PHYS010 How Things Work Homework 1-9
If the low-pressure neon vapor in a neon sign were replaced by a low-pressure mercury vapor, the sign would emit almost no visible light. Why not?
Excited mercury atoms emit primarily invisible ultraviolet light. Why: Electromagnetic waves in the ultraviolet spectrum have too short of wavelength to be detected by our eyes.
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
current induced in the aluminum by the approaching north pole produces a repelling north pole on the aluminum's surface. Why: Lenz's Law: the currents oppose the field change and repel the approaching north pole.
Your cat has chewed the cord to your desk lamp and has created a short circuit an electric connection from one wire to the other inside the cord. When you plug the lamp into the electric outlet,
current will bypass the bulb and the bulb will not light up. Why: The short circuit will provide a more effective path for the current heading toward the bulb through one wire and leaving through the other. Instead of flowing through the bulb's filament and lighting it up, most of the current will bypass the bulb by flowing through the short circuit.
Increasing the diameter of a camera lens, while keeping its focal length unchanged, does more than just brighten the image on the film so that you can use a shorter exposure. It also
decreases the depth of focus so that objects at different distances will no longer all be in focus at once. Why: The image appears brighter because there is more room for light to go into the lens which creates a larger range of angles. It focuses more critically then if the light is coming in all of the same directions.
When you drop a strong magnet through the center of a copper pipe, the magnet
descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet. Why: Lenz's Law: When dropping a magnet in a copper pipe, the currents of the pipe exert repulsive magnetic forces on the falling magnet.
A xerographic copier uses a special photoconductor surface that allows light from an original document to control the placement of black powder on white paper. The photoconductor only conducts electricity when it's exposed to light because, in the dark,
electrons in the photoconductor completely fill its valence levels and can't shift from one level to another in order to transport charge through the material.
When laser light reflects from a soap bubble, its photons can interfere with one another. In contrast, when sunlight reflects from a soap bubble, each photon can interfere only with itself. The reason for this difference is that
the photons in a laser beam are all part of a single wave, while those in sunlight are independent waves. Why: Laser light consists of a large number of absolutely identical photons which travel as one giant wave, leave the potential for large interference effects as each laser photon can interfere with every other laser photon. Why: Laser light consists of a large number of absolutely identical photons which travel as one giant wave, leave the potential for large interference effects as each laser photon can interfere with every other laser photon.
One type of home coffee grinder has a small blade that rotates very rapidly and cuts the coffee beans into powder. Nothing prevents the coffee beans from moving. The reason the beans don't get out of the way when the blade begins to push on them is
they have inertia. Why: The body at rest tends to stay at rest, a body in motion tends to stay in motion. Since beans were not moving, they remain at rest when the blade was moving
The steel ball in a pinball game rolls around a flat, tilted surface. If you flick the ball straight uphill, it gradually slows to a stop and then begins to roll downhill. Which direction is the ball accelerating (a) as it rolls uphill? (b) as it rolls downhill?
(a) downhill; (b) downhill Why: In order for the ball to stop, there is a downhill acceleration that decreases the velocity to a stop at a certain point. When it rolls down, the acceleration is also going in the same (downhill) because it will roll down with increasing velocity.
Most commercial airliners have static dissipaters on their wingtips. These sharp metal spikes extend from the end of each metal wing and point toward the rear of the plane. Suppose a plane had just flown through a negatively charged cloud and acquired a large negative charge. It is now flying through neutral air. Which of the following should you expect to happen near the static dissipaters?
A corona discharge at the dissipater tips will spray negative charge into the air. Why: Corona discharge is when negatively charged particles are so packed close together that they repel each other into the air.
Why is it important that the filament of a lightbulb have a much larger electrical resistance that the supporting wires that carry current to and from that filament?
A larger electrical resistance has a larger voltage drop and thus consumes more electrical power. Why: The filament's larger resistance ensures that it experiences the main voltage drop and receives most of the current's electrical power.
To read the magnetic strip on an ID or credit card, you must swipe it quickly past a tiny coil of wire. Why must the card be moving for the coil system to read it?
A moving magnet will induce a current in the coil so that the magnetic strip can be read. Why: Only a moving magnet or changing magnetic field will produce the electric field necessary to push currents through coil inside the playback head.
When you kick a soccer ball, which pushes on the other harder: your foot or the soccer ball?
