Physics 202 Midterm Exam 2 Oct 30th, 2012

Transcription

1 ID CODE: B Physics 202 Midterm Exam 2 Oct 30th, 2012 Name:...Yibin Pan... Student ID:... Section:... TA (please circle): James Buchannan Diptaranjan Das Ross Devol Yutao Gong Minho Kwon Greg Lau Andrew Loveridge Tao Peng Ben Stefanek Gandhari Wattal Instructions: 1. Donʼt forget to write down your name, student ID#, and section number. You need do this on (this page of) your test book and on your Scantron sheet as well. 2. Answer all multiple choice questions in this test book by indicating the best answer among choices. You must do this both on your test book and on your Scantron sheet. Follow instructions on the Scantron sheet on how to mark valid answers. 3. When you finish, you need to turn in both this test book and the Scantron sheet. 4: Use the blank side of question pages as additional draft spaces. You can also use empty space at the end of the test book. 5: Only one answer is allowed per problem/question. All problems have equal weight. Constants: k e = 9x10 9 Nm 2 /C 2 = 1/(4πε 0 ), ε 0 = 8.85x10-12 C 2 /(Nm 2 ) µ 0 =4πx10-7 Tm/A Please be very careful with the first question even though the answer will not count towards your grade: 1. ENTER THE ID CODE ABOVE IN THE UPPER RIGHT CORNER A. ID Code A B. ID Code B C. ID Code C D. ID Code D E. ID Code E

2 2. The figure below represents a section of a circular conductor of nonuniform diameter.the radius of cross-section A 1 is r 1 = cm. The current at A 1 is I 1 = 4.30 A. What is the magnitude of the current density j 1 across A 1? A x10 2 A/m 2 B x10 3 A/m 2 C x10 3 A/m 2 = I/A 1 = 4.3/(πr 2 1 ) This was in your HW5. D x10 4 A/m 2 E. None of above is within 10% of correct answer. 3. In the above figure, consider the cross-section A 2,which has twice as much radius compared to A 1. Consider the current density (j 2 ) and current (I 2 ) at A 2, which of the following statements is true? A. j 2 = j 1, I 2 = 2I 1 B. j 2 = j 1 /4, I 2 = I 1 I is conserved, j (=I/A) is not. C. j 2 = j 1, I 2 = 4I 1 D. j 2 = j 1 /2, I 2 = 2I 1 E. None of above is true. 4. An incandescent (i.e. resistive) light bulb ( bulb A ) is rated 80W at 120V. What is the resistance of light bulb A? A. 80 Ω B. 180 Ω P=ΔV 2 /R à R=ΔV 2 /P = /80 = 180 Ω C. 144 Ω D. 120 Ω E. None of above is within 5% of correct answer. 5. If in a lighting circuit the above light bulb A is connected in series with another incandescent light bulb (bulb B) which is rated 40W at 120V. If the brightness is proportional to the power a light bulb consumes, which of the following statements is true? A. Light bulb A is twice as bright as B B. Light bulb A is half as bright as B When in series, I is the same. P=I 2 R à P A /P B =R A /R B =1/2 C. Both are same as bright. D. Light bulb A is 1/4 as brighter E. None of above.

3 ** The figure below is referred to in the next four questions 6. In the circuit above, the currents through resistors R 1/2/3 are I 1 =1A, I 2 =3A, and I 3 =8A, respectively, in their actual directions shown. Other components are labeled as shown. (note: there is no ε 1 ) What is the current I 4? A. 12 A upwards B. 12 A downwards C. 4 A upwards Per junction rule. (first do junction above ε2, then the one above ε3 ) D. 4 A downwards E. None of above is within 0.1A from the correct answer. 7. In the circuit above, what is resistance R 1? A. 200 Ω Loop R 1 ε 2 R 2 : -I 1 R 1 -ε 2 +I 2 R 2 =0 à R 1 =200Ω B. 240 Ω C. 160 Ω D. 120 Ω E. None of above is within 5% from the correct answer. 8. Still in the above setting, what is emf ε 4? A. 160 V, the upper end is positive. B. 160 V, the upper end is negative. C. 80 V the upper end is positive. Loop R 3 ε 3 ε 4 R 4, for ε 4, first assume upper end is positive : I 3 R 3 -ε 3 -ε 4 +I 4 R 4 =0 à ε 4 =80V >0, so really upper end is positive. D. 50 V the upper end is negative. E. The above answers are either wrong in polarization or off by more than 10% in value. 9. Yet still in the above setting, what is the total power consumed in the circuit? A W B W = I 1 2 R 1 + I 2 2 R 2 + I 3 2 R 3 + I 4 2 R 4 (or P= I 2 ε 2 + I 3 ε 3 + I 4 ε 4 ) C W D W E. None of above is within 50 W from the correct answer.

