Physics Midterm 2
You have three batteries: a 12.0-volt battery (from a car) and two 1.5-volt batteries (from a flashlight). Using clips and wires, you can link these batteries together in various chains. The possible voltage rises that you can obtain with these batteries are
1.5 volts, 3.0 volts, 9.0 volts, 10.5 volts, 12.0 volts, 13.5 volts, and 15.0 volts.
The transformer in a power adapter has 120 turns in its primary coil and 10 turns in its secondary coil. With 120 volt AC power connected to the primary coil and a current of 1 ampere flowing through that primary coil, the transformer's secondary coil is supplying
10 volt AC power and a current of 12 amperes is flowing through that secondary coil.
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?
10 watts
The transformer in a power adapter has 120 turns in its primary coil and 12 turns in its secondary coil. With 120 volt AC power connected to the primary coil and a current of 1 ampere flowing through that primary coil, the transformer's secondary coil is supplying
12 volt AC power and a current of 10 ampere is flowing through that secondary coil.
The secondary coil of a large transformer near the street supplies your home with 120 volt alternating current. The transformer's primary coil has 100 times as many turns as its secondary coil. At this moment, the transformer is supplying electric power only to your home and that power is 1200 watts. The transformer's primary coil is being supplied with
12,000 volt alternating current and the current in that coil is 0.1 amperes.
The transformer in your garden illumination system has 120 turns in its primary coil and 12 turns in its secondary coil. With 120-volt AC power connected to the primary coil and a 2-ampere alternating current flows through that primary coil, the transformer's secondary coil is supplying
12-volt AC power and 20 amperes flows through that secondary coil.
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
2 volts.
The secondary coil of a large transformer near the street supplies your home with 120-volt alternating current electric power. The transformer's primary coil has 20 times as many turns as its secondary coil. At this moment, the transformer is supplying electric power only to your home and that power is 2400 watts. The transformer's primary coil is being supplied with
2,400-volt alternating current and the current in that coil is 1 ampere.
You have two identical electric space heaters, both of which are plugged into the same electrical outlet of your home. It's a cold winter day and you turn on one of the heaters. A current of 10 amperes flows through the wires of your home (to provide power to the heater) and wastes 1 watt in those wires. When you turn on the second heater, a current of
20 amperes flows through the wires of your home and wastes 4 watts in those wires
The transformer has 120 turns in its primary coil and 24 turns in its secondary coil. With 120 volt AC electric power connected to the primary coil and a current of 1 ampere flowing through that primary coil, the transformer's secondary coil is supplying
24 volt AC electric power and a current of 5 amperes is flowing through that secondary coil.
During your international travels as a celebrity chef, you mistakenly plug your electric frying pan into a 240-volt electric outlet. It was designed to operate properly only on 120-volt electric power. When you turn it on, twice the normal current flows through the appliance and that current experiences twice the normal voltage drop as it passes through the appliance. The power being delivered to the electric frying pan is
4 times normal.
The transformer in your home theater system has 120 turns in its primary coil and 40 turns in its secondary coil. With 120-volt AC power connected to the primary coil and a current of 5 amperes flowing through that primary coil, the transformer's secondary coil is supplying
40-volt AC power and a current of 15 amperes is flowing through that secondary coil.
A microwave oven's cooking chamber has 6 sides. What are those sides made of?
5 are metal and 1 is metal mesh.
The microwave oven's cooking chamber has 6 sides. What are those sides made of?
5 sides are metal and 1 side is metal mesh.
The secondary coil of a large transformer near the street supplies your home with 120 volt alternating current. The transformer's primary coil has 50 times as many turns as its secondary coil. At this moment, the transformer is supplying electric power only to your home and that power is 6000 watts. The transformer's primary coil is being supplied with
6,000 volt alternating current and the current in that coil is 1 ampere.
A particular AM radio station transmits a radio wave with a wavelength of 300 meters. It uses a simple monopole antenna, a vertical metal mast that rises upward above a horizontal conducting surface: the ground beneath the mast. For that mast to be resonant at the station's transmission frequency and as efficient as possible at emitting its radio wave, the mast is approximately
75 meters tall.
A particular AM radio station transmits a radio wave with a wavelength of 300 meters. It uses what is known as a simple monopole antenna: a vertical metal mast that rises upward above a horizontal conducting surface. The conducting surface below the mast is the ground itself. For that mast to be resonant at the station's transmission frequency and as efficient as possible at emitting its radio wave, the mast is approximately
75 meters tall.
You are recharging your cellphone and your power adapter contains a tiny transformer. Charges moving through the secondary coil of that transformer are picking up energy that is eventually used to recharge your phone. How is energy given to the charges in the coil?
A changing magnetic field in the transformer produces an electric field. That electric field pushes the charges forward as they move forward through the secondary coil and thus does work on them
During your semester abroad, you purchase a hair appliance designed to operate on 240-volt electric power. Its heating element is an electrical resistor, with a behavior that is accurately described by Ohm's law. When supplied with 240-volt electric power, a current of 5 amperes flows through the heating element and it consumes 1200 watts of electric power. When you supply it with 120-volt electric power upon your return to the United States, what happens?
A current of 2.5 amperes flows through it and it consumes 300 watts of electric power.
A sharp metal pin has a voltage of +1000 volts and it points toward a wide metal surface with a voltage of -1000 volts (see figure). What is the voltage midway between the surface and the pin?
