the beach, in which direction is current flowing through its battery and through its computer?
(A) Current flows toward lower voltage through the battery and toward lower voltage through the computer.
(B) Current flows toward higher voltage through the battery and toward higher voltage through the computer.
(C) Current flows toward lower voltage through the battery and toward higher voltage through the computer.
(D) Current flows toward higher voltage through the battery and toward lower voltage through the computer.
Problem 13:
When a long piece of wire is carrying a 5-ampere current, the wire exhibits a voltage drop of 1 volt. When that wire is carrying a 10-ampere current, the wire's voltage drop is
(A) 4 volts.
(B) 2 volts.
(C) 0.5 volts.
(D) 1 volt.
Problem 14:
You are talking on a cellphone when you enter an elevator. As the metal doors close, you lose reception and the call ends. Why?
(A) The vertical movement of the elevator prevents electromagnetic waves from reaching you.
(B) The metal surface of the elevator absorb the electromagnetic waves.
(C) The metal surfaces of the elevator reflect the electromagnetic waves.
(D) The closing doors cut the electromagnetic cord to your cellphone.
Problem 15:
Why does water heat up especially easily in a microwave oven?
(A) A water molecule spins rapidly, allowing it to absorb microwaves that more-slowly spinning molecules cannot.
(B) A water molecule has an oxygen atom and oxygen atoms absorb microwaves better than any other atom.
(C) A water molecule has an unusually large electric dipole, with its oxygen atom have a significant negative charge and its hydrogen atoms having significant positive charges.
(D) A water molecule is small compared to most molecules and its small size makes it better at absorbing microwaves.
Problem 16:
Your hairdryer is consuming 1800 watts of electric power. That power traveled to your city in high-voltage transmission wires carrying a total of 180 million watts (180,000,000 watts). What fraction of the electric charges passing through those transmission wires also pass through your hairdryer?
(A) 1 part in 1,000,000.
(B) 1 part in 100,000.
(C) zero.
(D) 1 part in 10,000.
Problem 17:
You put an empty mug and a mug full of coffee in a microwave oven and turn the oven on for 2 minutes. The full mug gets hot, but the empty mug stays cool. Why?
(A) The full mug is heavier and microwaves are absorbed in proportion to weight. The empty mug is too light to absorb significant microwave power.
(B) Heat flows into the water from its top surface, causing the full mug to become hot. The empty mug has no top surface, so it does not become hot.
(C) The microwaves vibrate each mug, but only the mug of coffee is resonant at the microwave frequency and retains the microwave power.
(D) The microwave electric fields polarize the water molecules, delivering power to the water molecules which turn it into thermal power. The empty mug doesn't absorb microwaves.
Problem 18:
A 120-volt AC electrical outlet in the United States has two slots: a long slot known as neutral and short slot known as power or load. Which slot sometimes has a voltage of +130 volts and which slot sometimes has a voltage of –130 volts?
(A) Neither slot ever has those voltages.
(B) Power can be +130 volts and neutral can be –130 volts.
(C) Neutral can be +130 volts and neutral can be –130 volts.
(D) Power can be +130 volts and power can be –130 volts.
Problem 19:
The WiFi base unit on your floor of a building has one antenna that is oriented vertically. You are 100 feet away from the base. How should you orient the antenna of your WiFi device?
(A) Orient the antenna horizontally.
(B) Orient the antenna vertically.
(C) It doesn't matter; all orientations are equally good.
(D) Point the antenna toward the base unit.
Problem 20:
A set of high-voltage power lines are conveying several million watts of electric power to Charlottesville. Near those power lines, there are
(A) neither electric nor magnetic fields.
(B) magnetic fields, but no electric fields.
(C) electric fields, but no magnetic fields.
(D) both electric and magnetic fields.
Problem 21:
A positively charged storm cloud is passing overhead as you step into your car in the middle of an open field. For the brief moment, while you are touch both the ground and the car, electric charge can move between the ground and car. When you are completely inside the car and the car's tires insulate it from the ground, what is the car's electric charge?
