Questions and Answers of Essential University Physics

On an energy-release-per-unit-mass basis, by approximately what factor do nuclear reactions exceed chemical reactions?
Explain and distinguish the roles of the control rods and moderator in a nuclear reactor.
Why is a water-moderated reactor intrinsically safer in a loss of coolant accident than a graphite-moderated reactor?
Is 238U fissionable? Is it fissile? Explain the distinction.
Why are fission fragments necessarily radioactive?
Nuclear waste comprises fission products and transuranics. Distinguish the two, including their implications for nuclear waste disposal.
What properties of fusion fuel require extreme values and thus present the greatest challenge to fusion energy technologies?
Explain the different approaches to the Lawson criterion taken by inertial-confinement and magnetic-confinement fusion schemes.
If you could extract all the deuterium from a gallon of seawater and use it as fusion fuel, how much gasoline would it be equivalent to in energy content?
Why are the energies given by Equations 34.12 negative? ke² 2a0\n° (energy levels, Bohr atom) (34.12a)
How might our everyday experience be different if Planck’s constant had the value 1 Js?
Energy–time uncertainty limits the precision with which we can know the mass of unstable particles (those that decay after a finite time). Why?
Why does the photoelectric effect suggest that light has particle-like properties?
Why are the lines of the Lyman series in the ultraviolet while some Balmer lines are in the visible?
How many spectral lines are in the entire Balmer series?
How are the uncertainty principle and wave-particle duality related?
Suppose the Compton effect were significant at radio wavelengths. What problems might this present for radio and TV broadcasting?
Why is the immediate ejection of electrons in the photoelectric effect surprising from a classical viewpoint?
What colors of visible light have the highest-energy photons?
Imagine an atom that, unlike hydrogen, had only three energy levels. If these levels were evenly spaced, how many spectral lines would result? How would their wavelengths compare?
Looking at the night sky, you see one star that appears red, another yellow, and another blue. Compare their temperatures.
The quantity E(vector)•B(vector) is invariant. What does this say about how different observers will measure the angle between E(vector) and B(vector) in a light wave?
An atom in an excited state emits a burst of light. What happens to the atom’s mass?
The rest energy of an electron is 511 keV. What’s the approximate speed of an electron whose total energy is 1 GeV?
If you took your pulse while traveling in a high-speed spacecraft, would it be faster than, slower than, or the same as on Earth?
Is matter converted to energy in a nuclear reactor? In a burning candle? In your body?
The Andromeda Galaxy is 2 million light years from the Milky Way. Although nothing can go faster than light, it would still be possible to travel to Andromeda in much less than 2 million years. How?
If you’re in a spaceship moving at 0.95c relative to Earth, do you perceive time to be passing more slowly than it would on Earth? Think! Is your answer consistent with the relativity principle?
Time dilation is sometimes described by saying that “moving clocks run slow.” In what sense is this true? In what sense does the statement violate the spirit of relativity?
Does relativity require that the speed of sound be the same for all observers? Why or why not?
What’s special about the special theory of relativity?
Why was it necessary to repeat the Michelson Morley experiment throughout the year?
Why was the Michelson Morley experiment a more sensitive test of motion through the ether than independent measurements of the speed of light in two perpendicular directions?
A prism bends blue light more than red. Is the same true of a diffraction grating?
Why does an oil slick show colored bands?
Why does a soap bubble turn colorless just before it dries up and pops?
Why don’t you see interference effects between the front and back of your eyeglasses?
You can hear around corners, but you can’t see around corners. Why?
In deriving the intensity in double-slit interference, why can’t you simply add the intensities from the two slits?
The primary maxima in multiple-slit interference are in the same angular positions as those in double-slit interference. Why, then, do diffraction gratings have thousands of slits instead of just two?
When the Moon passes in front of a star, the starlight intensity fluctuates before going to zero instead of dropping abruptly. Explain.
An LC circuit with a 20μF capacitor oscillates with period 5.0 ms. The peak current is 25 mA. Find (a) the inductance and (b) the peak voltage.
The resistance of a metal increases with increasing temperature, while the resistance of a semiconductor decreases. Why the difference?
You put a 1.5-V battery across a piece of material, and a 100-mA current flows. With a 9-V battery, the current increases to 400 mA. Is the material ohmic or not?
A solid conducting slab is inserted between the plates of a capacitor, not touching either plate. Does the capacitance increase, decrease, or remain the same?
A solid sphere contains positive charge uniformly distributed throughout its volume. Is the potential at its center higher than, lower than, or the same as at the surface?
The electric field of a flat sheet of charge is σ/2ε0. Yet the field of a flat conducting sheet even a thin one, like a piece of aluminum foil—is σ/2ε0. Explain this apparent discrepancy.
Why must the electric field be zero inside a conductor in electrostatic equilibrium?
An insulating sphere carries charge spread uniformly throughout its volume. A conducting sphere has the same radius and net charge, but of course the charge is spread over its surface only (assume
Does Gauss’s law apply to a spherical Gaussian surface not centered on a point charge, as shown in Fig. 21.31? Would this be a useful surface to use in calculating the electric field? FIGURE 21.31
You’re sitting inside an uncharged, hollow spherical shell. Suddenly someone dumps a billion coulombs of charge on the shell, distributed uniformly. What happens to the electric field at your
Why can’t you use Gauss’s law to determine the field of a uniformly charged cube? Why couldn’t you use a cubical Gaussian surface?