A negative voltage.
Why does water heat up especially easily in a microwave oven?
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.
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?
An electric current will flow through it, heating it up, and electric charge will also spray out of its sharp ends
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?
An electric current will flow through it, heating it up, and electric charge will also spray out of its sharp ends.
A capacitor consists of two conducting surfaces separated by an insulator. When those two conducting surfaces have equal but opposite electric charges, what is present in the separating insulator?
An electric field pointing toward the negatively charged surface
Which one of the following objects emits an electromagnetic wave that travels across the universe?
An electrically charged ball on top of a flagpole that is waving back and forth in the wind
Which one of the following objects emits an electromagnetic wave that travels across the universe?
An electrically charged ball on top of a flagpole that is waving back and forth in the wind.
Which one of the following objects emits an electromagnetic wave that travels across the universe?
An electrically charged ball that is going around in a circle at a constant speed on a plastic merry-go-round
Which one of the following objects emits an electromagnetic wave that travels across the universe?
An electrically charged ball that is going around in a circle at a constant speed on a plastic merry-go-round.
You are fidgeting while waiting for UVa to beat MSU. You throw a basketball upward and it rises for 2 seconds to a peak height of about 20 meters (66 feet) above your hand. After only 1 second of rising, how high above your hand was the ball?
Approximately 15 meters
You are at the gym, exercising on a step machine. You have one foot on each of the machine's pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push harder while moving downward than while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?
As that pedal moves downward
Someone has hidden a radio transmitter and antenna somewhere on the UVa grounds. You measure the transmitter's radio wave at several locations around grounds and find that, while the wave's electric field is always vertical, the wave's horizontal magnetic field points in different horizontal directions at different locations around grounds. Using a paper map, how might you find the transmitter and antenna?
At each measurement location, draw a line that is perpendicular to the magnetic field at that location. You will find the transmitter at the intersection of those lines.
There are two electrically charged coins on your desk. Although the coins have equal amounts of charge, the left coin's charge is positive and the right coin's charge is negative. Which statement correctly describes the electric field near those coins?
Between the coins, the electric field points to the right. To left of the left coin, the electric field points left. To the right of the right coin, the electric field points left
A hot ember in a campfire is glowing red. It's almost pure carbon (charcoal), so what determines the color of its reddish glow?
Charcoal is black, but like any black surface, it emits thermal radiation characteristic of its temperature -- red in this case.
When you place a ceramic mug of coffee in a microwave oven and turn that oven on, the coffee's temperature increases. What is it about coffee that causes it to heat in the microwave oven?
Coffee contains water molecules and each water molecule has a large electric dipole (a positive end and a negative end).
When you're using your smartphone on the beach, in which direction is current flowing through its battery and through its computer?
Current flows toward higher voltage through the battery and toward lower voltage through the computer.
Electric power is transmitted long distances as a relatively small current at a relatively high voltage. Why is that arrangement used?
Decreasing the current in each wire reduces the power those wires consume. Increasing the voltage difference between the wires allows those small currents to transmit large amounts of power.
When sunlight passes through a properly cut diamond, the diamond sparkles with light and color. How does the diamond manage to separate the colors of light in white sunlight?
Different frequencies of light travel at different speeds in the diamond and they refract differently as they enter and exit the diamond.
You are standing below a single electric wire that is carrying a constant current northward. At this moment, its voltage is +50,000 volts and the ground's voltage is 0 volts. In which direction is the electric field near you pointing?
Downward
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 alteration affect the coil and secondary circuit?
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.
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?
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.
Suppose both coils of an ordinary transformer are carrying constant electric currents. Why can't the transformer transfer electric power from one coil to the other?
Each coil's constant current produces a constant magnetic field, which produces no electric field and therefore cannot do work or negative work on charges in the coils' currents.
Suppose both coils of an ordinary transformer are carrying constant electric currents. Why can't the transformer transfer electric power from one coil to the other?
Each coil's constant current produces a constant magnetic field, which produces no electric field and therefore cannot do work or negative work on charges in the other coil's current.
A long bar magnet (a permanent magnet) has a north pole at its red-painted end and a south pole at its white-painted end. You break that magnet exactly in half, resulting in a red-painted half and a white-painted half. Which of the following correctly describes the two halves?
Each half has both a north pole and a south pole, and the net pole of each half is exactly zero.
A long bar magnet (a permanent magnet) has a north pole at its red-painted end and a south pole at its white-painted end. You break that magnet exactly in half, so that you have a red-painted half and a white-painted half. Which of the following correctly describes the two halves?
Each half has both a north pole and a south pole, and the net pole of each half is exactly zero.
When they are far away from any external magnetic influences, two blocks of pure iron exert no magnetic forces on one another. Why not?
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.
An electromagnet is a coil of wire that becomes magnetic due to electricity. What aspect of electricity makes it magnetic?
Electric current is magnetic
An electromagnet is a coil of wire that becomes magnetic due to electricity. What aspect of electricity makes it magnetic?
Electric current is magnetic.
A junkyard uses a huge electromagnet to pick up old steel automobiles. That electromagnet only binds magnetically to steel when it is supplied with electricity. What is it about electricity that causes that attraction between the electromagnet and the steel?
