13. A long solenoid of radius a has n loops of wire per unit length. For a time-dependent current I(t) = I₀ sin(ωt) through the solenoid, what is roughly the current on a circular conductor of radius b (b ＞ a) and resistance R outside the solenoid, assuming that the frequency is sufficiently low? Up to an overall constant factor, the current is (A) a² n I₀ ω cos(ωt)/R. (B) R n² I₀ cos(ωt). (C) a² n I₀ sin(ωt)/R. (D) R n² I₀ ω sin(ωt).

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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