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physics
modern physics

Questions and Answers of Modern Physics

Would you expect the molecule H2+ to be stable? If so, where would the single electron spend most of its time?
Explain why the carbon atom (Z = 6) usually forms four bonds with hydrogen-like atoms.
If conduction electrons are free to roam about in a metal, why don’t they leave the metal entirely?
Explain why the resistivity of metals increases with temperature whereas the resistivity of semiconductors may decrease with increasing temperature.
Figure 29-33 shows a €œbridge-type€ full-wave rectifier. Explain how the current is rectified and how current flows during each half cycle.
Compare the resistance of a pn junction diode connected in forward bias to its resistance when connected in reverse bias?
Explain how a transistor could be used as a switch?
What is the main difference between n-type and p-type semiconductors?
Describe how a pnp transistor can operate as an amplifier.
In a transistor, the base-emitter junction and the base-collector junction are essentially diodes. Are these junctions reverse-biased or forward-biased in the application shown in Fig 29-32?
A transistor can amplify an electronic signal, meaning it can increase the power of an input signal. Where does it get the energy to increase the power?
A silicon semiconductor is doped with phosphorus. Will these atoms be donors or acceptors? What type of semiconductor will this be?
Do diodes and transistors obey Ohm’s law? Explain.
Can a diode be used to amplify a signal? Explain.
Estimate the binding energy of a KC1 molecule by calculating the electrostatic potential energy when the K+ and CI– ions are at their stable separation of 0.28nm. Assume each has a charge of
The measured binding energy of KCI is 4.43eV. From the result of Problem 1, estimate the contribution to the binding energy of the repelling electron clouds at the equilibrium distance r0 = 0.28nm.
Estimate the binding energy of the H2 molecule, assuming the two H nuclei are 0.074nm apart and the two electrons spend 33% of their time midway between them.
Binding energies are often measured experimentally in kcal per mole, and then the binding energy in eV per molecule is calculated from that result. What is the conversion factor in going from kcal
(a) Apply reasoning similar to that in the text for the S = 0 and S = 1 states in the formation of the H2 molecule to show why the molecule He2 is not formed.(b) Explain why the He2+ molecular ion
Show that the quantity h2/I have units of energy.
The so-called “characteristic rotational energy,” h2/2I, for N2 is 2.48 X 10–4 eV calculate the N2 bond length.
(a) Calculate the characteristic rotational energy, h2/2I, for the O2 molecule whose bond length is 0.121nm,(b) What are the energy and wavelength of photons emitted in a L = 2 to L = 1 transition?
The equilibrium separation of H atoms in the H2 molecule is 0.74nm (Fig 29-8). Calculate the energies and wave-lengths of photons for the rotational transitions (a) L = 1 to L = 0,(b) L = 2 to L =
Calculate the bond length for the NaCI molecule give
(a) Use the curve of Fig. 29-17 to estimate the stiffness constant k for the H2 molecule. (Recall that PE = ½ kx2.)(b) Then estimate the fundamental wave-length for vibrational transitions using the
The spacing between “nearest neighbor” Na and CI ions in a NaCI crystal is 0.24nm. What is the spacing between two nearest neighbor Na ions?
Common salt, NaCI, has a density of 2.165 g/cm3. The molecular weight of NaCI is 58.44. Estimate the distance between nearest neighbor Na and CI ions.
Repeat Problem 13 for KCI whose density is 1.99g/cm3.
Explain on the basis of energy bands why the sodium chloride crystal is a good insulator.
A semiconductor, bombarded with light of slowly increased frequency, begins to conduct when the wave-length of light is 640mm. Estimate the size of the energy gap Eg.
Calculate the longest-wavelength photon that can cause an electron in silicon (Eg = 1.1eV) to jump from the valence hand to the conduction band.
The energy gap between valence and conduction bands in germanium is 0.72eV. What range of wavelengths can a photon have to excite an electron from the top of the valence band into the conduction hand?
