15. A circuit contains a resistance and a charged capacitance. The resistance is 20 ohms, and the capacitance is 5 F. If the circuit is switched off, how long will it take when the current decreases to a half of initial value? (ln2 = 0.69)
(A) 34 s
(B) 50 s
(C) 69 s
(D) 100 s
(E) 127 s

11. A Carnot engine takes 2000 J of heat from a reservoir at 500 K, does some work, and discards some heat to reservoir at 300 K. What is its efficiency? (A) 0.2 (B) 0.3 (C) 0.4 (D) 0.5 (E) 0.6

12. A series of 3 uncharged concentric spherical conducting shells surround a small central charge q. The potential at a point outside the third shell, at distance r from the center, and relative to V = 0 at ∞, is (A) −3keq/r (B) +3keq/r (C) − (ln 3)keq/r (D) + (ln 3)keq/r (E) +keq/r

13. A conductor of radius r, length ℓ , and resistivity ρ has resistance R. It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor. The resistance of the new conductor is . (A) R (B) R (C) R (D) 4 R (E) 16 R

複選題14. There is an electric dipole in an electric field, as shown below. Which of the following statement is incorrect? (A) The dipole rotates clockwise. (B) The torque on +q points up. (C) The electric dipole moment is proportional to the distance from –q to +q. (D) The dipole has a max potential energy when it is perpendicular to E. (E) The electric dipole moment points from –q to +q.

16. Using the rules of significant figures, calculate the following: 0.102 × 0.0821 × 273/1.01 (A) 2.2635 (B) 2.264 (C) 2.26 (D) 2.3 (E) 2.66351

17. Which of the following best describes an orbital? (A) Space where electrons are likely to be found in an atom. (B) Space which may contain electrons, protons, and/or neutrons. (C) The space in an atom where an electron is most unlikely to be found. (D) Small, walled spheres that contain electrons. (E) A single space within an atom that contains all electrons of that atom.

18. Which of the following is the correct electron configuration for OF− ? (A) (B) (C) (D) (E) None of the above.

19. Which of the following resonance structures is the most stable? (A) (B) (C) (D) (E) All of them are the same.

20. Which of the following does NOT contain at least one pi bond? (A) H2CO (B) CO2 (C) C2H2 (D) NO (E) All of the above (A-D) contain at least one pi bond.

21. Order the intermolecular forces (dipole-dipole, London dispersion, ionic, and hydrogen-bonding) from weakest to strongest. (A) dipole-dipole, London dispersion, ionic, and hydrogen-bonding (B) London dispersion, dipole-dipole, hydrogen-bonding, and ionic (C) hydrogen-bonding, dipole-dipole, London dispersion, and ionic (D) dipole-dipole, ionic, London dispersion, and hydrogen-bonding (E) London dispersion, ionic, dipole-dipole, and hydrogen-bonding