Electric currents produce magnetic fields
A junkyard uses a huge electromagnet to pick up old steel automobiles. That electromagnet only binds magnetically to steel when it is supplied with electricity. What is it about electricity that causes that attraction between the electromagnet and the steel?
Electric currents produce magnetic fields.
You are sitting on your porch, listening to a radio station. Suppose that station's radio wave is traveling toward you horizontally and you are looking directly at that oncoming radio wave. If the wave's electric field is directed vertically (up and down), in which direction is the wave's magnetic field?
Horizontally, left and right.
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?
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.
A transformer's primary coil receives power from the electric company and it provides power from its secondary coil. To increase the voltage of power provided from the secondary coil, what must you do?
Increase the number of turns in the secondary coil
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?
It is becoming thermal energy.
You toss a grapefruit straight up. Disregarding any effects of due to the air, what force or forces are acting on the grapefruit while it is above your hands?
Its weight.
If you put a thick metal spoon in a microwave oven, what will happen?
Microwaves in the oven will cause electric currents to flow in the metal spoon
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?
Orient the antenna vertically.
You are listening to a weak radio station as you walk through a city. You find that the reception depends on your location and that occasionally you get remarkably good reception. How are the structures around you acting to give you better reception than you would get if those structures weren't there?
Portions of the radio wave that reflect from those structures are producing constructive interference at the antenna of your radio receiver.
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?
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.
A 120-volt AC electrical outlet in the United States has two slots, known as neutral and power. Which slot sometimes has a voltage of +130 volts and which slot sometimes has a voltage of -130 volts?
Power can be +130 volts and power can be -130 volts.
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?
Power can be +130 volts and power can be -130 volts.
You are wading in a shallow pond and see a shiny ring on the lake bottom in front of you. You reach for the ring, but discover that it is not exactly where it appears to be. In fact, the portion of your arm that's under water is bent relative to the portion of your arm that's above water. What effect is causing these optical problems?
Refraction
A recycling plant uses a magnet to separate steel scraps from aluminum scraps. Why does the magnet attract steel but not aluminum?
Steel has magnetic domains that align with the magnetic field, but aluminum has no magnetic domains.
Different LED lightbulbs have color temperatures 2700 K, 3000 K, and 5500K. Which one emits more bluish light?
The 5500 K lightbulb emits more bluish light.
You are dragging an old carpet across the floor in a straight line at a steady speed. Which of the following statements about the forces acting on the carpet is correct?
The amount of force that you're exerting on the carpet must be equal to the amount of force that friction is exerting on it.
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?
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.
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?
The car is negatively charged.
You hold a positively charged stick near an electrically neutral glass ball and then near an electrically neutral metal ball. What force does the charged stick experience in these two cases?
The charged stick is attracted to both the glass ball and the metal ball.
Red light travels slightly faster in water than does violet light. In a rainbow, red appears on the outside of the arc and violet appears on the inside of the arc. If red light instead traveled slower in water than did violet light, how would the colors of a rainbow be arranged?
The colors would be reversed: violet on the outside and red on the inside.
Your electric hairdryer was designed to operate on 120 volt alternating current. You are traveling in South Asia and your friend connects it to the local electric power, which is 240 volt alternating current. What happens to your electric hairdryer?
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.
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?
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.
A downhill skier is descending a snow-covered mountain. The skier steps off of a level region of the mountain and onto a steep slope. The skier begins to accelerate rapidly downhill on the slope. What force is causing the skier to accelerate downhill?
The downhill ramp force that is the sum of the skier's weight and the support force exerted on the skier by the snow-covered slope.
A radio station's vertical transmitting antenna is located several miles west of you and there is nothing but open grassland between that antenna and you. As the radio wave from that antenna travels past you on its way eastward, in which directions do its electric and magnetic fields point?
The electric field points up and down. The magnetic field points north and south.
You are boating on a large lake and you are talking to a friend by radio. Each of you holds a small radio transmitter-receiver with its antenna pointing straight up toward the sky. If your friend is directly south of you, along which directions do the electric and magnetic fields of your friend's radio wave point?
The electric field points up-down and the magnetic field points east-west.
A motionless hydrogen atom has one electron and one proton. The electron has -1 fundamental unit of electric charge and the proton has +1 fundamental unit of electric charge. Which of the following statements correctly describes the energy of the electron in this atom?
The electron's energy is determined by its quantum standing wave. Since only certain quantum standing waves are possible in the atom, the electron can have only certain specific energies.
Which force is your weight? [The force that is your weight and not a force that is equal to your weight.]
The force that causes you to accelerate downward when you are high above the surface of a trampoline.
You are pushing a desk across a tile floor. The desk is moving in a straight line at a steady speed. Compare the force you exert on the desk to the frictional force the floor exerts on the desk. [Assume that those two forces are the only horizontal forces acting on the desk.]
The force you exert desk is equal in amount but opposite in direction to the frictional force the floor exerts on the desk.
When light that was traveling through empty space enters a gas, the light slows down slightly. What characteristic of the gas is responsible for slowing the light?
The gas contains electric charges
You use a converging lens to project the image of a flower on the wall of your dorm room. How does that lens form a sharp image of the flower on the wall?
The lens bends the light rays from a single point on the flower so that they converge to a single point on the wall.
You are talking on a cellphone when you enter an elevator. As the metal doors close, you lose reception and the call ends. Why?
