Physics Test 5 Final HW Questions
Does the voltage output increase when a generator is made to spin faster? Defend your answer.
-As we know, the frequency of the voltage induced in the coils increases as the relative motion between the conductor and the magnet increases. -->Thus, the voltage of the generator increases as it spins faster.
How much energy is given to each coulomb of charge that flows through a 1.5-V battery?
1.5 joules 1 volt = 1 joule/coulomb
A droplet of ink in an industrial ink-jet printer carries a charge of 1.6 X 10^-10 C and is deflected onto paper by a force of of 3.2 X 10^-4 N. Show that the strength of the electric field to produce this force is 2 million N/C.
Electrical field is force divided by charge e= f/q
Since atoms are mostly empty space, why don't we fall through a floor we stand on?
Electrons in one atom repel the electrons in another.
Why does a piece of iron in a current-carrying loop increase the magnetic field strength?
The electromagnetic field of the current-carrying loop is amplified because it aligns the domains in the iron.
What relationship between electron orbits and light emission did Bohr postulate?
The energy difference between two electron orbits would equal the energy of an emitted photon.
In Chapter 22, we learned that the direction of the electric field about a point charge is radial to the charge. What is the direction of the magnetic field surrounding a current-carrying wire?
The magnetic field is directed along concentric circles surrounding the wire.
CH 24 What is the source of magnetic force?
The source of magnetic force is the motion of charged particles, usually electrons.
What do we mean when we say that outer space is not really empty?
The vacuum of space is full of electromagnetic wave energy.
What is the evidence for the claim that iron exists in the relatively cool outer layer of the Sun?
When a spectrum of the Sun is compared with the spectrum of the element iron, the iron lines overlap and perfectly match certain Fraunhofer lines. This is evidence for the presence of iron in the Sun.
A straight current-carrying wire is horizontal and oriented south to north. When a compass needle is placed below or above it, in what direction does the compass needle point?
When placed above the needle points east and when placed below it points west. As you move the compass is goes in the direction of the magnetic field.
What is a quantum of light called?
a photon
Show that it would take about 2.5 s for a pulse of laser light to reach the Moon and to bounce back to Earth.
time = distance/speed 2*(38500 x 10^3 / 3 x 10^8) = 2.5 s
Find the voltage change when (a) an electric field does 12 J of work on a 0.0001-C charge and (b) the same electric field does 24 J of work on a 0.0002-C charge
(a) So 12 J / 0.0001 C equals 120,000 V. (b) same as (a), since 24 J / 0.0002 C = 120,000 V
How does the charge of one electron compare with the charge of another electron? How does it compare with the charge of a proton?
- all electrons have the same charge and mass - protons carry an amount of positive charge equal to the negative charge of electrons.
The potential difference between a storm cloud and the ground is 100 million V. If a charge of 2 C flashes in a bolt from the cloud to Earth, what is the change in the potential energy of the charge?
-PE\Q=V -PE=VxQ -PE= 100millionV X 2C -PE= 200million Joules
Would electromagnetic waves exist if changing magnetic fields could produce electric fields but changing electric fields could not, in turn, produce magnetic fields? Explain.
Electromagnetic waves depend on mutual field regeneration. If the induced electric fields did not in turn induce magnetic fields and transfer energy to them, the energy would be localized rather than "waved" into space. Electromagnetic waves would not exist.
When a gas glows, discrete colors are emitted. When a solid glows, the colors are smudged. Why?
Emitting electrons interact with nearby neighboring atoms in a solid. In a gas, there are few nearby atoms. Think of the clear frequency of a single ringing bell, vs the smudged sound of a crowded box of bells.
A typical wavelength of infrared radiation emitted by your body is 25 mm (2.5 X 10^-2m). Show that the energy per photon of such radiation is about 8.0 X 10^-24 J.