The energy gap Eg in germanium is 0.72eV. When used as a photon detector, roughly how many electrons can be made to jump from the valence to the conduction band by the passage of a 760-keV photon
We saw that there are 2N possible electron states in the 3s band of Na, where N is the total number of atoms. How many possible electron states are there in the(a) 2s band(b) 2p band and(c) 3p
Suppose that a silicon semiconductor is doped with phosphorus so that one silicon atom in 106 is replaced by a phosphorus atom. Assuming that the “extra” electron in every phosphorus atom is
At what wavelength will an LED radiate if made from a material with an energy gap Eg = 1.4eV?
If an LED emits light of wavelength λ = 650nm, what is the energy gap (in eV) between valence and conduction bands?
A silicon diode, whose current-voltage characteristics are given in Fig. 960-Ω resistor. What battery voltage is needed to produce a 12-mA current?
Suppose that the diode of Fig. 29-28 is connected in series to a 100-Ω resistor and a 2.0-V battery. What current flows in the circuit?
Sketch the resistance as a function of current, for V > 0, for the diode shown in Fig. 29-28.
An ac voltage of 120V rms is to be rectified. Estimate very roughly the average current in the output resistor R (25kΩ) for (a) A half-wave rectifier (Fig. 29-29), and (b) A full-wave
A silicon diode passes significant current only if the forward-bias voltage exceeds about 0.6V. Make a rough estimate of the average current in the output resistor R of (a) A half-wave rectifier
A 120-V rms 60-Hz voltage is to be rectified with a full-wave rectifier (Fig. 29-30), where R = 21kΩ, and C = 25μF. (a) Make a rough estimate of the average current.
From Fig. 29-32, write an equation for the relationship between the base current (IB), the collector current (IC), and the emitter current (IE, not labeled in the figure).
Estimate the binding energy of the H2 molecule by calculating the difference in kinetic energy of the electrons between when they are in separate atoms and when they are in the molecule, using the
The average translational kinetic energy of an atom or molecule is about KE = 3/2 kT (Eq. 13-8), where k = 1.38 X 10–23 J/K is Boltzmann’s constant. At what temperature T will KE be on the order
In the ionic salt KF, the separation distance between ions is about 0.27nm.(a) Estimate the electrostatic potential energy between the ions assuming them to be point charges (magnitude) 1e), (b) It
Consider a mono atomic solid with a weakly bound cubic lattice, with each atom connected to six neighbors, each bond having a binding energy of 3.9 x 10€“3 eV. When this solid melts, its
For O2 with a bond length of 0.121nm, what is the moment of inertia about the center of mass?
A diatomic molecule is found to have an activation energy of 1.4eV. When the molecule is disassociated, 1.6eV of energy is released. Draw a potential energy curve for this molecule.
When EM radiation is incident on diamond, it is found that light with wavelengths shorter than 226nm will cause the diamond to conduct. What is the energy gap between the valence band and the
A TV remote control emits IR light. If the detector on the TV set is not to react to visible light, could it make use of silicon as a “window” with energy gap Eg = 1.14eV? What is the
For an arsenic donor atom in a doped silicon semiconductor, assume that the “extra” electron moves in a Bohr orbit about the arsenic ion. For this electron in the ground state, take into account
Most of the sun’s radiation has wavelengths shorter than 1000nm. For a solar cell to absorb all this, what energy gap ought the material have?
For a certain semiconduc5tor, the longest wavelength radiation that can be absorbed is 1.92mm. What is the energy gap in this semiconductor?
Green and blue LEDS became available many years after red LEDS were first developed. Approximately what energy gaps would you expect to find in green (525nm) and in blue (465nm) LEDS?
A zener diode voltage regulator is shown in Fig. 29-35. Suppose that R = 1.80kΩ and that the diode breaks down at a reverse voltage of 130V. (The current increases rapidly at this point, as
What do different isotopes of a given element have in common? How are they different?
What are the elements represented by the X in the following;(a) 232/92 X;(b) 18/7 X;(c) 1/1 X;(d) 82/38X;(e) 247/97X?
How many protons and how many neutrons do each of the isotopes in Question 2 have?
Identify the element that has 88 nucleons and 500 neutrons.