Both push equally Why: Newton's Third Law: For every action, there is an equal but opposite reaction
You're cutting wood with a handsaw. You have to push the saw away from you as it moves away from you and pull the saw toward you as it moves toward you. When are you doing positive work on the saw?
Both while pushing and pulling Why: Since the saw is going the same direction as you push and pull the saw, you are doing positive work. Both displacement and force vectors are in the same direction.
What is the main reason a DVD can hold more information than a CD, aside from the number of layers of data storage medium?
DVD's use shorter-wavelength light than CD's. Why: CD players use infrared light while DVD's use red light. Electromagnetic waves cannot read structures much smaller than their own wavelength and infrared light has a larger wavelength than red light.
After you extract two pieces of adhesive tape from a tape dispenser, those pieces will repel one another. Explain their repulsion.
Each piece of tape will acquire like charges while being extracted from the tape dispenser. Why: The pieces of tape will have the same charge as one another and will therefore repel.
When your receiver is too far from an AM radio station, you can only hear the loud parts of the transmission. When it's too far from an FM station, you lose the whole sound all at once. Explain the reasons for this difference.
In an AM transmission, the radio wave is strongest during the loudest portions of the broadcast but in an FM transmission, the radio wave intensity is constant. Why: The amplitude of the wave is bigger for the louder portion of the broadcast but it is constant for a radio station. Therefore, AM reaches farther than FM and through different hills/mountains
The two prongs of a power cord are meant to carry current to and from a lamp. If you were to plug only one of the prongs into an outlet, the lamp wouldn't light at all. Why wouldn't it at least glow at half its normal brightness?
No current flows because by using only one prong, an open circuit is created and thus the lamp cannot glow. Why: The lamp needs one connection to receive the charge and one to return the charge to the power company
As you jump across a small stream, does a horizontal force keep you moving forward? If so, what is the force?
No, there is no horizontal force. You continue to move forward because of inertia. Why: The stream acts like a frictionless surface so it is inertia that is moving you. (No forward force)
Thermal energy can shift some of the electrons in a hot semiconductor from valence levels to conduction levels. What effect do these shifts have on the semiconductor's ability to conduct electricity?
Semiconductor's ability to conduct electricity increases. Why: Laser light consists of identical photons which travel as ONE giant wave
Fancy makeup mirrors allow users to choose either fluorescent or incandescent illumination to match the lighting in which they'll be seen. Why does the type of illumination affect their appearances?
Since incandescent light has less blue than fluorescent light, incandescent light reflected from a person's skin would also have less blue in it. Skin can't reflect light that isn't there.
Suppose you are driving north and suddenly hit your brakes to avoid a dog in the road. As you come to a stop your acceleration is directed
South Why: The breaks will slow you down so the motion is opposite from the direction you are going.
An ion generator clears smoke from room air by electrically charging the smoke particles. Why will those charged smoke particles stick to the walls and furniture?
The charged smoke particles polarize the neutral surfaces. Why: Although the surfaces are neutral, the charged smoke particles will polarize the wall by attracting the opposite charge that the particles have in the walls which will make the particles stick to the surfaces.
Which way does the electric field point around the positive terminal of an alkaline battery?
The electric field points away from the positive terminal's surface. Why: Electro field lines are always pointed away from positive charges
Hammering or heating a permanent magnet can demagnetize it. What happens to the magnetic domains inside it during these processes?
The magnetic domains will lose their uniform orientations and become more randomly oriented. Why: Since a permanent magnet has a uniform magnetic orientation, the hammering or heating will disturb this uniformity and new domains are created.
Do more magnetic flux lines begin or end on a button magnet, or are those numbers equal?
The magnetic flux lines beginning and ending on a button magnet are equal. Why: Each magnetic flux line begins from the north pole of a magnetic material and ends at the south pole. Therefore, the numbers are equal.
When astronauts walked on the surface of the moon, they could see the stars even though the sun was overhead. Why can't we see the stars while the sun is overhead.
The scattered light from the sun is so bright that it overwhelms the dim light of the stars Why: Since there is blue light scattered across the sky from the sun's light, it overpowers the light that is being admitted by the stars since they are so far away.
Sports photographers often use large aperture, long focal length lenses. What limitations do these lenses impose on the photographs?
