GRE Sample Physics Problems (6)

76. A steady current I flows through a hollow cylinder of radius a and is uniformly distributed around the tube. Let r be the distance from the axis of symmetry of the tube to a given point.

The magnitude of the magnetic field B at a point inside the tube is (in Gaussian, cgs units):

A) 0 .....B) 2I/ rc.......C) 2Ir/ a²  c.......D)  4 π  (r  - a) I /r²   ...E) I/ π a²


77. What is the magnitude of the magnetic field B, at a point outside the tube? (Gaussian units)

A) 0.....B) I/ π a²    .... C) 2 π  I /r²    ....D) 2Ir/ a²  c.....E) 2I/ rc

78. A certain capacitor consists of two identical square conducting surfaces of area A, separated by a distance d from each other. The dielectric between the capacitor plates is air whose dielectric constant can be taken equal to unity.  What is the capacitance of the capacitor (MKS units):

A) d/ A....B)   e_o A/ d......C) A/  d e_o    .....D) Ad/ e_o    ....E) d e_o   / A

79. If a potential difference V is established between the plates of the capacitor what is the resulting potential energy of the capacitor? (Let C denote the capacitance of the capacitor).

A) CV......B) C/ 2 V² ........C) 1/2 C V²......D) 1/2 C²  V......E) Q² /  2C

80. What is the force between the plates of the capacitor? (MKS units)

A) e_o A V².....B)  e_o A (d/ 2V) .....C) e_o (V² /A d² )

D) e_o A (V² / 2 d² )

E) e_o A (V/ d)

81. An infinitely long conducting plane is defined by the equation x = 0. (That is, it consists of the y-z  plane). A point charge q is located at the point x = a, y= z = 0)  What is the magnitude of the force exerted between the point charge and the infinitely conducting plane (cgs units)?

A) 0.....B) q/ a.....C) q/ 2a.....D) q² / a²    ...E) q² /4 a²   

82. What is the potential energy of the system in cgs units?

A) - q² / a  ....B)  q² / 4a    ....C) -  q² / 4a    .....D) q² / a    ...E)- q² / 4a    

83. A small object lies 4 inches to the left of the vertex of a concave mirror whose radius of curvature is 12 inches.  The magnification of the image is:

A) 1.5......B) 2.0......C) 3.0.......D) 4.0......E) 6.0

84. A tank is filled with water to a depth of 6 inches and a coin is placed at the bottom of the tank.  The apparent distance of the coin from the surface is (take the index of refraction of water as 4/3)

A) 3 inches....B) 4.5 inches... C) 5 inches.....D) 6 inches...E) 7.5 inches

85. How does the focal length f of a thin lens change when it is immersed in water? (Take n = 1.50 for lens)

A) unchanged  ...B)increased to 1.33f.....C) increased to 1.50f...D)increased to 1.50/ 1.33f

E) increased to 3.91 f

86. In the operation of a laser which one of these is a necessary condition?

A) destructive interference  B) inversion  C) polarization  D)diffraction  E) refraction

87. A refrigerator has a coefficient of performance = 6, and removes 2,400 calories from the food stored in it. The electric motor operating the refrigerator must perform an amount of work in calories equivalent to:

A) 6......B) 200.....C) 400......D) 2, 400....E) 14, 400

88. Consider the matrix:

(0....0......3)
(0....0......0)
(3....0......0)

Which of the following statements regarding the matrix is not true?

A) It is symmetric....B) It can be diagonalized......C) It has non-degenerate eigenvalues

D) It is equal to its transpose.....E) It has an inverse

89. When an electron and positron annihilate each other, which of the following is usually created?

A) Nothing.....B) One photon.....C) Two photons.....D) Two neutrons   E) A neutrino

90. A planet has a period about the Sun equal to T and a mean distance from the Sun equal to R. T²  varies inversely as:

A) 1/ R....B) R^{2   ......}C) R........D)   R³     .......E) R^1/3