【GR8677】 The Solution for GRE Physics Practice Test [81-100]

The Solution for GRE Physics Practice Test [81-100]

 

추천글 : 【GRE】 Solution for GRE Physics Practice Test 

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81. A small circualr wire loop of radius a is located at the center of a much larger circular wire loop of radius b as shown below. The larger loop carries an alternating current I = I₀ cos ωt, where I₀ and ω are constants. The magnetic field generated by the current in the large loop induces in the small loop an emf that is approximately equal to which of the following? (Either use mks units and let μ₀ be the permeability of free space, or use Gaussian units and let μ₀ be 4π / c².)

 

 

⑴ answer: B

⑵ Φ_B = πa² · μ₀ I₀ cos ωt / 2b 

⑶ emf = -N dΦ_B / dt = πa² · μ₀ I₀ / 2b · ω sin ωt 

 

 

82. The emission spectrum of an atomic gas in a magnetic field differs from that of the gas in the absence of a magnetic field. Which of the following is true of the phenomenon?

 

 

⑴ answer: E

 

 

83. A spectral line is produced by a gas that is sufficiently dense that the mean time between atomic collisions is much shorter than the mean lives of the atomic states responsible for the line. Compared with the same line produced by a low-density gas, the line produced by the higher-density gas will appear

 

 

⑴ answer: C

 

 

84. Sodium has eleven electrons and the sequence in which energy levels fill in atoms is 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc. What is the ground state of sodium in the usual notation ^2S+1L_J? 

 

 

⑴ answer: B

⑵ electron configuration : 1s² 2s² 2p⁶ 3s¹ 

⑶ valence electron : 3s¹

⑷ S (total spin quantum number) = | s₁ + s₂ + ∙∙∙ | = | 1/2 | = 1/2

⑸ L (total orbital angular momentum quantum number) = | l₁ + l₂ + ∙∙∙ | = | 0 | = 0

⑹ J (total angular momentum quantum number) = L + S = 1/2

⑺ 2S+1 = 2

⑻ L = 'S'

 

 

85. The figure below shows the photon interaction cross sections for lead in the energy ranger where the Compton, photoelectric, and pair production processes all play a role. What is the correct identification of these cross sections?

 

 

⑴ answer: B

 

 

86. The exponent in Coulomb's inverse square law has been found to differ from two by less than one part in a billion by measuring which of the following?

 

 

⑴ answer: E

 

 

87. In a gas of N diatomic molecules, two possible models for a classical description of a diatomic molecule are:

 

 

Which of the following statements about this gas is true?

 

 

⑴ answer: E

⑵ Model Ⅰ has a specific heat of 5/2 Nk because the degree of freedom is 5 (3 for translational and 2 for rotational).

⑶ Model Ⅱ has a specific heat of 7/2 Nk because of the addition of vibrational freedom.  

 

 

88. Consider a system of N noninteracting particles confined in a volume V at a temperature such that the particles obey classical Boltzmann statistics. If the temperature is lowered to the point at which quantum effects become important, the pressure of the gas may differ depending on whether the particles are fermions or bosons. Let P_F be the pressure exerted by the particles if they are fermions, P_B be the pressure if they are bosons, and P_C be the pressure the particles would exert if quantum effects are ignored. Which of the following is true?

 

 

⑴ answer: B 

 

 

89. A system containing two identical particles is described by a wave function of the form 

 

 

where x₁ and x₂ represent the spatial coordinates of the particles and α and β represent all the quantum numbers, including spin, of the states that they occupy. The particles might be 

 

 

⑴ answer: E

⑵ two identical particles...

 

 

90. The figure below shows one of the possible energy eigenfunctions ψ(x) for a particle bouncing impenetrable walls located at x = -a and x = +a. The potential energy equals zero for | x | ＜ a. If the energy of the particle is 2 electron volts when it is in the quantum state associated with this eigenfunction, what is its energy when it is in the quantum state of lowest possible energy?

 

 

⑴ answer: C 

 

 

91. When a narrow beam of monoenergetic electrons impinges on the surface of a single metal crystal at an angle of 30 degrees with the plane of the surface, first-order reflection is observed. If the spacing of the reflecting crystal planes is known from x-ray measurements to be 3 ångströms, the speed of the electrons is most nearly

 

 

⑴ answer: D

⑵ interference condition : 2d sin θ = 2 × (3 × 10^-10) × 0.5 = nλ = 1 × λ ⇔ λ = 3 × 10^-10 m

⑶ De Broglie matter wave : p = m_ev = h / λ = 2.21 × 10^-24 kg·m/s

⑷ v = p / m_e = 2.42 × 10⁶ m/s  

 

 

92. Which of the following is NOT compatible with the selection rule that controls electric dipole emission of photons by excited states of atoms?