Why are the atomic masses of many elements (see the periodic table) not close to whole numbers?
How do we know there is such a thing as the strong nuclear force?
What are the similarities and the differences between the strong nuclear force and the electric force?
What is the experimental evidence in favor of radioactivity being a nuclear process?
The isotope 64/29Cu is unusual in that it can decay by λ, β –, and β+, emission. What is the resulting nuclide for each case?
A 238/92U nucleus decays to a nucleus containing how many neutrons?
Describe, in as many ways as you can, the difference between a, β and y rays?
What element is formed by the radioactive decay of? (a) 24/11Na (β –) (b) 22/11Na (β +) (c) 210/84Po (α)?
What element is formed by the decay of? (a) 32/15P (β–); (b) 35/16S (β–) (c) 211/83Bi (α)
Fill in the missing particle or nucleus; (a) 45/20Ca →? + e – + v (b) 58/29Cu →? + y (c) 46/24Cr → 46/23V +? (d) 234/94Pu →? + α (e) 239/93Np → 239/94Pu +?
Immediately after a 238/92 U nucleus decays to 234/90Th + 4/2He, the daughter thorium nucleus still has 92 electrons circling it. Since thorium normally holds only 90 electrons, what do you suppose
When a nucleus undergoes either β– or β+ decay, what happens to the energy levels of the atomic electrons? What is likely to happen to these electrons following the decay?
The alpha particles from a given alpha-emitting nuclide are generally mono energetic; that is, they all have the same kinetic energy. But the beta particles from a beta-emitting nuclide have a
Do isotopes that undergo electron capture generally lie above or below the line of stability in Fig. 30-2?
Can hydrogen or deuterium emit an α particle? Explain.
Why are many artificially produced radioactive isotopes rare in nature?
An isotope has a half-life of one month. After two months, will a given sample of this isotope have completely decayed? If not, how much remains?
Why are none of the elements with Z > 92 stable?
A proton strikes a 6/3Li nucleus. As a result, an α particle and another particle are released. What is the other particle?
Can 14/6C dating be used to measure the age of stone walls and tablets of ancient civilizations? Explain
In both internal conversion and β decay and electron is emitted. How conversion and β decay, an electron is emitted. How could you determine which decay process occurred?
A pi meson has a mass of 139 MeV/c2. What is this in atomic mass units?
What is the approximate radius of an alpha particle (4/2He)?
What is the rest mass of a bare α particle in MeV/c2?
(a) What is the approximate radius of a 64/29Cu nucleus?(b) Approximately what is the value of A for a nucleus whose radius is 3.9 X 10–15 m?
(a) Show that the density of nuclear matter is essentially the same for all nuclei.(b) What would be the radius of the Earth if it had its actual mass but had the density of nuclei?(c) What would be
(a) What is the fraction of the hydrogen atoms mass that is in the nucleus?(b) What is the fraction of the hydrogen atom’s volume that is occupied by the nucleus?
Approximately how many nucleons are there in a 1.0-kg object? Does it matter what the object is made of? Why or why not?
How much energy must an α particle have to just “touch” the surface of a 238/92U nucleus?
Estimate the total binding energy for 40/20Ca, using Fig. 30-1.
Use Fig. 30-1 to estimate the total binding energy of (a) 238/92U, and (b) 84/36Kr.
Use Appendix B to calculate the binding energy of 2/1H (deuterium).
Calculate the binding energy per nucleon for a 14/7N nucleus.
Determine the binding energy of the last neutron in a 40/19K nucleus.
Calculate the total binding energy, and the binding energy per nucleon, for (a) 6/3Li,(b) 208/82Pb. Use Appendix B.
Compare the average binding energy of a nucleon in 23/11Na to that in 24/11 Na.
How much energy is required to remove? (a) A proton.(b) A neutron, from 16/8O? Explain the difference in you answers.
(a) Show that the nucleus 8/4Be (mass = 8.005305u) is unstable and will decay into two α particle (b) Is 12/6C stable against decay into three α particles? Show why or why not.
How much energy is released when tritium, 3/1H, decays by β– emission?

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