Frequency is speed/wavelength: f = (3 x 10^8 m/s) / (2.5 x 10^-2 m) =1.2 x 10^10 Hz Photon energy is Planck's constant x frequency, E=hf = (6.63 x 10^34 J x s)(1.2 x 10^10 Hz) = 8.0 x 10^-24 J
Green light is emitted when electrons in a substance make a particular energy-level transition. If blue light were instead emitted from the same substance, would it correspond to a greater or lesser change of energy in the atom?
Greater because blue is higher than green.
Since every object has some temperature, every object radiates energy. Why, then, can't we see objects in the dark?
Human eyes are not sensitive to anything in the infrared spectrum.
How is the fact that an electromagnetic wave in space never slows down consistent with the conservation of energy?
If light slowed down, its energy would decrease, thereby violating the law of conservation of energy.
How is the fact that an electromagnetic wave in space never speeds up consistent with the conservation of energy?
If light speeded up, its energy would increase, thereby violating the law of conservation of energy.
CH 11 Where in the atom is most of its mass concentrated?
In the nucleus as protons and neutrons
CH 30 In a neon tube, what occurs immediately after an atom is excited?
It de-excites and emits light. Electrons are boiled off electrodes at tube ends, jostling at high speeds by AC voltage; smashing boosts orbital e- into higher energy levels. The energy is radiated as red light (neon) and e- fall back to stable orbits (ground state).
An X-ray photon can hit an electron without that electron picking up all the photon's energy. Make a hypothesis about the frequency of the photon that "leaves the scene" of the collision. (This phenomenon is called the Compton effect.)
It goes into making another photon of light another electromagnetic wave the photon loses energy, so its frequency decreases one photon is absorbed and another, lower-energy, photon is emitted
What are the three ways in which voltage can be induced in a loop of wire?
Move a magnet near the loop, move the loop near a magnet, change current in a nearby loop
Is it necessary for a charged body actually to touch the ball of the electroscope for the leaves to diverge? Defend your answer.
No, because the the charges interact without touching
According to Niels Bohr, can a single electron in one excited state give off more than one photon when it jumps to a lower energy state?
No. There can be only one!
When an automobile moves over a wide, closed loop of wire embedded in a road surface, is the magnetic field of Earth within the loop altered? Is a pulse of current produced? Can you cite a practical application for this at a traffic intersection?
Part of the Earth's magnetic field is enclosed in the wide loop of wire imbedded in the road. If this enclosed field is somehow changed, then in accord with the law of electromagnetic induction, a pulse of current will be produced in the loop. Such a change is produced when the iron parts of a car pass over it, momentarily increasing the strength of the field. A practical application is triggering automobile traffic lights. (When small ac voltages are used in such loops, small "eddy currents" are induced in metal of any kind that passes over the loop. The magnetic fields so induced are then detected by the circuit.)
Does the photoelectric effect support the wave theory or the particle theory of light?
Particle theory. The photoelectric effect proves conclusively that light has particle properties. Einstein attributed quantum properties to light itself, viewed radiation as a hail of particles. The number of photons in a light beam affects the brightness of the whole beam, whereas the frequency of the light controls the energy of each individual photon.
Do radio waves travel at the speed of sound, at the speed of light, or somewhere in between?
Radio waves are light and therefore travel at the speed of light.
What is the principal difference between a radio wave and light? Between light and an X-ray?
Radio waves have a lower frequency and longer wavelength than visible light waves. Light waves have a lower frequency and longer wavelength than X-rays.
Which has the lower energy quanta: red light or blue light? Radio waves or X-rays?
Red, radio. Relatively low frequency of waves ensures low energy per photon. UV photons deliver more energy than red to a molecule because the frequency of UV radiation is greater than red. X-rays, with even higher frequencies, can deliver even more.
Consider just four of the energy levels in a certain atom, as shown in the diagram below. How many spectral lines will result from all possible transitions among these levels? Which transition corresponds to the highest-frequency light emitted? To the lowest-frequency light emitted?