The field of an infinite charged line decreases as 1/r. Why isn’t this a violation of the inverse-square law?
A point charge is located a fixed distance outside of a uniformly charged sphere. If the sphere shrinks in size without losing any charge, what happens to the force on the point charge?
In a certain region the electric field points to the right and its magnitude increases as you move to the right, as shown in Fig. 21.30. Does the region contain net positive charge, net negative
Why can a bird perch on a high-voltage power line without getting electrocuted?
One proton is accelerated from rest by a uniform electric field, another proton by a nonuniform electric field. If they move through the same potential difference, how do their final speeds compare?
Would a free electron move toward higher or lower potential?
The electric field at the center of a uniformly charged ring is obviously zero, yet Example 22.6 shows that the potential at the center isn’t zero. How is this possible?
Must the potential be zero at any point where the electric field is zero? Explain.
Must the electric field be zero at any point where the potential is zero? Explain.
The potential is constant throughout an entire volume. What must be true of the electric field within that volume?
In considering the potential of an infinite flat sheet, why isn’t it useful to take the zero of potential at infinity?
“Cherry picker” trucks for working on power lines often carry electrocution hazard signs. Explain how this hazard arises and why it might be more of a danger to someone on the ground than to a
Can equipotential surfaces intersect? Explain.
Is the potential at the center of a hollow, uniformly charged spherical shell higher than, lower than, or the same as at the surface?
Two equal but opposite charges form a dipole. Describe the equipotential surface on which V = 0.
The electric potential in a region increases linearly with distance. What can you conclude about the electric field in this region?
Two positive point charges are infinitely far apart. Is it possible, using a finite amount of work, to move them until they’re a small distance d apart?
How does the energy density at a certain distance from a negative point charge compare with the energy density at the same distance from a positive point charge of equal magnitude?
A dipole consists of two equal but opposite charges. Is the total energy stored in the dipole’s electric field zero? Why or why not?
Charge is spread over the surface of a balloon, which is then allowed to expand. What happens to the energy of the electric field?
Does the superposition principle hold for electric-field energy densities? That is, if you double the field strength at some point, do you double the energy density as well?
A student argues that the total energy associated with the electric field of a charged sphere must be infinite because its field extends throughout an infinite volume. Critique this argument.
A capacitor is said to carry a charge Q. What’s the net charge on the entire capacitor?
Does the capacitance describe the maximum amount of charge a capacitor can hold, in the same way that a bucket’s capacity describes the maximum amount of water it can hold? Explain.
Is a force needed to hold the plates of a charged capacitor in place? Explain.
Two capacitors contain equal amounts of energy, yet one has twice the capacitance. How do their voltages compare?
A parallel-plate capacitor is connected to a battery that imposes a potential difference V between its plates. If a dielectric slab is inserted between the plates, what happens to(a) the potential
Explain the difference between the current and current density.
A constant electric field generally produces a constant drift velocity. How is this consistent with Newton’s assertion that force results in acceleration, not velocity?
When caught in the open in a lightning storm, a person should crouch low with feet close together rather than lie flat on the ground. Why?
Good conductors of electricity are often good conductors of heat. Why might this be?
Why can current persist forever in a superconductor with no applied voltage?
Does an electric stove burner draw more current when it’s first turned on or when it’s fully hot?
A person and a cow are standing in a field when lightning strikes the ground nearby. Why is the cow more likely to be electrocuted?
A 50-W and a 100-W lightbulb are both designed to operate at 120 V. Which has the lower resistance?
Equation 24.8a suggests that no power can be dissipated in a superconductor because R = 0. But Equation 24.8b suggests the power should be infinite. Which is right, and why? P = 1²R (24.8a) v2
What’s wrong with this news report: “A power-line worker was injured when 4000 volts passed through his body”?
Are household electrical outlets connected in series or parallel? How do you know?
All the resistors in Fig. 25.24 have the same resistance. In which circuits does the battery supply the same current? (a) (b) Lww- ww (c) (d) FIGURE 25.24 For Thought and Discussion 2 ww ww ww
Can the voltage across a battery’s terminals differ from the battery’s rated voltage? Explain.
Can the voltage across a battery’s terminals be higher than the battery’s rated voltage? Explain.
In some cities, streetlights are wired in such a way that when one burns out, they all go out. Are the lights in series or parallel?
When the switch in Fig. 25.25 is open, what’s the voltage across the resistor? Across the switch? FIGURE 25.25 For Thought and Discussion 6
Two identical resistors in series dissipate equal power. How can this be, when electric charge loses energy in flowing through the first resistor?
When a large electric load such as a washing machine or oven comes on, lights throughout a house often dim. Why?
How would you connect a pair of equal resistors across an ideal battery in order to get the greatest power dissipation?
You have a battery whose voltage and internal resistance are unknown. Using an ideal voltmeter and an ideal ammeter, how would you determine each of these characteristics?
A student who’s confused about voltage and current hooks a nearly ideal ammeter across a car battery. What happens?

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