NAME: PHYSICS 6B SPRING 2011 FINAL EXAM ( VERSION A )

Transcription

1 NAME: PHYSCS 6B SPRNG 2011 FNAL EXAM ( VERSON A ) Choose the best answer for each of the following multiple-choice questions. There is only one answer for each. Questions 1-2 are based on the following situation, pictured at right: Two protons are initially at rest and separated by a distance d = 10-6 m. They are surrounded by a uniform B field of 0.05 T (created by an external magnet). 1. What is the magnitude of the electric force exerted on charge 2 by charge 1? B + + d 1 2 A) N B) N C) N D) N 2. Due to the repulsive electric force, the protons acquire a velocity moving away from each other. What is the direction of the magnetic force exerted by the external B field on proton 1? A) out of the page ( ) B) into the page ( ) C) toward proton 2 D) away from proton 2 3. An empty aluminum box (cubical shape, each side is 1 meter long) floats with 50% of its volume above the water surface. How much mass would you have to add to it to make it barely sink? (Assume ρ water = 1000 kg / m 3 ) A) 200 kg B) 800 kg C) 500 kg D) 650 kg 4. When a human is standing, where in the body is blood pressure the highest? (gnore the motion of the blood assume it is standing still) A) head B) stomach C) wrists D) feet Questions 5-6 are based on the following situation: A bar magnet is dropped through a horizontal copper ring (radius = 10 cm). At some instant during the fall, the induced current in the ring is 0.05 A. 5. What is the magnitude of the induced B field at the center of the ring? A) T B) T C) T D) T S N 6. The induced current in the ring exerts a force on the magnet as it falls. What is the direction of this force? A) upward B) downward C) to the east D) to the west 7. A bat flies straight toward a brick wall at a speed of m/sec. t emits a sonar signal of frequency 40,000 Hz toward the wall. The waves reflect back to the bat. What is the frequency of the reflected waves, as heard by the bat? (Assume the speed of sound in air is 340 m/sec.) A) 43,197 Hz B) 41,753 Hz C) 42,424 Hz D) 41,034 Hz

2 + + Questions 8-9 are based on the situation pictured at right: A wire loop ( radius 2.0 cm ) carries a current of 0.50 A as shown. t is in a uniform externallycreated B field of 0.60 T, and its axis makes an angle of 30 with the B field. 8. What is the magnitude of the torque exerted on the loop by the B field? A) N m B) N m C) N m D) N m 30 B 9. What is the magnetic flux of the externally-created B field through the loop? A) Weber B) Weber C) Weber D) Weber Questions are based on the following situation: A parallel-plate capacitor has a capacitance of 100 mf. The capacitor is fully charged, using a battery with an EMF of 10.0 V. The capacitor is then disconnected from the battery. 10. An electron is brought very close to the negative plate and released from rest. t accelerates toward the positive plate. What is the electron's kinetic energy just before it reaches the positive plate? A) J B) J C) J D) J 11. The initial energy stored in the capacitor is U 0. An external force now pulls the plates apart, causing the distance between the plates to double. What is the new amount of energy stored in the capacitor? A) 2U 0 B) U 0 / 2 C) 4 U 0 D) U 0 / 4 Questions are based on the circuit shown at right: There is no internal resistance in the batteries. 2.0 Ω 12. What is the total current through the 1.0 Ω resistor? A) 1.0 A B) 2.0 A C) 3.0 A D) 4.0 A 6.0 V 1.0 Ω 4.0 V 13. f the 2.0 Ω resistor is changed to a 4.0 Ω resistor, what happens to the current drawn from the 6.0 V battery? A) increases B) decreases C) is unchanged D) goes to zero Questions are based on the situation shown at right: Two parallel wires are each 1.50 meters long. The wires are separated by a distance of 5.00 cm. They carry parallel currents: 1 = 1.00 A and 2 = 2.00 A 14. What is the force exerted by wire 1 on wire 2? A) N B) N C) N D) N There is a location where the B fields produced by 1 and 2 cancel each other (producing a total B field of zero). How far is this location from wire 1? A) 1.67 cm B) 2.50 cm C) 3.33 cm D) 1.25 cm