Their depths of focus are relatively small. Why: When the depths of focus are small, the lens is small and there is a large amount of background blur
Why can't an audio amplifier operate without batteries or a power supply?
To produce a more powerful copy of an input signal, the amplifier needs a source of energy, current, and voltage.
Compare how a potato cooks in a microwave oven with how it cooks in an ordinary oven.
In a microwave oven, the potato's water absorbs microwaves and that microwave energy becomes thermal energy and the potato's temperature rises uniformly. In an ordinary oven, heat flows gradually into the potato through its surface and its temperature rises nonuniformly with the middle of the potato last to become cooked. Why: In a microwave, the waves get into the potato, even the middle because the water molecules are going back and forth. In the oven, the thermal energy will be absorbed first and will start moving towards the center
Some of the laser light striking a DVD's reflective layer hits the flat region around a pit and reflects back toward the photodiode. How does this reflected wave actually reduce the amount of light detected by the photodiode?
Waves reflected by pits and flats interfere destrictively.
A MOSFET doesn't change instantly from a perfect insulator to a perfect conductor as you vary the charge on its gate. With intermediate amounts of charge on its gate, the MOSFET acts as a resistor with a moderate electrical resistance. This flexibility allows the MOSFET to control the amount of current flowing in a circuit. Explain why a MOSFET becomes warm as it controls that current.
When current flows through a MOSFET, the MOSFET acts as a resistor which results in a voltage drop across the MOSFET and thus some of the current's energy is converted into thermal energy. Why? The MOSFET is effictively an electrically controllable resister. When there is a small amount of charge on its gate, it has relatively large resistance. As the charge on the gate increases, its resistance goes down. A the resistance incresaes, the charges lose voltage. That energy is being converted to heat.
Why are most microwave TV dinners packaged in plastic rather than aluminum trays?
Why: Since aluminum reflects the waves, then the food will not be warmed up properly. Plastic containers would absorb the waves and therefore will heat up.
Porous, unglazed ceramics can absorb water and moisture. Why are they unsuitable for use in a microwave oven?
Why: Since the water molecules are in ceramic, it warms up until it becomes extremely hot and the ceramic can possibly shatter.
Explain the strategy of putting highly radioactive materials in a storage place for many years as a way to make them less hazardous. That wouldn't work for chemical poisons, so why does it work for radioactive materials?
With time, radioactive nuclei will exponentially decay and the materials become less and less hazardous. Why:The radioactive decay of nuclei is characterized by their half-life. This is time in which half of the nuclei will decay. So, if the half life is one year, half will have decayed away.
An extremely fast-moving charged particle traveling in a magnetic field can radiate X-rays, a phenomenon known as synchrotron radiation. Why is the magnetic field essential to this emission?
Without the magnetic field, the charged particle would travel at a constant velocity and would not radiate electromagnetic waves because of its constant velocity.
If you're taking a photographic portrait of a friend and want objects in the foreground and background to appear blurry, how should you adjust the camera's aperture and shutter speed?
You should use the largest available aperture and a short shutter speed. Why: You should use a larger aperture if you would like to have a brighter looking photo and the object to be the only thing in focus in the picture. Think of a shutter speed as if a person was running and you would like to get a burst of photos, this is a fast shutter speed.
An engineer at the company you are working for has just reported finding an unusual electromagnetic wave. This wave consists only of an electric field, with no magnetic field accompanying it. You are certain that the engineer is mistaken because
an electromagnetic wave must have a magnetic field that changes with time to produce its electric field. Why: An accelerating charge transfers energy back and fourth between electric and magnetic fields
If electrons had four different internal states that could be distinguished from one another, how many electrons could occupy the same orbital without violating the Pauli exclusion principle?
any number from 0 to 4 electrons Why: Up to four electrons could then occupy the same level. The Pauli exclusion principle only prevents indistinguishable Fermi particles from occupying the same level
Fission chain reactions in uranium are sustained by neutrons, each of which is released by one fission and may induce a subsequent fission. Each fission also releases other particles, which include protons. However, proton-containing fission fragments are not as effective at causing subsequent fissions because they
are repelled as they try to approach other uranium nuclei. Why: Neutrons have no charge and therefore no electromagnetic interactions and can collide strongly with the nucleus. In contrast, charged particles will interact electromagnetically and begin to interact weekly at larger distances.
