PRACTICE EXAM 2 for Midterm 2

Transcription

1 PRACTICE EXAM 2 for Midterm 2

2 Multiple Choice Questions 1) In the circuit shown in the figure, all the lightbulbs are identical. Which of the following is the correct ranking of the brightness of the bulbs? A) B and C have equal brightness, and A is the dimmest. B) A and B have equal brightness, and C is the dimmest. C) A is brightest, C is dimmest, and B is in between. D) A is the brightest, and B and C have equal brightness but less than A. E) All three bulbs have the same brightness. Answer: D 2) An RC circuit is connected across an ideal DC voltage source through an open switch. The switch is closed at time t = 0 s. Which of the following statements regarding the circuit are correct? (There may be more than one correct choice.) A) The capacitor charges to its maximum value in one time constant and the current is zero at that time. B) The potential difference across the resistor and the potential difference across the capacitor are always equal. C) The potential difference across the resistor is always greater than the potential difference across the capacitor. D) The potential difference across the capacitor is always greater than the potential difference across the resistor E) Once the capacitor is essentially fully charged, there is no appreciable current in the circuit. Answer: E 3) An electron moving in the direction of the +x-axis enters a magnetic field. If the electron experiences a magnetic deflection in the -y direction, the direction of the magnetic field in this region points in the direction of the A) +z-axis. B) -z-axis. C) -x-axis. D) +y-axis. E) -y-axis. Answer: B

3 4) A vertical wire carries a current vertically upward in a region where the magnetic field vector points toward the north. What is the direction of the magnetic force on this current due to the field? A) downward B) toward the north C) toward the south D) toward the east E) toward the west Answer: E 5) A long straight conductor has a constant current flowing to the right. A wire rectangle is situated above the wire, and also has a constant current flowing through it (as shown in the figure). Which of the following statements is true? A) The net magnetic force on the wire rectangle is upward, and there is also a net torque on the it. B) The net magnetic force on the wire rectangle is zero, and the net torque on it is zero. C) The net magnetic force on the wire rectangle is downward, and there is also a net torque on the it. D) The net magnetic force on the wire rectangle is zero, but there is a net torque on it. E) The net magnetic force on the wire rectangle is downward, and the net torque on it is zero. Answer: E

4 6) A very long, solid, conducting cylinder of radius R carries a current along its length uniformly distributed throughout the cylinder. Which one of the graphs shown in the figure most accurately describes the magnitude B of the magnetic field produced by this current as a function of the distance r from the central axis? A) 1 B) 2 C) 3 D) 4 E) 5 Answer: A

5 7) A coil lies flat on a tabletop in a region where the magnetic field vector points straight up. The magnetic field vanishes suddenly. When viewed from above, what is the direction of the induced current in this coil as the field fades? A) counter-clockwise then clockwise B) clockwise then counter-clockwise C) clockwise D) counter-clockwise E) There is no current induced in the coil. Answer: D 8) In the figure, a copper bar is in contact with a pair of parallel metal rails and is in motion with velocity ν. A uniform magnetic field is present pointing downward, as shown. The bar, the rails, and the resistor R are all in the same plane. The induced current through the resistor R is A) from a to b. B) from b to a. C) There is no induced current through the resistor. Answer: C 6) In the figure, the current in a solenoid having no appreciable resistance is flowing from b to a and is decreasing at a rate of 9.6 A/s. The self-induced emf in the solenoid is found to be 8.4 V. Which point, a or b is at higher potential? A) a B) b Answer: A 10) At what rate would the current in a 100-mH inductor have to change to induce an emf of 1000 V in the inductor? A) 100 A/s B) 1 A/s C) 1000 A/s D) 10,000 A/s E) 10 A/s Answer: D

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7 Note that part (b) of the above problem is technically chapter 31 material, and would not appear on the test.