The metal surfaces of the elevator reflect the electromagnetic waves
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?
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.
What is the difference between a radio wave and a microwave?
The microwave has a shorter wavelength than the radio wave.
By themselves, two ordinary steel paperclips do not attract one another magnetically. If you pick up one paperclip with a strong permanent magnet, however, the free end of that paperclip will be able to pick up a second paperclip. What has changed in the first paperclip that now allows it to pick up the second paperclip?
The paperclip's many magnetic domains originally pointed in random direction, but the nearby permanent magnet has aligned the domains and thereby magnetized the paperclip
You are sitting in the lobby of a hotel, watching 4 people who are wearing backpacks. Which person is doing work on their backpack at this moment?
The person who is riding upward at constant velocity in a glass elevator.
Why is it energy-inefficient to transmit power long distances as large currents in two wires having a small difference in voltage?
The power a wire wastes as thermal power is proportional to the square of the current it carries
Light's speed in Pyrex glass is the same as its speed in salad oil. Suppose you fill a Pyrex measuring cup with salad oil and place it in the sun. When a ray of sunlight passes from the Pyrex glass and enters the salad oil, will that ray partially reflect from the interface between glass and oil, and will the ray's path bend (refract) as it moves from glass to oil?
The ray will not partially reflect and its path will not bend.
A local radio station employs the AM (amplitude modulation) technique to deliver a talk radio program to the radio receiver in your kitchen. How does this work?
The station varies the intensity of its radio wave to convey sound information to your receiver, which moves its speaker with each intensity change and thereby reproduces the sound
An FM radio station has an official frequency of 100 MHz (100 million cycles per second) and plays music you like. What aspect of the station's radio wave is conveying the information your receiver needs to reproduce the music?
The time interval between the radio wave's electric field peaks varies slightly to represent the music's air pressure variations.
An FM radio station has an official frequency of 102 MHz (102 million cycles per second) and plays music you like. What aspect of the station's radio wave is conveying the information your receiver needs to reproduce the music?
The time interval between the radio wave's electric field peaks varies slightly to represent the music's air pressure variations.
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.]
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.
Your flashlight is switched on and a wire inside it is carrying current from the positive battery terminal to the lamp input. Compare the voltage at the positive battery terminal to the voltage at the lamp input.
The voltage at the positive battery terminal is slightly greater than the voltage at the lamp input.
When you plug a toaster into a household electric outlet, the two metal blades of its plug connect with the two metal slots in the outlet. You start cooking toast and the outlet supplies 120-volt alternating current to the toaster. What is happening at those two connections?
The voltage difference between the two connections reverses many times per second and the current flowing through each connection also reverses many times per second.
When you plug your hairdryer into a household electric outlet, the two metal blades of its plug connect with the two slots in the outlet. You turn the hairdryer on and the outlet supplies 120-volt alternating current to the hairdryer. What is happening at those two connections?
The voltage difference between the two connections reverses many times per second and the current flowing through each connection also reverses many times per second.
A metal wire extends from left to right and current is flowing through it toward the right. Compare the voltage of the left part of the wire to the voltage on the right part of the wire.
The voltage on the right is less than the voltage on the left.
A typical radio wave that is traveling from Charlottesville to Richmond has a vertical electric field and horizontal magnetic field. Suppose we used some filtering system to eliminate the wave's magnetic field, but made no effort to get rid of its electric field. What would happen to the wave?
The wave could not exist without its magnetic field
A typical radio wave that is traveling from Charlottesville to Richmond has a vertical electric field and horizontal magnetic field. Suppose we used some filtering system to eliminate the wave's magnetic field, but made no effort to get rid of its electric field. What would happen to the wave?
The wave could not exist without its magnetic field.
The power outlet in your room supplies 120 volt alternating current. That outlet has two metal slots through which power is provided. One slot is longer than the other. Which of the following statements is true about those two metal slots?
There are moments during which the voltage difference between those two slots is zero.
The power outlet in your room supplies 120-volt AC electric power. That outlet has two metal slots through which power is provided. One slot is longer than the other. Which of the following statements is true about those two metal slots?
There are moments during which the voltage difference between those two slots is zero.
You are about to push two coins off a horizontal table simultaneously, a US quarter (big coin) and US dime (little coin). They start side-by-side near the edge of the table and you push them equally hard for equal times. Off they go! Which of the following choices correctly describes what happens to the coins?
They hit the floor at the same time, but the dime hits the ground farther from the table than does the quarter
The sign in the window of a diner is made from glass tubes that glow various colors as electric current passes through them. Those tubes are actually fluorescent lamps, but instead of emitting white light, they emit colored light. What change did the lamp manufacturer make to produce a green fluorescent lamp instead of a white one?
They replaced phosphors that emit white light with phosphors that emit green light.
You are arm-wrestling a friend. While you are gradually winning the competition, compare the torque that your arm exerts on your friend's arm to the torque your friend's arm exerts on your arm.
Those two torques are equal in amount but opposite in direction
As shown in the figure below, a sharp metal pin has a voltage of +1000 volts and it points toward a wide metal surface with a voltage of -1000 volts. Where in this system can you find a position where the voltage is 0 volts?
To the right of the midway point.
Why does the United States' electric power grid use alternating current rather than direct current?
Transferring power from circuit to circuit using transformers can only work if the current changes with time
A microwave oven heats food primarily through its water content. What aspect of water makes it especially susceptible to heating by microwaves?