If you are backing up but slowing down, your acceleration is directed
forwards Why: The breaks will slow you down so the motion is opposite from the direction you are going.
An FM radio station instructs your radio receiver how to move its speaker cone by varying the
frequency of the radio wave it produces Why: Air pressure fluctuations are encoded as small changes in the frequency of the radio wave, while the amplitude does not vary.
Your class is rather unhappy with the instructor, and they pitch in and decide to fund a sabbatical for him to go to Mars. Unbeknownst to the students the following semester, the instructor has made long distance course arrangements so the "show can go on". The instructor begins talking about mass, weight and related things. He correctly makes which following statement:
his mass is still essentially unchanged but his weight is less than on earth. Why: Mass is unchanging Scalar quantity but weight is not because force of gravity on planets are different than Earth's so the quantity would change.
In a typical laser, the right color of light bounces back and forth between two mirrors and
is amplified by the excited atoms in the laser medium. Why: When the laser medium is placed between the two mirrors, there is a chance that a photon, emitted spontaneously by one of the excited systems, will bounce off a mirror and return toward the laser medium. As that returning photon passes through the laser medium, it's amplified.
A projectile is thrown directly upward and caught again. At the top of its path
its vertical velocity is zero Why: Since the projectile is at its peak height, there is no velocity because when it is thrown up, the velocity decreases until it becomes zero at the peak.
A photoconductor's electrical properties go from insulating to conducting when it is exposed to light because,
light supplies the energy needed to move electrons from the valence band to the conduction band Why: A photoconductor is insulating in the dark because its valence band is full and its conduction band is empty. If you shine light on the photoconductor, light energy promotes some of the valence level electrons to conduction levels and suddenly the electrons can move about.
For a nuclear weapon to explode normally, the fissionable material in its core must be assembled very quickly. For technical reasons, a nuclear bomb developed by a terrorist group would probably not achieve such rapid assembly. As a result of its slow assembly, such a bomb would
overheat and push itself apart during the assembly, producing only a weak explosion. Why: A nuclear explosion requires that a critical mass is achieved. This means that each fission induce at least an additional fission. This requires the fissionable material to be compressed, which is difficult to maintain as the heat will tend to push itself apart.
A battery works because it
pumps positive charge from its negative terminal to its positive terminal Why: The battery adds energy to the current and raises its voltage. As it pumps current, the battery consumes its store of chemical potential energy so that it eventually runs out and "dies."
A dolphin can leap several meters above the ocean's surface. Gravity does not stop the dolphin from leaving the water because
the dolphin has inertia as it approaches the surface. Why: Intertia of the dolphin helps leave the water but gravity will pull it back to the water when it gets into the air.
Many sophisticated cameras have zoom lenses. When you select the telephoto setting, the objects in front of you appear much closer. In this setting,
the focal length of the lens is longer than normal. Why: Imagine zooming in closer on an object with a camera.
You have covered a grounded metal surface with a layer of photoconductor. Working in the dark, you sprinkle negative charge onto this surface. If you now expose only the left half of the photoconductor to light, you will find that
the left half becomes neutral while the right half remains negatively charged. Why: It transforms from an insulator into a conductor. The charges leave when there is light exposed to the area.
A glass fiber can act as a pipe for light-light that enters the fiber at its end follows the fiber almost indefinitely without escaping through the fiber's surface. The light follows the fiber because
whenever it tries to leave the glass at a shallow angle, it's perfectly reflected. Why: All the light rays trying to leave the glass fiber hits the surface with a shallow angle, therefore, they are reflected perfectly, i.e. total internal reflection, but not refracted so that they can successfully get out.
Which of the following can cause a charged particle to accelerate?
A gravitational field and an electric field Why: Coulomb's Law
You remove the batteries from a working flashlight, turn both of them around as a pair, and reinsert them in the flashlight. They make good contact with the flashlight's terminals at both ends, so that there is no mechanical problem preventing the flashlight from working. If you now switch on the flashlight, it will
work properly, although current will now be flowing backward through its circuit. Why: The current continues to flow but in the opposite direction because the current still derives the energy from the battery to the bulb's filament.
A speedboat is pulling a water-skier with a rope, exerting a large forward force on her. The skier is traveling in a straight line at constant speed. The net force she experiences is
zero Why: No net force because there is zero acceleration due to constant velocity.