Six transitions are possible. The highest-frequency transition is from quantum level 4 to level 1. The lowest-frequency transition is from quantum level 4 to level 3.
How can elements with low atomic numbers have so many spectral lines?
Spectral lines are formulated when electrons make transitions between their own energy levels. When an electron is excited it doesn't stay in its lowest state it jumps to a different state to make up for the extra energy.
The nearest star beyond the Sun is Alpha Centauri, 4.2 3 1016 m away. If we were to receive a radio message from this star today, show that it would have been sent 4.4 years ago.
T=D/V T= time (4.4 years) D= Distance (4.2x10^16) V= Speed of radio waves (3 x 108 m/s) = 1.4x10^8 Convert to years: 1.4x10^8 x 1hr/36000s x 1day/24hrs x 1year/365 days = 4.4 years
An electroscope is a simple device consist-ing of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbances in a jar, as shown in the sketch. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Why? (Electroscopes are useful not only as charge detectors but also for measuring the quantity of charge: The more charge transferred to the ball, the more the leaves diverge.)
That is because the 2 leaves will have like charges, and as such will repel each other, and spread apart.
What is the relationship between the energy differences of orbits in an atom and the light emitted by the atom?
The energy difference between orbits is equal to the sum of the energies of photons emitted by an electron going from one orbit to the other.
How is the energy of a photon related to its vibrational frequency?
The energy is proportional to the frequency. E~f "The frequency of the photon is directly proportional to its energy. E=hf (Planck's)
How does the difference in energy between energy levels relate to the energy of the photon that is emitted by a transition between those levels?
The energy of the photon is equal to the difference in energy between the energy levels. E~f Electrons dropping from hi to low energy levels in an excited atom emit with each jump a throbbing pulse of electromagnetic radiation (photon) with frequency related to the energy transition of the jump. E=hf
When astronomers observe a supernova explosion in a distant galaxy, they see a sudden, simultaneous rise in visible light and other forms of electromagnetic radiation. Is this evidence to support the idea that the speed of light is independent of frequency? Explain.
The fact that the different parts of the electromagnetic spectrum emitted in the explosion are received simultaneously is evidence of the frequency independence of the speed of light. If wave speed depended on frequency, different frequencies would be received at different times.
If you place a free electron and a free proton in the same electric field, how will the forces acting on them compare?
The forces on the electron and proton will be equal in magnitude, but opposite in direction. Because of the greater mass of the proton, its acceleration will be less than that of the electron, and be in the direction of the electric field. How much less? Since the mass of the proton is nearly 2000 times that of the electron, its acceleration will be about 1/2000 that of the electron. The greater acceleration of the electron will be in the direction opposite to the electric field. The electron and proton accelerate in opposite directions.
How does the frequency of induced voltage relate to how frequently a magnet is plunged in and out of a coil of wire?
The frequencies are the same.
Why are spectral lines often referred to as "atomic fingerprints"?
The spectrum of the element can be used as a "fingerprint" to identify the element
How is the wavelength of light related to its frequency?
The wavelength is inversely proportional to the frequency.
What relative direction between a magnetic field and a current-carrying wire results in the greatest force?
The wire should be perpendicular to the field.
We hear people talk of "ultraviolet light" and "infrared light." Why are these terms misleading? Why are we less likely to hear people talk of "radio light" and "X-ray light"?
These terms are misleading because light is what we can see. We are less likely to hear people talking about 'radio light' or 'x-ray light' because their wavelengths are so far away from visible light
At some automobile toll booths, a thin metal wire protrudes from the road, making contact with cars before they reach the toll collector. What is the purpose of this wire?
To drain off the static electricity charge built up by car rolling down the road, so the toll collector doesn't get a shock every time someone hands over some money
How is it possible to take photographs in complete darkness?
UV rays
The frequency of violet light is about twice that of red light. How does the energy of a violet photon compare with the energy of a red photon?
