9. With a 50 Ω input impedance oscilloscope, one observes a 1 GHz noise with 20 mV amplitude through a coaxial cable. Approximately how many electrons are running back-and-forth at the input end due to this noise signal? (A) 10³ (B) 10⁶ (C) 10⁹ (D) 10¹²

4. A point-like bug of mass m lies on the rim of a uniform disk of mass M that can rotate freely about its center. Initially the bug and disk rotate together with an angular velocity ω₀. Then the bug walks half way to the center of the disk. What is ratio of the new kinetic energy of the system to its initial kinetic energy? (A) (4m+2M)/(m+2M) (B) (m+2M)/(4m+2M) (C) (2m+4M)/(2m+M) (D) (2m+M)/(2m+4M)

9. With a 50 Ω input impedance oscilloscope, one observes a 1 GHz noise with 20 mV amplitude through a coaxial cable. Approximately how many electrons are running back-and-forth at the input end due to this noise signal?
(A) 10³
(B) 10⁶
(C) 10⁹
(D) 10¹²

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