A car battery is labeled as providing 12 V. Compare the electrostatic potential energy of positive charge on the battery's negative terminal with that on its positive terminal.
A positive charge on the battery's positive terminal will have more electrostatic potential energy than on its negative terminal. Why: If you bring a positive charge towards the battery's positive terminal, then there will have to be positive work to bring the positive charge towards another positive charge. If you bring a charge towards the battery's negative terminal, the positive is attracted to a negative so there is not as much work involved.
The magnetostatic forces between two button magnets decrease surprisingly quickly as their separation increases. Use Coulomb's law for magnetism and the dipole character of each button magnet to explain this effect.
As separation distance increases, the distance separating the two pairs of poles is very small and therefore the attractive and repulsive forces nearly cancel. Why: Coulomb's Law
One type of microphone has a permanent magnet and a coil of wire that move relative to one another in response to sound waves. Why is the current in the coil related to the motion?
As the coil moves through the magnetic field, electric charges in the coil experience forces and thus flow through the coil as current that is proportional to the coil's velocity through the magnetic field.
On a rainy day you can often see oil films on the surfaces of puddles. Why do these films appear brightly colored?
Light reflects from the top and bottom surfaces of an oil film and the two reflections interfere with one another and the type of interference, constructive or destructive, depends on the film's thickness as well as the light's wavelength.
If most of the highest energy valence levels in a diode laser's p-type anode weren't empty, it would become relatively inefficient and probably wouldn't emit laser light at all. Why not?
Electrons in the highest valence levels can absorb the laser light while undergoing radiative transitions to the conduction levels and that absorption would trap the laser radiation and reduce the diode's ability to emit laser light. Why: Electrons in the highest valence levels can absorb the laser light while undergoing radiative transitions to the conduction levels. That absorption would trap the laser radiation and reduce the diode's ability to emit laser light.
The Empire State Building has several FM antennas on top, added in part to increase its overall height. These antennas aren't very tall. Why do short antennas, located high in the air, do such a good job of transmitting FM radio?
FM radio transmission involves high-frequency, short-wavelength radio waves and since a good antenna is one-quarter wavelength long, FM radio transmission requires relatively short antennas. Why: A good length for an antenna is one quarter of a wavelength and a FM wave length is generally one meter so it is short
The neighborhood joker comes up to you and is really on a roll this morning. After several "good ones" they then say "OK so you're taking physics, right? Here's one - if the acceleration of gravity is 9.8 m/s2 then why isn't everything accelerating? HAHAHA". Being a dedicated student you give this question some real thought and come up with a reasonable response. You might say
Gravity is just part of the story and it is the total net external force that determines acceleration. Why: Newton's Third Law. Since gravity is acting upon you, there is an equal but opposite force that the ground is imposing on you, so there is no acceleration.
Diamond has an index of refraction of 2.42. If you put a diamond in water, you see reflections from its surfaces. However, if you put it in a liquid with an index of refraction of 2.42, the diamond is invisible. Why is it invisible, and how is this effect useful to a jeweler or gemologist?
If the liquid and diamond have the same index of refraction, then light doesn't change speed on entry to or exit from the diamond and no light reflects from the interface between the two. Why: The reflection occurs because the electric part of the wave loses energy as it accelerates charges in the material, while the magnetic part of the wave is unaffected
Why doesn't increasing the current passing through an LED affect the color of its light?
Increasing the current will result in a brighter LED but the color of light produced by the LED is determined primarily by the anode's band gap in the LED's semiconductor. Why: The level differences corresponds to a particular energy difference, which is emitted as a photon with a particular energy, frequency (and color).
When the brakes on his truck fail, the driver steers it up a runaway truck ramp. As the truck rolls up the ramp, it slows to a stop. What happens to the truck's kinetic energy, its energy of motion?
It converts into gravitational potential energy Why: When the truck goes up the hill, the energy is kinetic energy that is converting into gravitational potential energy. This happens when the truck loses its velocity going uphill.
Suppose you have a car traveling down the road at constant speed and not changing direction. It is experiencing gravity, wind resistance and frictional forces from the road. What can be said about the car's acceleration?
It is not accelerating because it has constant velocity. Why: The three forces acting on the car does not affect the acceleration. The fact that the car is going constant velocity and not speeding up says that there is no acceleration.