Using E(photon) = hf, we conclude that the energy of a violet photon is about twice the energy of a red photon.
What is it, exactly, that waves in a light wave?
What waves in a light wave are the electric and magnetic fields. Their oscillation frequency is the frequency of the wave.
Is the current that is produced by a common generator ac or dc?
ac
How do the surface temperatures of reddish, bluish, and whitish stars compare?
bluish star is the hottest, then white and red
What produces a magnetic field?
electric charges in motion
CH 22 What term is used for "electricity at rest"?
electrostatics
Which contributes more to an atom's mass: electrons or protons? Which contributes more to an atom's size?
protons electrons
Two point charges are separated by 6 cm. The attractive force between them is 20 N. Find the force between them when they are separated by 12 cm. (Why can you solve this problem without knowing the magnitudes of the charges?)
since the separation distance is doubled, the force will be 1/2 squared= 1/4 the original value. 1/4 (20N)= 5N Remember there is an inverse relationship between force and distance.
Which have the longest wavelengths: light waves, X-rays, or radio waves?
radio waves
What is the color of visible light of the lowest frequencies? Of the highest frequencies?
red violet
Which has shorter wavelengths: ultraviolet or infrared? Which has higher frequencies?
ultraviolet ultraviolet
How does the magnitude of the electrical force com-pare between a pair of charged particles when they ar brought to half their original distance of separation? To one-quarter their original distance? To four times their original distance? (What law guides your answers?)
-half = 4 times (1/4th) -one-quarter = 1/16th -four times = 1/16th (1/4 distance = 4 times)
Why are the tires for trucks carrying gasoline and other flammable fluids manufactured to be electrically conducting
-to prevent charge from heat/gasoline building up
How does one coulomb of charge compare with the charge of a single electron?
1 C is associated with 6.25 × 1018 electrons. That's 6.25 billion billion electrons!
What is the wavelength of a wave that has a frequency of 1 Hz and travels at 300,000 km/s?
300,000 km
CH 26 What does a changing magnetic field induce?
A changing electric field
What does a changing electric field induce?
A changing magnetic field
What is induced by the rapid alternation of a magnetic field?
A rapidly alternating electric field
What is induced by the rapid alternation of an electric field?
A rapidly alternating magnetic field
What is a spectroscope, and what does it accomplish?
A spectroscope displays the spectrum of light as brightness versus wavelength.
What kind of force field surrounds a stationary electric charge? What additional field surrounds it when it moves?
An electric field surrounds a stationary charge. A magnetic field and an electric field surround a moving electric charge.
How does an absorption spectrum differ in appearance from an emission spectrum?
An emission spectrum consists of bright lines against a dark background, whereas an absorption spectrum consists of dark lines against a bright rainbow background.
What produces an electromagnetic wave?
An oscillating or accelerating electric charge
Suppose that the strength of the electric field about an isolated point charge has a certain value at a distance of 1 m. How will the electric field strength compare at a distance of 2 m from the point charge? What law guides your answer?
At twice the distance the field strength will be 1/4. the inverse-square law
Which has the higher frequency: red or blue light? Which has the greater energy per photon: red or blue light?
Blue light, blue light. "A photon in a beam of red light carries an amount of energy corresponding to its frequency." Low frequency = low energy. "A photon of twice the frequency has twice the energy; found in UV part of spectrum, blue." Higher frequency = hi energy.
How can you charge an object negatively with only the help of a positively charged object?
By induction; Bring the positively charged object near the object to be charge and the far side of the uncharged object will become positively charged. If you then touch the far side you will in effect remove this charge because electrons will flow form your body to the positive charge. If you remove your finger, the object has negative charge.
What kind of charging occurs when you slide your body across a plastic surface?
Charging by friction occurs. Electrons are transferred when one object rubs against another.
In what way are magnetic poles very different from electric charges?
Electrical charges produce electrical forces, regions called magnetic poles give rise to magnetic forces.
