Physics 126 Fall 2004 Practice Exam 1. Answer will be posted about Oct. 5.

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1 Physics 126 Fall 2004 Practice Exam 1. Answer will be posted about Oct Which one of the following statements best explains why tiny bits of paper are attracted to a charged rubber rod? A) Paper is naturally a positive material. B) Paper is naturally a negative material. C) The paper becomes polarized by induction. D) Rubber and paper always attract each other. E) The paper acquires a net positive charge by induction. 2. Three charges are positioned as indicated in the figure. What are the horizontal and vertical components of the net force exerted on the +15 µc charge by the +11 µc and +13 µc charges? horizontal vertical A) 95 N 310 N B) 76 N 310 N C) 250 N 130 N D) 95 N 130 N E) 76 N 370 N Page 1

2 3. Four point charges are held fixed at the corners of a square as shown in the figure. Which of the five arrows shown below most accurately shows the direction of the net force on the charge Q due to the presence of the three other charges? A) B) C) D) E) Use the following to answer question 10: 4. The figure shows an equilateral triangle ABC. A positive point charge +q is located at each of the three vertices A, B, and C. Each side of the triangle is of length a. A point charge Q (that may be positive or negative) is placed at the mid-point between B and C. Determine an expression for the magnitude and sign of Q so that the net force on the charge at A is zero newtons. Page 2

3 5. Two particles of the same mass carry charges +3Q and 2Q, respectively. They are shot into a region that contains a uniform electric field as shown. The particles have the same initial velocities in the positive x direction. The lines, numbered 1 through 5, indicate possible paths for the particles. If the electric field points in the negative y direction, what will be the resulting paths for these particles? A) path 1 for +3Q and path 4 for 2Q B) path 3 for +3Q and path 2 for 2Q C) path 4 for +3Q and path 3 for 2Q D) path 2 for +3Q and path 5 for 2Q E) path 5 for +3Q and path 2 for 2Q Page 3

4 6. The figure shows the electric field lines in the vicinity of two point charges. Which one of the following statements concerning this situation is true? A) q 1 is negative and q 2 is positive. B) The magnitude of the ratio (q 2 / q 1 ) is less than one. C) Both q 1 and q 2 have the same sign of charge. D) The magnitude of the electric field is the same everywhere. E) The electric field is strongest midway between the charges. 7. What is the electric flux passing through a Gaussian surface that surrounds a C point charge? A) 8.5 x 10 9 Nm 2 /C B) 6.8 x 10 8 Nm 2 /C C) 1.3 x 10 7 Nm 2 /C D) 4.9 x 10 6 Nm 2 /C E) 7.2 x 10 5 Nm 2 /C 8. Which one of the following statements best explains why it is possible to define an electrostatic potential in a region of space that contains an electrostatic field? A) Work must be done to bring two positive charges closer together. B) Like charges repel one another and unlike charges attract one another. C) A positive charge will gain kinetic energy as it approaches a negative charge. D) The work required to bring two charges together is independent of the path taken. E) A negative charge will gain kinetic energy as it moves away from another negative charge. Page 4

5 9. A charge q = 4.0 µc is moved 0.25 m horizontally to point P in a region where an electric field is 150 V/m and directed vertically as shown. What is the change in the electric potential energy of the charge? A) 2.4 x 10 3 J B) 1.5 x 10 4 J C) zero joules D) +1.5 x 10 4 J E) +2.4 x 10 3 J Use the following to answer questions : Two point charges are held at the corners of a rectangle as shown in the figure. The lengths of sides of the rectangle are m and m. Assume that the electric potential is defined to be zero at infinity. 10. Determine the electric potential at corner A. A) +6.0 x 10 4 V B) 2.4 x 10 5 V C) +4.6 x 10 5 V D) 7.8 x 10 5 V E) zero volts Page 5

