10. A sample of a monatomic ideal gas containing 6.00 × 10²³ atoms in a volume of 8.00 × 10⁻³ m³ is initially in thermal equilibrium with its surroundings at 300 K. The gas is made to expand until the final volume is 6.40 × 10⁻² m³, and the final temperature is 50 K. The change in entropy of the gas should be (A) positive (B) negative (C) zero (D) undetermined.

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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