About how much of the measured electromagnetic spectrum does light occupy?
Less than 1 millionth of 1%
At the security area, people walk through a large coil of wire and through a weak ac magnetic field. What is the result of a small piece of metal on a person that slightly alters the magnetic field in the coil?
The coil with AC current is built into the security doors which detect metal objects. The presence of metal (more specifically, magnetic material) on a person which passes through a door with a coil, alters the inductance of the coil. This, in turn, changes the AC frequency, which enables the system to detect metal objects. Large enough metallic object, and the alarm would sound.
How is the direction of an electric field defined?
The direction of the field is the direction of the force on a positive test charge.
What happens to the direction of the magnetic field about an electric current when the direction of the cur-rent is reversed?
The magnetic field reverses direction at every point. A clockwise pattern of concentric circles becomes a counterclockwise pattern of concentric circles and vice verse
CH 25 What must change in order for electromagnetic induction to occur in a wire coil?
The magnetic field through the coil must change.
How can a hydrogen atom, which has only one electron, have so many spectral lines?
The many spectral lines from the element hydrogen are the result of the many energy states the single electron can occupy when excited.
Which part of an atom is positively charged, and which part is negatively charged?
The proton(positive charge) is stuck in the nucleus of an atom and the electron (negative charge) dashes around and does all sorts of things (surrounding nucleus)
Does atomic excitation occur in solids as well as in gases? How does the radiant energy from an incandescent solid differ from the radiant energy emitted by an excited gas?
Yes, excitation happens to any solid, liquid, gas. The radiant energy differs.
Figure 32.9 shows three transitions among three energy levels that would produce three spectral lines in a spectroscope. If the energy spacing between the levels were equal, would this affect the number of spectral lines?
Yes, they would move down to 2
If a beam of red light and a beam of blue light have exactly the same energy, which beam contains the greater number of photons?
a beam of red light
If we double the frequency of light, we double the energy of each of its photons. If we instead double the wave-length of light, what happens to the photon energy?
it decreases by half
How is the direction of an electric field indicated with electric field lines?
it's always in the direction that a positive charge would move
Is the rule for the interaction between magnetic poles similar to the rule for the interaction between electrically charged particles?
like poles repel, unlike poles attract and the force is proportional to the inverse square of the distance between two poles
A beam of electrons passes through a magnetic field without being deflected. What can you conclude about the orientation of the beam relative to the magnetic field? (Ignore any other fields.)
parallel to the field lines.
Which requires a physical medium in which to travel: light, sound, or both? Explain.
sound- vibrations
In 1676, the Danish astronomer Ole Roemer had one of those "aha" moments in science. He concluded from accumulated observations of eclipses of Jupiter's moon at different times of the year that light must travel at finite speed and needed 1300 s to cross the diameter of Earth's orbit around the Sun. Using 300,000,000 km for the diameter of Earth's orbit, calculate the speed of light based on Roemer's 1300-s estimate. How does it differ from a modern value for the speed of light?
speed = distance\time = 300,000,000 km\ 1300 s = 231,000 km/s. This value is 77 percent of the modern value.
The Sun is 1.50 3 1011 m from Earth. How long does it take the Sun's light to reach Earth? How long does it take light to cross the diameter of Earth's orbit? Compare this time with the time measured by Roemer in the 17th century (treated in Problem 33).
t= d/v---> d/t=v t= 500 seconds
How does the frequency of a radio wave compare to the frequency of the vibrating electrons that produce it?
they are the same
Will a pair of parallel current-carrying wires exert forces on each other?
yes, each will experience a force
In the diagram, the energy difference between states A and B is twice the energy difference between states B and C. In a transition (quantum jump) from C to B, an electron emits a photon of wavelength 600 nm. (a) What is the wavelength emitted when the photon jumps from B to A? (b) When it jumps from C to A?
λ(wavelength) = 300 nm λ = 200 nm