6 11. What is the electric potential energy of a +3.0 _C charge placed at corner A? A) 0.10 J B) 0.18 J C) 2.3 J D) 3.6 J E) zero joules 12. Which one of the following statements best describes the equipotential surfaces surrounding a point charge? A) The equipotential surfaces are planes extending radially outward from the charge. B) The equipotential surfaces are curved planes surrounding the charge, but only one passes through the charge. C) The equipotential surfaces are concentric cubes with the charge at the center. D) The equipotential surfaces are concentric spheres with the charge at the center. E) The equipotential surfaces are concentric cylinders with the charge on the axis at the center. 13. A parallel plate capacitor with plates of area A and plate separation d is charged so that the potential difference between its plates is V. If the capacitor is then isolated and its plate separation is decreased to d/2, what happens to the potential difference between the plates? A) The potential difference is increased by a factor of four. B) The potential difference is twice it original value. C) The potential difference is one half of its original value. D) The potential difference is one fourth of its original value. E) The potential difference is unchanged. 14. The potential difference across the ends of a wire is doubled in magnitude. If Ohm's law is obeyed, which one of the following statements concerning the resistance of the wire is true? A) The resistance is one half of its original value. B) The resistance is twice its original value. C) The resistance is not changed. D) The resistance increases by a factor of four. E) The resistance decreases by a factor of four. Page 6

7 15. When a light bulb is connected to a 4.5 V battery, a current of 0.16 A passes through the bulb filament. What is the resistance of the filament? A) 440 Ohms B) 28 Ohms C) 9.3 Ohms D) 1.4 Ohms E) 0.72 Ohms 16. Which one of the following statements concerning resistors in series is true? A) The voltage across each resistor is the same. B) The current through each resistor is the same. C) The power dissipated by each resistor is the same. D) The rate at which charge flows through each resistor depends on its resistance. E) The total current through the resistors is the sum of the current through each resistor. 17. A 4.5-V battery is connected to two resistors connected in series as shown in the drawing. Determine the total power dissipated in the resistors. A) W B) W C) 0.15 W D) 0.60 W E) 4.7 W Page 7

8 18. Two resistors are arranged in a circuit that carries a total current of 15 A as shown in the figure. Which one of the entries in the following table is correct? A) A B) B C) C D) D E) E Use the following to answer questions : The figure shows a simple RC circuit consisting of a 10.0-µF capacitor in series with a resistor. Initially, the switch is open as suggested in the figure. The capacitor has been charged. The potential difference between its plates is V. At t = 0 s, the switch is closed. The capacitor discharges exponentially so that 2.0 s after the switch is closed, the potential difference between the capacitor plates is 37 V. In other words, in 2.0 s the potential difference between the capacitor plates is reduced to 37 % of its original value. Page 8

9 19. Calculate the electric potential energy stored in the capacitor before the switch is closed. A) 0.01 J B) 0.02 J C) 0.03 J D) 0.04 J E) 0.05 J 20. Determine the numerical value of the resistance R. A) Ohms B) Ohms C) Ohms D) Ohms E) Ohms 21. Which one of the following statements concerning the magnetic force on a charged particle in a magnetic field is true? A) It is a maximum if the particle is stationary. B) It is zero if the particle moves perpendicular to the field. C) It is a maximum if the particle moves parallel to the field. D) It acts in the direction of motion for a positively charged particle. E) It depends on the component of the particle's velocity that is perpendicular to the field. 22. An electron traveling due north enters a region that contains a uniform magnetic field that points due east. In which direction will the electron be deflected? A) east B) west C) down D) up E) south Page 9

10 23. Two charged particles of equal mass are traveling in circular orbits in a region of uniform, constant magnetic field as shown. The particles are observed to move in circular paths of radii R 1 and R 2 with speeds v 1 and v 2, respectively. As the figure shows, the path of particle 2 has a smaller radius than that of particle 1. Which one of the following statements about this system is false? A) v 1 /Q 1 < v 2 /Q 2 B) Particle 2 carries a positive charge. C) Particle 1 carries a negative charge. D) Neither particle gains energy from the magnetic field. E) The particle velocities have no components parallel to the magnetic field. 24. Two loops carry equal currents I in the same direction. The loops are held in the positions shown in the figure and are then released. Which one of the following statements correctly describes the subsequent behavior of the loops? A) Both loops move to the left. B) The loops remain in the positions shown. C) The top loop moves to the right; the bottom loop moves to the right. D) The loops repel each other. E) The loops attract each other. Page 10

P202 Practice Exam 1 Spring 2004 Instructor: Prof. Sinova

P202 Practice Exam 1 Spring 2004 Instructor: Prof. Sinova Name: Date: 1. Each of three objects has a net charge. Objects A and B attract one another. Objects B and C also attract one another, but objects