Use the concepts of refraction, reflection, and dispersion to explain why a diamond emits a spray of colored lights when sunlight passes through its cut facets.
Light refracts and disperses as it enters the diamond, reflects from the back surface, and refracts and disperses as it exits the diamond.
Dish-shaped reflectors are used to steer microwaves in order to establish communications links between nearby buildings. Those reflectors are often made from metal mesh. Why don't they have to be made from solid metal sheets?
Microwaves will not respond to the holes in the metal that are smaller than their wavelengths.
When two medium-sized nuclei are stuck together during an experiment at a nuclear physics lab, the result is usually a large nucleus with too few neutrons to be stable. The nucleus soon falls apart. Why could more neutrons make it stable?
Neutrons would experience the attractive nuclear force while diluting the repulsion between protons. Why: Since protons repel each other, attractive nuclear force is more possible with less electrons
A typical bar magnet has an "N" stamped on its north pole end and an "S" stamped on its south pole end. With the right tools, you might be able to change this bar magnet in one or more of the following ways: (1) remove its magnetic poles altogether, (2) reverse its magnetic poles so that it has a north pole at the end stamped "S" and a south pole at the end stamped "N", (3) convert its north pole into a south pole so that it has south poles at both ends and no north poles. Which of those three options is physically possible?
Only (1) and (2) Why: You cannot have two of the same poles on opposite ends. North and south always appear in equal pairs.
If you're wearing polarizing sunglasses and want to see who else is wearing polarizing sunglasses, you only have to turn your head sideways and look to see which people now have sunglasses that appear completely opaque. Why does this test work?
Polarizing sunglasses normally block horizontally polarized light, so when you look at someone's eyes when they are wearing polarizing sunglasses, you see only vertically polarized light. Tilting your head sideways will allow your sunglasses to block vertically polarized light and thus you will see no light coming from the eyes of other people wearing polarizing sunglasses.
Holding your hand on a static generator (e.g., a Van de Graff generator) can make your hair stand up, but only if you are standing on a good electrical insulator. Why is that insulator important?
Standing on an insulator allows you to retain the charges transferred from the static generator. Why: If you are not standing on a good insulator, then you will act as an object that transfers charges from the Van de Graff machine to the ground, like a wire.
An acorn falls from a branch located 9.8 m above the ground. After 1 s of falling, the acorn's velocity will be 9.8 m/s downward. Why hasn't the acorn hit the ground?
The acorn's average speed for the 1 s is less than 9.8 m/s. Why: Since the acorn started at 0 speed, then the average speed would be less than 9.8 so it is not possible for the acorn to go 9.8 meters in one second.
If you bring a button magnet near an iron pipe, they will soon begin attracting one another. Why don't they repel?
The button magnet's magnetic field polarizes the iron pipe and the pipe develops an opposite pole near the approaching pole of the button magnet and thus attracts the magnet. Why: Since there is an opposite pole is developed in iron, the force will be attractive but not repulsive
If you hold your camera up to a small hole in a fence, you will be able to take a picture of the scene on the other side. However, the exposure time will have to be quite long, and the depth of focus will be surprisingly large. Explain.
The hole effectively reduces the diameter of the camera lens and small diameter lenses have large depths of focus because the few light rays that manage to get through the small opening are already close to one another but they require longer exposures because so little light passes through the narrow lens opening. Why: the hole reduces the diameter of the camera lens
The electric field around an electrically charged hairbrush diminishes with distance form the hairbrush. Use Coulumb's law to explain this decrease in magnitude of the field.
The electrostatic forces between the hairbrush and a positive charge diminish with increasing separation distance which results in a smaller electric field. Why: Coulomb's law says that force diminishes between two objects as the distance becomes larger. Electrostatic force is inversely proportional to distance's squares.
A Ping-Pong ball contains an enormous number of electrically charged particles. Why don't two Ping-Pong balls normally exert electrostatic forces on each other?
The electrostatic forces cancel due to zero net charges on each ball. Why: When having electronically charged particles in objects, this can mean that each object can have the same amount of positive as they do negative charges which can make the objects neutral in charges. So, there would be no electrostatic charge between the two objects.
The gate of a MOSFET is separated from the channel by a fantastically thin insulating layer. This layer is easily punctured by static electricity, yet the manufacturers continue to use thin layers. Why would thickening the insulating layer spoil the MOFSET's ability to respond to charge on its gate?