4 10. A simple RC series circuit is shown below. Initially the switch is open (i.e. disconnected) and the capacitor is not charged. It is known that the capacitance is C=2.0 µf. At t=0, the switch is closed and the capacitor is being charged. If it takes 5.0s to charge the capacitor to the 70% of full charge. What is the resistance R? A. R=2.1 x10 6 Ω 0.7= 1-exp(-t 0.7 /τ) à τ = - 5s/ln(1-0.7) = 4.19s =RC à R=2.08 MΩ B. R=7.0 x10 6 Ω C. R=4.2 x10 6 Ω D. None of above is within 10% of correct answer. E. Insufficient information as ΔV is not given. 11. As shown below, a solenoid coil is connected with a battery and placed near a bar magnet. Consider the magnetic forces in between them, which of the following statements is true? A. The coil is subject to a force to the left, the magnet is subject a force to the right B. The coil is subject to a force to the right, the magnet is subject a force to the left Per right hand rule, the coil has south end at left. C. Both are subject forces to the left D. Both are subject forces to the right E. None of above.

5 12. At t=0, a negative point charge of q=-1.0 µc enters a uniform magnetic field in the +k direction. The initial velocity of the charge is v= 6.0 i j m/s. The strength of the magnetic field is 2.0 T. The mass of the charge is 1.0x10-9 kg. (see picture below). What is the magnetic force on the charge at t=0? A x 10-6 N B. (-4.0 i j)x10-6 N C. (4.0 i j)x10-6 N F=qvxB, see review exercise #4. D. (-4.0 i j)x10-6 N E. None of above. 13. In the above setting, be convinced that the particle will be moving in circular path under the influence of the magnetic field. Which of the followings best describes its circular motion? A. circling clockwise in xy plane B. circling counter-clockwise in xy plane (per right hand rule or use F above) C. circling in xz plane D. circling in yz plane E. The circular path falls in none of the planes mentioned above. 14. Still in above setting, what is the radius of the circular path? A. 4.8 mm B. 3.2 mm =mv/(qb), v=sqrt(v x 2 +v y 2 ) C. 7.4 mm D. 8.7 mm E. None of above is within 0.5 mm of correct answer. 15. Yet still in the above setting, what is the works done to the charged particle in the time duration in which that the particle completes 10 circles? A. 0 (magnetic force never does work) B. 1.7 x10-6 J C. 5.4 x10-6 J D. 8.9 x10-6 J E. none of above is within 10% of correct answer.

6 16. As shown, three concentric conducting cylinders have radius 1.0m, 2.0m, and 4.0m, respectively. They all have length of infinity. The currents carried by the cylinders are 1A into page, 2A out of page, and 3A into page, respectively from the inner to the outer ones. What is the magnetic field at d=5.0m on the right from the center? A. 0.4 x10-7 T, downward B. 0.6 x10-7 T, upward C. 0.8 x10-7 T, downward, Per Ampere s Law, B=µ 0 I enc /(2πr), I enc Use Right Hand Rule (RHR) for direction. D. 1.0 x10-7 T, upward E. The above answers are either wrong in direction or off from true magnitude by more than 10%. 17. In the above setting, what is the magnetic field at d=3.5 m on the right from the center? A. 0.4 x10-7 T, downward B. 0.6 x10-7 T, upward, same as in the last one, different I r=3.5m C. 0.8 x10-7 T, downward D. zero E. The above answers are either wrong in direction or off from true magnitude by more than 10%.

7 18. Figures below illustrate possible configurations of a Velocity Selector. By adjusting E and B fields, a velocity selector allows charged particles with only certain velocity to pass through. Which of the following statements is true? A. Config. A works only for positive charge and config. B only for negative charge. B. Config. A works only for negative charge and config. B only for positive charge. C. Config. A won t work at all D. Config. B won t work at all per right hand rule, for either charge F B and F E are opposite in config. A (good), but in config. B F B and F E are in the same dir. (bad) E. The two configurations work for both (positive/negative) charges. 19. The figure below shows the trajectory of a charged particle of velocity v after entering into a B field. Is the charge of the particle A. positive? B. negative? Use right hand rule. C. not enough information to determine.

8 20. Two long, parallel conductors, separated by 14.0 cm, carry currents in the direction as shown. The first wire carries a current I 1 = 6.00 A, and the second carries I 2 = 8.00 A. (See figure below. Assume the conductors lie in the plane of the page.) Let F 1 and F 2 denote magnetic forces on wire 1 and 2, respectively. What can we say about the directions of these two forces? A. F 1 upward, but F 2 downward B. F 1 downward but F 2 upward Use RHR to verify that they attract each other. C. both upward D. both downward E. None of above, the forces are in left or right direction. 21. In the setting above, the ratio between the (magnitudes of) two forces, F 1 :F 2, is: A. 4:3 B. 3:4 C. 16:9 D. 9:16 E. None of above (=1:1, per Newton s 3 rd law, or note F ~ I 1 I 2. )