(A) The car is negatively charged.
(B) The car is positively charged.
(C) The car's charge cannot be determined without measuring it.
(D) The car is electrically neutral.
Problem 22:
Sunlight appears white to our eyes because its electromagnetic waves
(A) have the specific frequency that we perceive as white, rather than red, green, or blue.
(B) are a mixture of frequencies that include frequencies we perceive as red, green, and blue.
(C) have a frequency that is higher than those waves we perceive as blue.
(D) have a frequency that is lower than those waves we perceive as red.
Problem 23:
The electricity powering your clock radio consists of a 2-ampere current that experiences a voltage drop of 5 volts. How much electric power is your clock radio consuming?
(A) 10 watts
(B) 0.4 watts
(C) 2.5 watts
(D) 5 watts
Problem 24:
As you walk through a building with lots of metal surfaces in it, many reflected partial waves reach your cellphone. You find that your cellphone reception varies significantly with location. Why?
(A) Your reception is only as good as the strongest of the partial waves.
(B) If they travel in the same direction, the partial waves work together to give excellent reception. If they travel in opposite directions, they can cancel to give poor reception.
(C) If their electric fields point in the same direction, the partial waves work together to give excellent reception. If they point in opposite directions, they can cancel to give poor reception.
(D) Your reception is only as good as the weakest of the partial waves.
Problem 25:
An AM radio station broadcasts at 1000 kilohertz (1,000,000 cycles-per-second) using a 150 meter tall vertical antenna. You are only 1 mile away from that antenna and yet you have almost no reception -- the wave passing you is incredibly weak. Where are located?
(A) You are in a plane directly above the broadcast antenna.
(B) You are directly North of the broadcast antenna.
(C) You are directly East of the broadcast antenna.
(D) You are Northeast of the broadcast antenna.
Problem 26:
Different LED lightbulbs have color temperatures 2700 K, 3000 K, and 5500K. Which one emits more bluish light?
(A) There is not enough information here to know which lightbulb emits more bluish light.
(B) The 2700 K lightbulb emits more bluish light.
(C) The 3000 K lightbulb emits more bluish light.
(D) The 5500 K lightbulb emits more bluish light.
Problem 27:
In the US, an AM radio station must keep its electromagnetic waves to within 10 kHz (10,000 cycles-per-second) of its official carrier frequency. An FM radio station's waves must be within 100 kHz of its carrier frequency. Why is FM better for music than AM?
(A) An FM wave has more pressure than an AM wave, so it sounds louder.
(B) FM stands for "fantastic music", whereas AM stands for "awful music".
(C) With 10 times as much bandwidth, the FM station can convey 10 times as much sound information per second.
(D) More sound can ride on an FM wave than on an AM wave.
Problem 28:
A battery charger recharges a battery by
(A) pushing current through the battery from its negative terminal to its positive terminal.
(B) pushing current through the battery from its positive terminal to its negative terminal.
(C) connecting a wire between the battery's positive terminal and its negative terminal.
(D) removing positive charges from the battery's positive terminal and negative charges from its negative terminal.
Problem 29:
Two coins slide horizontally off a tabletop side-by-side at the same instant and begin to fall. The US quarter was moving twice as fast as the US dime when the two coins left the tabletop. Where and when do the coins hit the level floor beneath the table? [neglect any effects due to the air.]
(A) The two coins hit the ground at the same time and land about the same distance from the table.
(B) The two coins hit the ground at the same time, but the quarter lands about twice as far from the table as does the dime.
(C) The dime hits the ground before the quarter, but the quarter lands about twice as far from the table as does the dime.
(D) The quarter hits the ground before the dime, but the two coins land about the same distance from the table.
Problem 30:
When a light wave is travelling through empty space, midway between a distant star and your eye, that wave consists of
(A) either an electric field or an electric charge, and either a magnetic field or a magnetic pole.
(B) an electric field and either a magnetic field or a magnetic pole.
(C) a magnetic field and either an electric field or an electric charge.
(D) an electric field and a magnetic field, but no electric charges or magnetic poles.