 

 

⑴ answer: D

⑵ Δs = 0

 

 

93. An electric sander has a continuous belt that rubs against a wood surface as shown schematically below. The sander is 100 percent efficient and draws a current of 9 amperes from a 120-volt line. The belt speed is 10 meters per second. If the sander is pushing against the wood with a normal force of 100 newtons, the coefficient of friction is most nearly 

 

 

⑴ answer: D

⑵ 120 × 9 = F × 10 = μ × 100 × 10 ⇔ μ = 1.08  

 

 

94. In the circuit shown below, R₂ = 3R₁ and the battery of emf ℇ has negligible internal resistance. The resistance of the diode when it allows current to pass through it is also negligible. At time t = 0, the switch S is closed and the currents and voltages are allowed to reach their asymptotic values. Then at time t₁, the switch is opened. Which of the following curves most nearly represents the potential at point A as a function of time t?

 

 

⑴ answer: B

⑵ t = 0⁺ : The sudden change of electrical current can maximize the voltage of the inductor.

⑶ steady state : After a certain period of time, the inductor becomes similar to closed wire.

⑷ time constant of RL circuit : L/R

⑸ The time constant of the later circuit is less than that of the initial circuit, so the plot of the later circuit is more steep.

 

 

95. In the cycle shown below, KL and NM represent isotherms, while KN and LM represent reversible adiabats. A system is carried through the Carnot cycle KLMN, taking in heat Q₂ from the hot reservoir T₂ and releasing heat Q₁ to the cold reservoir T₁. All of the following statements are true EXCEPT: 

 

 

⑴ answer: C

⑵ ΔS_hot = -Q₂ / T₂

⑶ ΔS_cool = Q₁ / T₁

⑷ ΔS_tot = 0 

 

 

96. A particle of mass M is in an infinitely deep square well potential V where 

 

 

A very small perturbing potential V' is superimposed on V such that 

 

 

If ψ₀, ψ₁, ψ₂, ψ₃, ··· are the energy eigenfunctions for a particle in the infinitely deep square well potential, with ψ₀ being the ground state, which of the following statements is correct about the eigenfunction ψ₀' of a particle in the perturbed potential V + V'?

 

 

⑴ answer: B

 

 

97. Two uniform cylindrical disks of identical mass M, radius R, and moment of intertia 1/2 MR², as shown below, collide on a frictionless, horizontal surface. Disk 1, having an initial counterclockwise angular velocity ω₀ and a center-of-mass velocity v₀ = 1/2 ω₀R to the right, makes a grazing collision with disk II initially at rest. If after the collision the two disks stick together, the magnitude of the total angular momentum about the point P is 

 

 

⑴ answer: A

⑵ angular momentum made by initial rotation : Iω₀ = 1/2 MR²ω₀ (CCW)

⑶ angular momentum made by translational movement about P : R × Mv₀ = 1/2 MR²ω₀ (CW) 

 

 

98. The long thin cyclindrical glass rod shown below has length ℓ and is insulated from its surroundings. The rod has an excess charge Q uniformly distributed along its length. Assume the electric potential to be zero at infinite distances from the rod. If k is the constant in Coulomb's law, the electric potential at a point P along the axis of the rod and a distance ℓ from one end is kQ/ℓ multiplied by 

 

 

⑴ answer: D

⑵ electric field

 

 

⑶ electric potential 

 

 

 

99. The positronium "atom" consists of an electron and a positron bound together by their mutual Coulomb attraction and moving about their center of mass, which is located halfway between them. Thus the positronium "atom" is somewhat analogous to a hydrogen atom. The ground-state binding energy of hydrogen is 13.6 electron volts. What is the ground-state binding energy of positronium?

 

 

⑴ answer: B

⑵ positronium

 

 

100. The screen of a pinhole camera is at a distance D from the pinhole, which has a diameter d. The light has an effective wavelength λ. (λ ≪ D) For which of the following values of d will the image be sharpest?

 

 

⑴ answer: A

 

입력: 2023.10.06 01:00