3 Questions are based on the following situation: A guitarist is attempting to tune 2 identical strings: One is to be tuned to a fundamental frequency of 200 Hz, the other is to be tuned to 300 Hz. 16. What should be the ratio of the tensions in the two strings? A) 1.50 B) 2.25 C) 1.22 D) Which of the following frequencies is a harmonic of both strings? A) 300 Hz B) 500 Hz C) 600 Hz D) 800 Hz Questions are based on the following situation: An LC circuit is constructed using a 5.0 mf capacitor and a 20.0 mh inductor. At the instant t = 0, there is no current in the inductor and the capacitor has a charge of 0.05 C. Assume the circuit has no electrical resistance. 18. What is the maximum current in the inductor? A) 5.0 A B) 3.0 A C) 7.5 A D) 4.0 A 19. At which of the following times is there more energy stored in the magnetic field than the electric field? A) t = 15.7 msec B) t = 31.4 msec C) t = 62.8 msec D) t = 94.2 msec Questions are based on the following situation: A proton with a speed of 4000 m/sec enters a magnetic field of 0.04 T and moves in a circle around the field lines. 20. What is the radius of this circle? A) 3.91 mm B) 5.31 mm C) 7.22 mm D) 1.04 mm 21. Moving at the same speed, an electron would move in a circle with compared to the proton. A) smaller radius B) same radius C) larger radius D) zero radius Questions are based on the following situation: A 3.0 kg mass is attached to a spring with constant k = N / m. t oscillates on a frictionless horizontal surface. 22. What is the period of this vibration? A) 0.78 sec B) 1.09 sec C) 1.63 sec D) 0.33 sec 23. f the maximum kinetic energy of the oscillation is 24.0 J, what is the amplitude of the oscillation? A) 0.28 m B) 0.51 m C) 0.69 m D) 1.32 m 24. The parts of a generator are similar to those of a. A) battery B) resistor C) capacitor D) motor

4 Questions are based on the following situation: Two capacitors ( 1.0 mf and 2.0 mf ) are connected in series. The total voltage drop across the combination is 6.0 V. 25. What is the voltage drop across the 2.0 mf capacitor? A) 2.0 V B) 0.6 V C) 1.2 V D) 4.0 V 26. What is the total energy stored by the two capacitors? A) 2.8 mj B) 4.4 mj C) 8.9 mj D) 12.0 mj Questions are based on the following situation: An inductor ( L = 20.0 mh ) and a resistor ( R = 10.0 Ω ) are connected in series. At the instant t = 0, they are connected to a battery with no internal resistance and an EMF of 8.0 V. 27. At the instant just after t = 0, what is the current through the circuit? A) 0.0 A B) 1.0 A C) 2.0 A D) 3.0 A 28. After a long time, the current has reached a steady value. What is the current at this time? A) 0.32 A B) 0.80 A C) 1.31 A D) 0.58 A Questions are based on the following situation: A transformer has 10 turns of wire in the primary coil and 40 turns of wire in the secondary coil. The primary coil is attached to an AC generator which has a voltage amplitude of 100 V. 29. What is the voltage amplitude of the secondary coil? A) 100 V B) 50 V C) 400 V D) 200 V 30. Assume the transformer is 100% efficient. What is the ratio of the power output (secondary coil) to the power input (primary coil)? A) 2 B) 1 C) 4 D) 16 PLEASE HAVE YOUR PCTURE.D. READY TO SHOW AS YOU HAND N THE EXAM!! HAVE A GOOD SUMMER!

5 CONSTANTS e = C k = N m 2 C 2 0 = N m A m proton = kg m electron = kg F = k q 1 q 2 r 2 B = 0 2 r FORMULAS: F = qv Bsin F L = r U = q V B center = 0 2R = N A B sin B = B Acos R = mv q B v wave = F T f L = v±v L v±v S f S v = f n = 2L n C = 0 A d = k m Q = C V U = 1 2 C V 2 U = Q 2 2C V = R 1 = 1 1 C eq C 1 C 2 C eq = C 1 C 2 R eq = R 1 R 2 T = 2 PE = 1 2 k x2 KE = 1 2 mv 2 1 R eq = 1 R 1 1 R 2 = L t U = 1 2 L 2 = 1 LC V 2 V 1 = N 2 N 1 F bouyant = fluid g Vol displaced P 1 g y v 12 =P 2 g y v 2 2