Water molecules are polar (have an electric polarization), so microwaves exert fluctuating torques on them
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?
With 10 times as much bandwidth, the FM station can convey 10 times as much sound information per second.
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?
You are in a plane directly above the broadcast antenna.
Your pet rabbit chews through the extension cord supplying power to your computer and produces a short circuit: the two metal wires in the cord become electrically connected to one another. If no safety mechanisms existed, what would happen to the extension cord and your computer?
Your computer would receive approximately zero power and the cord would become hot.
Your pet hamster chews through the extension cord supplying power to your lamp and produces a short circuit: the two metal wires in the cord become electrically connected to one another. If no safety mechanisms existed, what would happen to the extension cord and your lamp?
Your lamp would receive approximately zero power and the cord would become hot.
Your roommate hammers the extension cord supplying power to your lamp and produces a short circuit: the two metal wires in the cord become electrically connected to one another. If no safety mechanisms existed, what would happen to the extension cord and your lamp?
Your lamp would receive approximately zero power and the cord would become hot.
You have a typical motor/generator, consisting of a rotating permanent magnet and a stationary coil of wire. There is a crank attached to the rotating magnet and you are exerting a torque on that crank. The device acts as
a generator when you twist it in the direction of its rotation and it acts as a motor when you twist it opposite the direction of its rotation
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 smaller current wastes less power in the wires
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 smaller current wastes less power in the wires.
The difference between a metal and an insulator lies in their band structures. An insulator is unable to conduct electric current in response to an electric field because
all of its electrons reside in filled bands and thus can't shift into empty levels so as to move across the material.
As it travels through otherwise empty space, a radio wave consists of
an electric field and a magnetic field
When a light wave is midway between a distant star and your eye, that wave consists of
an electric field and a magnetic field, but no electric charges or magnetic poles.
When a light wave is travelling through empty space, midway between a distant star and your eye, that wave consists of
an electric field and a magnetic field, but no electric charges or magnetic poles.
The Fermi level in a particular solid material lies in the middle of a band of levels. In other words, the highest-energy electrons in that material partly fill a band of levels. This material is
an electrical conductor
If the speed of light in water did not depend on the wavelength of that light, a rainbow would
appear white.
Sunlight appears white to our eyes because its electromagnetic waves
are a mixture of frequencies that include frequencies we perceive as red, green, and blue.
You have two bowling balls, one of which weighs twice as much as the other. The balls roll off a horizontal table at the same time and at the same velocity. In this situation, both balls hit the floor
at approximately the same time and at the same distance from the table
You have two bowling balls, one of which weighs twice as much as the other. The balls roll off a horizontal table at the same time, but the heavier ball leaves the table at twice the velocity of the lighter ball. In this situation, both balls hit the floor
at approximately the same time, but the heavier ball hits considerably farther from the table than the lighter ball.
Two billiard balls are rolling forward on a horizontal table, but the green ball is traveling twice as fast as the red ball. The two balls roll off the edge of the table simultaneously. The green ball hits the ground
at the same time as the red ball, but the green ball lands twice as far from the table as the red ball
Two pool balls are rolling forward on a horizontal table, but the green ball is traveling twice as fast as the red ball. The two balls roll off the edge of the table simultaneously. The green ball hits the ground
at the same time as the red ball, but the green ball lands twice as far from the table as the red ball.
A charged red balloon clings to a neutral window and a charged blue balloon clings to a neutral wall. If you bring those two balloons close together, they will
attract if they have opposite charges or repel if they have like charges.
The electric motor in a toy car has a permanent magnet rotor and stationary electromagnets. When the motor is propelling the car, the poles of its electromagnets
attract the rotor's poles as they approach and repel the rotor's poles as they separate
The electric motor in a toy car has a permanent magnet rotor and stationary electromagnets. When the motor is propelling the car, the poles of its electromagnets
attract the rotor's poles as they approach and repel the rotor's poles as they separate.
Two identical magnetic stickers, offering pizza delivery information, are clinging to your refrigerator. If you bring those two stickers near one another they will
attract, repel, or neither, depending on how their magnetic poles are positioned relative to one another.
When the MOSFET device that we discussed in class and in the book has zero charge on its gate surface, its middle portion (the "channel") cannot conduct current. Putting positive charge on the gate surface can allow the MOSFET to conduct current by
attracting electrons into the channel, where they fill some of the conduction levels
When the MOSFET device that we discussed in class and in the book has zero charge on its gate surface, its middle portion (the "channel") cannot conduct current. Putting positive charge on the gate surface can allow the MOSFET to conduct current by
attracting electrons into the channel, where they fill some of the conduction levels.
The electric field near a positive charge points
away from the charge and becomes weaker with increasing distance from the charge.
On a bright day, the pupil of your eye (the circular opening through which you see) becomes very small in diameter. One consequence of having a tiny pupil is that
both distant and nearby objects appear relatively sharp at the same time. In other words, your depth of focus is large.
A set of high-voltage power lines are conveying several million watts of electric power to Charlottesville. Near those power lines, there are
both electric and magnetic fields
A set of high tension power lines are conveying several million watts of electric power to Charlottesville. Near those power lines, there are
both electric and magnetic fields.