The gate's charge must be very near the channel in order to attract opposite charges and draw the charge into the channel. Why: A MOSFET turns from insulating to conducting as charge is attracted to its gate. It is attracted by having charge on the opposite of the insulating layer.
Increasing the power to an incandescent bulb makes its filament hotter and its light whiter. Why doesn't increasing the power to a neon sign change its color?
The incandescent lamp is emitting thermal radiation with a roughly blackbody spectrum. The neon sign is not a thermal light source, so increasing the power simply brightens its light but does not change its spectrum. Why: In contract, the light from a neon sign come from atomic transitions between different orbitals
A CD player uses a beam of laser light to read the disc, focusing that light to a spot less that 1 µm (10-6 m) in diameter. Why can't the player use a cheap incandescent lightbulb for this task, rather than a more expensive laser?
The incandescent lightbulb's photons are all different and therefore will not focus to the same tiny spot. Why: The light from an incandescent light bulb comes from the acceleration of individual charges
When you look into the front of a square glass vase filled with water, its sides appear to be mirrored. Why do the sides appear so shiny?
The light that you are seeing is experiencing total internal reflection. Why: That light tried to escape from the glass at too shallow an angle and reflected back into the glass instead.
If you hold a permanent magnet the wrong way in an extremely strong magnetic field, its magnetization will be permanently reversed. What happens to the magnetic domains inside the permanent magnet during this process?
The magnetic domains of the permanent magnet that are aligned with the applied magnetic field grow while the domains opposite to the applied magnetic field shrink. Why: Because there is a stronger magnet, the magnetic field forces magnetic dipoles in the permanent magnet to align with itself.
A discharge in a mixture of gases is more likely to emit a full white spectrum of light than a discharge in a single gas. Why?
The more different atoms and molecules present in a mixture of gases, the more variety there is in the possible radiative transitions and thus, the more likely that a full spectrum of white light will be emitted by the discharge. Why: Different mixtures of gases give different atoms and molecules and when electrons are in their excited state they produce light when trying to move to its ground state, producing light.
If you connect the output of one inverter to the input of a second inverter, how will the output of the second inverter be related to the input of the first inverter?
The output of the second inverter will be the same as the input to the first inverter. Why: Once you input a number for the first time, then the inverter will say the other number. For example, if your original number is 0, then the next number will be 1.
Although a particular AM radio station claims to transmit 50,000 W of music power, that's actually its average power. There are times when it transmits more power than that and times when it transmits less. Explain.
The power being transmitted is changed in order to represent air pressure fluctuations with the radio wave: compressions are represented by strengthening the radio wave and rarefactions are represented by weakening the radio wave. Why: The power transmitted depends on the air pressure because more charges are accelerated when there is a high air pressure.
Why can light from a blue laser form a narrower beam waist than light from an infrared laser?
The size of the beam waist depends largely on the wavelength of the light and since blue light has a shorter wavelength than infrared light, blue light can form a narrow beam waist. Why: Blue light has a shorter wavelength than infrared light so blue light can form a narrow beam waist Why:
Why is it difficult or impossible to make very small atomic bombs?
The smaller the nuclear fuel component, the more likely it is that a neutron will escape before it causes a fission and the less efficient the fuel is at using fission neutrons to sustain a chain reaction. Why: The fission reaction works as a chain reaction. Each neutron decayed from an atom starts a new reaction (neutrons hit other atoms and break them apart) that will create more neutrons.
When a sodium atom is in its lowest energy excited state, it can emit light. Why?
The sodium atom can undergo a radiative transition to its ground state, thereby emitting a photon of light. Why: In its ground state, there would be no place where an atom can go to a lower state so there is no light emitted
In an n-channel MOSFET, the source and drain are connected by a thin strip of p-type semiconductor. Why is this device labeled as having an n-channel rather than a p-channel?
The strip of p-type semiconductor becomes effectively n-type when extra electrons are drawn into it by nearby positive charge on its gate. Why: When you put positive charge on the gate, it attracts negative charge into the p-type material at the center of the MOSFET. These additional electrons fill the vacant valence levels and some electrons enter the conduction levels. Once the conduction levels in the central semiconductor have begun to fill, that material is effectively n-type semiconductor. The MOSFET is then composed entirely of n-type semiconductor and it conducts electricity.
