11. A point charge Q is surrounded by a spherical conducting shell of inner radius a and outer radius b. The conducting shell is grounded. Which of the following statement is correct ? (A) The total charge on the outer surface of the conducting shell is zero. (B) The electric potential V is proportional to 1/r for 0 ＜ r ＜ a. (C) The total charge on the conducting shell is zero. (D) The magnitude of the electric field is nonzero and inversely proportional to r² for r ＞ b.

9. In a model gasoline engine, the air-gasoline mixture can be treated approximately with a thermodynamic cycle known as the Otto cycle. In the first process (1→2), the gas undergoes an isentropic compression (the engine's compression stroke), during which the temperature rises from 300 K to 600 K. The next process (2→3) is an isochoric pressure increase, and the gas temperature reaches 1500 K as the gasoline burns. Next (3→4), the gas undergoes isentropic expansion (the engine's power stroke) during which its temperature drops to 800 K. Finally (4→1), the gas experiences an isochoric pressure decrease during which its temperature drops back to 300 K. Please estimate the efficiency of this model engine. (A) 10%~20% (B) 20%~30% (C) 30%~40% (D) 40%~50%.

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