The shiny surface of a CD or DVD is essentially a comb of microscopic mirrored strips that are separated by non-reflective gaps (something like this: ||||||||||||||||||||). When sunlight reflects from these rows of tiny mirrors, you see colors because the reflective strips
break each light wave up into pieces that can then interfere with one another
A coconut falls 20 meters from top of a tall palm tree to the ground. This fall takes about 2 seconds [assume the acceleration due to gravity is 10 m/s2]. After it has fallen for only 1 second, the coconut is still falling and is located
considerably more than 10 meters above the ground
An acorn falls 20 meters from the high branch of an oak tree to the ground. This fall takes about 2 seconds. After it has fallen for only 1 second, the acorn is still falling and is located
considerably more than 10 meters above the ground.
As a camera takes a photograph of a bird, many different light rays from the tip of the bird's beak pass through the camera's lens. Assuming the camera is properly focused on the bird's beak, those light rays will
converge to one point on the camera's image sensor.
An LED (light emitting diode) produces a photon (a single quantum wave of light) when an electron
crosses the diode's junction and then makes a radiative transition from a conduction level to an empty valence level.
When your cellphone's battery is being recharged,
current is flowing through the battery from its higher voltage end to its lower voltage end.
Like the back of a credit card, your ID card has a magnetic strip on it. The pattern of magnetization on that strip represents information about you. The magnetic strip is made from a material that is
difficult to magnetize and difficult to demagnetize.
If you carry a portion of positive charge from a region of high voltage to a region of low voltage, that charge will
do work on you during that move.
A strong bar magnet has one north pole and one south pole. Dropping the magnet onto the floor breaks it in half, so that one piece is the former north pole and the other piece is the former south pole. You examine those two pieces and find that
each piece has zero net pole.
A microwave heats food primarily through the food's water. Liquid water heats up when exposed to intense an microwave because the fluctuating
electric field in the microwave twists water molecules back and forth rapidly.
Compared to a pure semiconductor, an n-type semiconductor has some extra electrons and a p-type semiconductor has some missing electrons. Suppose you have two electrically neutral disks, one of n-type semiconductor and one of p-type semiconductor. When you touch the two disks,
electrons migrate from the n-type semiconductor to the p-type semiconductor and form a depletion region (a region that resembles pure semiconductor)
Radio and microwave devices use tank circuits to help them move enough electric charge to produce strong electromagnetic waves. At its most basic level, a tank circuit consists of a capacitor (two surfaces that can store separated electric charge) and an inductor (an electromagnet that can carry current). When a tank circuit is helping a device produce an electromagnetic wave
energy in the tank circuit is shifting back and forth rhythmically between the capacitor's electric field and the inductor's magnetic field
Radio and microwave devices use tank circuits to help them move enough electric charge to produce strong electromagnetic waves. At its most basic level, a tank circuit consists of a capacitor (two surfaces that can store separated electric charge) and an inductor (an electromagnet that can carry current). When a tank circuit is helping a device produce an electromagnetic wave,
energy in the tank circuit is shifting back and forth rhythmically between the capacitor's electric field and the inductor's magnetic field.
A one-battery flashlight is turned on and is producing light. The current flowing through its lightbulb
enters the lightbulb at higher voltage and exits at lower voltage. The current flowing through its battery enters the battery at lower voltage and exits at higher voltage.
You have two bar magnets that are identical except that one has twice the mass of the other. The net magnetic pole of the larger magnet is
equal to the net magnetic pole of the smaller magnet.
You have two bar magnets that are identical except that one is 10 times as heavy as the other. The net magnetic pole of the larger magnet is
equal to the net magnetic pole of the smaller magnet.
When you connect an electromagnet to a source of alternating current, its magnetization reverses direction repeatedly. An electric charge near that AC electromagnet
experiences a force because the AC electromagnet's changing magnetic field produces an electric field
When you connect an electromagnet to a source of alternating current, its magnetization reverses direction repeatedly. An electric charge near that AC electromagnet
experiences a force because the AC electromagnet's changing magnetic field produces an electric field.
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
forces between electric charges become weaker with increasing distance.
What characteristic distinguishes pure red light from pure blue light? A red light wave
has a smaller frequency than a blue light wave.
Compared to an ordinary AA battery, an ordinary D battery
has the same voltage but can supply more total electrical energy
At this moment in time, a water bottle is touching only one thing: the top of a table. The downward force the water bottle is exerting on the table right now
is greater than, less than, or equal to the water bottle's weight, depending on the situation.
A speedboat is pulling a water-skier with a rope and the skier is traveling forward in a straight line path at a constant speed. The net force experienced by the skier
is zero
Two of your friends are pulling on opposite ends of a rope in the game of tug-o-war. At this moment, your friend on the left is winning: both of them are moving to the left at constant velocity. The net force on the rope
is zero.
You are playing basketball and have managed to get your hands on the ball that an opponent is still holding. At this moment, the two of you are pulling the ball in opposite directions and you seem to be winning: the ball is moving toward you at constant velocity. At this moment, the net force on the ball
is zero.
Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand takes less energy from you because
it doesn't move downward as you push downward on it, so you do almost zero work on it.
The difference between a metal and an insulator lies in their band structures. A metal is able to conduct electric current in response to an electric field because
its electrons only partially fill the highest energy band they occupy so those electrons can move in response to electric fields.
