【GR8677】 The Solution for GRE Physics Practice Test [21-40]

The Solution for GRE Physics Practice Test [21-40]

 

추천글 : 【GRE】 Solution for GRE Physics Practice Test 

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21. Two observers O and O' observe two events, A and B. The observers have a constant relative speed of 0.8 c. In units such that the speed of light is 1, observer O obtained the following coordinates:

 

Event A: x = 3, y = 3, z = 3, t = 3

Event B: x = 5, y = 3, z = 1, t = 5

 

What is the length of the space-time interval between two events, as measured by O'?

 

 

⑴ answer: C

⑵ Lorentz coordinate conversion : The space-time interval does not change according to observers.

 

 

 

22. Which of the following statements most accurately describes how an electromagnetic field behaves under a Lorentz transformation?

 

 

⑴ answer: B

 

 

23. Which of the following statements concerning the electrical conductivities at room temperature of a pure copper sample and a pure silicon sample is NOT true?

 

 

⑴ answer: E

⑵ The addition of an impurity, called doping, can increase the conductivity as the nubmer of charge carriers increases.

 

 

24. The battery in the diagram below is to be charged by the generator G. The generator has a terminal voltage of 120 volts when the charging current is 10 amperes. The battery has an emf of 100 volts and an internal resistance of 1 ohm. In order to charge the battery at 10 amperes charging current, the resistance R should be set at 

 

 

⑴ answer: C

⑵ total current = 1 = (120 - 100) / (R + 1) → R = 1 Ω

 

 

25. A charged particle is released from rest in a region where there is a constant electric field and a constant magnetic field. If the two fields are parallel to each other, the path of the particle is a 

 

 

⑴ answer: E

⑵ The particle goes perpendicular to E and B, making electric force and Lorentz force cancel each other.

 

 

26. A nickel target (Z = 28) is bombarded with fast electrons. The minimum electron kinetic energy needed to produce x-rays in the K series is most nearly  

 

 

⑴ answer: D

 

 

27. The hypothesis that an electron possesses spin is qualitatively significant for the explanation of all of the following topics EXCEPT the 

 

 

⑴ answer: D

⑵ 🄐 Aufbau principle

⑶ 🄑 related to the degree of freedom

⑷ 🄒 Zeeman effect 

⑸ 🄓 The fact only tells that the electron has charge.

⑹ 🄔 related to Zeeman effect

 

 

28. Eigenfunctions for a rigid dumbbell rotating about its center have a ϕ dependence of the form ψ(ϕ) = A exp(im ϕ), where m is a quantum number and A is a constant. Which of the following values of A will properly normalize the eigenfunction?

 

 

⑴ answer: D 

 

 

 

29. A negative test charge is moving near a long straight wire in which there is a current. A force will act on the test charge in a direction parallel to the direction of the current if the motion of the charge is in a direction

 

 

⑴ answer: A

⑵ The direction of magnetic field by the wire current is determined by Ampere's law. 

⑶ Lorentz force : F = qv × B = -ev × B || I (wire current)

 

 

30. The configuration of the potassium atom in its ground state is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Which of the following statements about potassium is true? 

 

 

⑴ answer: E

⑵ 🄐 3d orbitals are not filled.

⑶ 🄑 4s subshell is partially fileld.

⑷ 🄒 Its least tightly bound electron, 4s¹, has ℓ = 0.

⑸ 🄓 Its atomic number is 19.

⑹ 🄔 Due to the spherical nature of s orbital, the shape of the electron distribution determined by the outermost electron is also spherical. 

 

 

Questions 31-33 refer to the following apparatus used to study the photoelectric effect. 

 

 

In this apparatus, the photocathode and the collector are made from the same material. The potential V of the collector, measured relative to ground, is initially zero and is then increased or decreased monotonically. The effect is described by Einstein's photoelectric equation

 

|eV| = hν - W.

 

31. When the photoelectric equation is satisfied and applicable to this situation, V is the 

 

 

⑴ answer: A

⑵ Electrons collect at the collector and emit to photocathod without the current. 

 

 

32. The photoelectric equation is derived under the assumption that 

 

 

⑴ answer: D

 

 

33. The quantity W in the photoelectric equation is the 

 

 

⑴ answer: D 

 

 

Questions 34-36

The potential energy of a body constrained to move on a straight line is kx⁴ where k is a constant. The position of the body is x, its speed v, its linear momentum p, and its mass m.

 

34. The force on the body is 

 

 

⑴ answer: B

⑵ F = -dU / dx = -4kx³ 

 

 

35. The Hamiltonian function for this system is 

 

 

⑴ answer: A

 

 

36. The body moves from x₁ at time t₁ to x₂ at time t₂. Which of the following quantities is an extremum for the x - t curve corresponding to this motion, if end points are fixed? 

 

 

⑴ answer: A

 

 

37. The figure below represents a point mass m attached to the ceiling by a cord of fixed length ℓ. If the point mass moves in a horizontal circle of radius r with uniform angular velocity ω, the tension in the cord is 

 

 

⑴ answer: E

 

 

 

38. If logical 0 is 0 volts and logical 1 is +1 volt, the circuit shown below is a logic circuit commonly known as

 

 

 

⑴ answer: A

⑵ truth table 

 

Input 1	Input 2	Output
0	0	0
0	1	1
1	0	1
1	1	1

Table. 1. truth table

 

 

39. The gain of an amplifier is plotted versus angular frequency ω in the diagram below. If K and a are positive constants, the frequency dependence of the gain near ω = 3 × 10⁵ second^-1 is most accurately expressed by 

 

 

⑴ answer: E

⑵ Bode plot

⑶ The slope, i.e. | log Gain | divided by | log ω |, is about -2 near ω = 3 × 10⁵ second^-1.   

 

 

 

40. An experimenter measures 9934 counts during one hour from a radioactive sample. From this number the counting rate of the sample can be estimated with a standard deviation of most nearly 

 

 

⑴ answer: A

⑵ The experimenter follows Poisson distribution.

⑶ The standard deviation of the random variable is square root of the mean of it.

⑷ √9934 ≃ 100

 

입력: 2023.07.29 14:31