The voltage at the start of an ordinary wire is 10 volts and a current of 5 amperes enters that wire. The voltage at the end of the wire is
less than 10 volts and a current of 5 amperes exits the wire.
At the point where a current of 1 ampere enters an ordinary wire, the voltage is 100 volts. The wire is electrically isolate from everything else. At the point where current exits that same wire, the voltage is
less than 100 volts and a current of 1 ampere exits the wire
The voltage at the start of an ordinary wire is 120 volts and a current of 10 amperes enters the wire. The voltage at the end of the wire is
less than 120 volts and a current of 10 amperes exits the wire.
The voltage at the start of an ordinary wire is 20 volts and a current of 1 ampere enters that wire. The voltage at the end of the wire is
less than 20 volts and a current of 1 ampere exits the wire.
The power adapter for your clock radio contains a transformer, several diodes, and a capacitor. The purpose of the diodes is to convert
low-voltage alternating current into low-voltage direct current.
A typical gasoline-powered home generator has two electric outlets: one that provides 120 volt alternating current and one that provides 240 volt alternating current. Each outlet is connected to a coil of wire in the generator, a coil that resembles the secondary coil of a transformer. Compared to the coil supplying the 120 volt outlet, the coil supplying the 240 volt outlet
makes twice as many turns around the iron core of the generator.
A battery charger recharges a battery by
making current flow through the battery from its positive terminal to its negative terminal
A red LED and a red laser pointer both emit red light. A key difference between those two devices is that the LED emits
many small light waves and the laser pointer emits one large light wave
You are standing on a cliff at the edge of a lake and dropping pebbles into the water. Each pebble takes 2 seconds to reach the water. After it has fallen for only 1 second, one of these pebbles is located
much closer to you than to the water.
Unlike pure semiconductor, n-type semiconductor has some filled conduction levels and p-type semiconductor has some empty valence levels. Suppose you have two electrically neutral disks, one of n-type semiconductor and one of p-type semiconductor. When you touch the two disks, electrons migrate across the boundary between the disks and each disk becomes electrically charged. An electric field appears between the two disks. During the migration, electrons move from the
n-type semiconductor to the p-type semiconductor and the resulting electric field points from the n-type semiconductor toward the p-type semiconductor.
Unlike pure semiconductor, n-type semiconductor has some filled conduction levels and p-type semiconductor has some empty valence levels. Suppose you have two electrically neutral disks, one of n-type semiconductor and one of p-type semiconductor. When you touch the two disks, electrons migrate from the
n-type semiconductor to the p-type semiconductor and the resulting electric field points from the n-type semiconductor toward the p-type semiconductor.
A rocking chair has damaged the cord of your desk lamp. One of the two wires in the cord is completely cut in half and cannot carry any current. However, the other wire still connects the lamp to the electric socket. If you switch on the lamp,
no current will flow through either wire and the lamp will remain dark.
You accidently pinch the cord of your desk lamp under your rocking chair and cut through one of the two wires in the lamp's cord. Only one wire now connects the lamp to the electric socket. If you switch on the lamp,
no current will flow through either wire and the lamp will remain dark.
A racquetball court has rigid, immovable walls. You hit the rubber ball forward, toward the front wall of the court, and the ball bounces back at you. Before hitting the wall, the ball carried both energy and forward momentum. During its bounce, the ball transferred to the wall
no energy and more forward momentum than it had before the bounce.
You are watching a baseball game and the pitcher has just thrown the ball toward the batter at home plate. Neglect any effects due to the air. Once the ball has left the pitcher's hand and is heading forward toward home plate, it experiences
no horizontal force in the forward direction.
The electric cord providing power to your desk lamp has two wires inside it. Your rocking chair crushes the cord while study and cuts through one of the cord's wires. That wire now has an insulating gap in it. Your desk lamp now emits
no light.
You accidentally leave a thick metal spoon in a glass bowl of soup that you put in the middle of a microwave oven. When you turn on the microwave oven,
nothing unusual happens because the spoon is a good electrical conductor and has smooth ends.
The positive terminal and negative terminal of a 9 volt battery are about 1/4 inch apart. In keeping with its name, the battery's positive terminal has a voltage that is 9 volts greater than the voltage of its negative terminal. Midway between those two terminals, the electric field
points away from the positive terminal and toward the negative terminal
You shoot a free throw at the basketball court and the ball travels in an arc toward the hoop. Neglecting any effects due to the air, the net force on the ball when it is midway between you and the hoop
points straight down
When an alternating current flows through the primary coil of a transformer, the magnetization of the transformer's iron core reverses rapidly. That changing magnetization can provide power to current in the transformer's secondary coil by
producing an electric field that can do work on charges flowing through the secondary coil.
Electric power is transmitted long distances as a relatively small current at a relatively high voltage because the power wasted in the transmission wires is
proportional to the square of the current, so reducing that current dramatically reduces the wasted power.
A battery charger recharges a battery by
pushing current through the battery from its positive terminal to its negative terminal
A battery charger recharges a battery by
pushing current through the battery from its positive terminal to its negative terminal.
You are standing on a skateboard in the middle of a bus and that bus is moving forward at constant velocity. The skateboard prevents the floor from exerting any horizontal forces on you. When you let go of the post you were holding, so that you experience no horizontal forces, you
remain in the middle of the bus
Liquid water heats faster in the microwave oven than solid ice because only the molecules in liquid water can
rotate in response to the torques exerted on them by the microwave's electric field
The sky is blue rather than red because the atmosphere scatters blue light more effectively than red light. Blue light scatters more than red light because blue light has
shorter wavelengths than red light.
When a beam of white sunlight passes through the crystal chandelier hanging over the ballroom of your new Hollywood mansion, it produce spots of light all over the room. Some of those spots look like tiny rainbows. The key physical effect that allows the chandelier's perfectly transparent crystals to form those colored rainbow spots is
that different wavelengths of light travel at slightly different speeds in the crystals
You make a sharp left turn in your car and your cellphone slips off the dashboard and out the right passenger window. The cellphone exited the car because
the car did not exert enough leftward force on the cellphone to make the cellphone accelerate with the car
Two identical children run along a horizontal platform side by side and jump into a swimming pool at the same moment. They both jump equally hard, but one child jumps upward while the other child jumps forward. You watch them fall and see that
the child who jumped forward reaches the water before the child who jumped upward
The voltage difference between the two ends of a metal wire is proportional to
the current passing through that wire
Your hairdryer was designed to operate on 120 volt alternating current, but you are traveling in Europe and your friend connects it to 230 volt alternating current. The hairdryer quickly burns out because
the electric field in the hairdryer's heating element is too strong and that field pushes too much current through the heating element
A microwave oven reheats your coffee by exposing that coffee to an intense microwave, an electromagnetic wave with a wavelength of about 12 cm. The coffee becomes hotter because
the microwave's alternating electric field exerts torques on the polar water molecules in the coffee and those molecules rotate and rub against one another.
A permanent magnet sticks for years to your steel refrigerator because
the permanent magnet's magnetic field causes the steel's magnetic domains to rearrange so that they attract the permanent magnet.
Compared to an ordinary AA battery, an ordinary D battery has
the same voltage but can supply more total electrical energy
You are watching children play a game of tug-o-war with a 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 left suddenly pulls hard toward the left. The team on the right has anticipated this threat and is able to keep their end of the rope from moving. The left end of the rope stretches toward the left and the rope breaks. Breaking the rope required energy and that energy was provided by
the team on the left
You are heating water in a hotpot that is plugged into an electric outlet. Like most electric conductors, the heating element in the hotpot obeys Ohm's law. At this moment, the current flowing through that heating element is proportional to
the voltage difference between the two ends of the heating element.
The current passing through a metal wire is proportional to
the voltage difference between the two ends of the wire
The current passing through a metal wire is proportional to
the voltage difference between the two ends of the wire.
Your hairdryer was designed to operate on 120 volt alternating current, but you are traveling in Europe and your friend connects it to 230 volt alternating current. The hairdryer quickly burns out because
the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element.
Your hairdryer was designed to operate on 120-volt alternating current, but you are traveling in Europe and your friend connects it to 230-volt alternating current. The hairdryer quickly burns out because
the voltage gradient in the hairdryer's heating element is too large and too much current flows through the heating element.
The sky is blue rather than pink at midday because
tiny particles in the air are more efficient at Rayleigh scattering violet light waves than they are at scattering red light waves
If you put a magnetic compass in a uniform magnetic field, the compass needle will experience a
torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field
If you put a magnetic compass in a uniform magnetic field, the compass needle will experience a
torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field.
At each point in otherwise empty space, the electric field surrounding a negative charge is directed
toward the negative charge and weakens with increasing distance from the negative charge.
The electric power grid in the United States uses alternating current because
transformers can move power from one circuit to another only if those circuits are carrying alternating current.
The electric power grid in the United States uses alternating current because
transformers can move power from one circuit to another only if those circuits are carrying alternating currents.
A motor/generator is a single device that can act as a motor or a generator. Suppose that you are twisting the rotor of a motor/generator with your hand while that rotor is rotating. The device acts as a generator when you
twist the rotor in the direction of its rotation.
The electrons in a large atom aren't all in the lowest energy orbital (the orbital closest to the atom's nucleus). That's because
two identical electrons can never be in the same quantum wave
For practical reasons, most AM radio stations have a single vertical transmitting antenna. If your AM radio receiver used a single rod-shaped metal receiving antenna, you would receive the transmission best by orienting that receiving antenna so that it is
vertical
You are boating on a vast lake and listening to an AM radio station. You can see the station's vertical transmitting antenna on the shore, directly to your north. As that station's radio wave passes by you, its electric field points
vertically up and down and its magnetic field points horizontally east and west
You are listening to an AM radio station as you drive across the plains of Kansas. It is so flat that you can see the station's vertical transmitting antenna, even though that antenna is 10 miles north of you. As that station's radio wave passes by you, its electric field points
vertically up and down and its magnetic field points horizontally east and west.
You are boating on a vast lake and listening to an AM radio station. You can see the station's vertical transmitting antenna on the shore, directly to your west. As that station's radio wave passes by you, its electric field points
vertically up and down and its magnetic field points horizontally north and south.
Your hairdryer is consuming 1800 watts of electric power. That power traveled to Charlottesville 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 toaster?
zero.
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?
zero.
Your toaster is consuming 1000 watts of electric power. That power traveled to Charlottesville in high-voltage transmission wires carrying a total of 100 million watts (100,000,000 watts). What fraction of the electric charges passing through those transmission wires also pass through your toaster?
zero.
