PHYSICS 30 ELECTRIC FIELDS ASSIGNMENT 1 55 MARKS

Transcription

1 For each of the following questions complete communication must be shown. Communication consists of an introduction to the physics of the situation, diagrams, word explanations and calculations in a well laid out formula, substitution, answer format. 1. In a demonstration, a student drops a neutral polystyrene sphere toward the top of the dome of a negatively charged Van de Graaff generator. The polystyrene sphere is coated with an electrically conductive surface. (10 marks) Observation I The student initially observes the neutral sphere fall toward the Van de Graaff generator, as shown below. Observation II The student next observes the sphere touch the top of the generator and then move upward, as shown below. Use the concepts associated with electric field, electrical force, and charging by induction and charging by conduction to analyze the demonstration and observations given above. In your analysis of the demonstration above, - draw and label field lines that show the shape and direction of the electric field around the negatively charged dome of a Van de Graaff generator In your analysis of observation I, - describe the rearrangement of charge that occurs on the neutral sphere as it falls toward the top of the Van de Graaff generator, and explain why this occurs - predict the change in the magnitude of the acceleration of the neutral sphere as it falls toward the top of the Van de Graaff generator, and justify your prediction In your analysis of observation II, - explain why the sphere moves upward away from the Van de Graaff generator 1

2 2. A small conductive sphere is held in a fixed position by a fine silk thread and the force due to a charged Perspex rod, as shown in the diagram. (10 marks) a) The perspex rod is similar to ebonite. Explain the charge on the rod, and how the rod is capable of holding the sphere in position. Three forces, the weight (F g ) of the sphere, the tension (T) in the thread, and the electrostatic force (F e ) due to the charged rod, act on the sphere which is being held in a stationary position. b) Draw labelled arrows, on the diagram above, to show the direction of each of the three forces acting on on the sphere which is being held in a stationary position. c) Draw a labelled vector diagram of the three forces. d) If the mass of the sphere is 2.00 x 10-3 kg, calculate the magnitude of the weight (F g ) of the sphere, the tension (T) in the thread, and the electrostatic force (F e ) due to the charged rod, 3. In a modified Millikan apparatus, a small charged object that has a mass of 2.5 x 10-6 kg is suspended by the electric field between charged parallel plates. The students conducting the experiment measured the balancing voltage required for different plate separations. (10 marks) Plate Separation (mm) Balancing Voltage (V) a) Plot a graph of balancing voltage and plate separation, with the manipulated variable on the horizontal axis. b) Calculate the slope of the graph. What physical quantity or quantities does this slope represent? c) Use a suitable averaging technique to determine the magnitude of the charge on the suspended object. 2

3 Choose the best response and place your answers, using HB pencil, on the Scantron sheet provided. 1. What is the net static electric charge on a metal sphere having an excess of +3 elementary charges? a C b C c C d C 2. A lightning strike delivers 20 C of charge in 3.0 ms. The number of electrons transferred to the ground, during the strike is a.b x 10 cd electrons. The values of a, b, c, and d, respectively, are,,, and. (Record all four digits of your answer on the answer sheet.) 3. Rules Governing the Redistribution of Charges on Objects I. The charge distribution at a pointed part of the surface will be greater than over smoothly curved portions. II. Electrostatic forces of repulsion cause excess charges to move to the outside surfaces of conducting objects. III. Electrostatic forces of attraction cause excess charges to move to the inside surfaces of insulating objects. IV. Electrostatic forces of repulsion cause charges to spread out evenly along the surfaces of conductors. The rules governing the redistribution of excess charges on two oppositely charged objects are: a. I, II, and III b. I, III, and IV c. I, II, and IV d. II, III, and IV 4. Given an ebonite rod, fur, and an electroscope, steps in an experimental procedure to determine the unknown charge on a sphere, are stated below: 1. Rub the ebonite rod with the fur. 2. Touch the ebonite rod to the electroscope. 3. Bring the electroscope close to the sphere with an unknown charge. 4. Observe the leaves on the electroscope. The methods of charging objects in steps 1, 2, and 3, respectively, are: a. friction, conduction, and induction b. conduction, friction, and induction c. induction, conduction, and friction d. conduction, induction, and friction 3

4 Use the following information to answer the next three questions. To determine the electric force on a 2.5 x 10-4 kg neutral pith ball, a student charges a Van de Graaff generator and suspends the pith ball by an insulating thread. 5. When the neutral pith ball is placed near the charged Van de Graaff generator, the pithball is attracted to the generator as a result of a. induction b. grounding c. conduction d. induction and grounding 6. The direction of the electrical force on the pith ball is a. b. c. d. 7. The magnitude of the electrical force exerted on the pith ball by the charged Van de Graaff generator is a N b N c N d N 8. 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 A and C repel one another. Which one of the following table entries is a possible combination of the signs of the net charges on these three objects? A B C A B C A B C a b c d e f. 9. An ebonite rod that has been rubbed with fur is brought near the knob of a positively charged electroscope. In response, the leaves of the electroscope will: a. spark b. converge together c. diverge further d. remain unchanged 4

5 10. A hollow metal can has been charged negatively. A positively charged sphere, suspended by an insulating thread, is placed at positions A (outside the can), and B (inside the can). The positively charged sphere, when it is placed at the positions shown, would: a. swing to the right at A and to the left at B b. swing to the left at A and to the right at B c. swing to the right at A and remain stationary at B d. remain stationary at A and swing to the left at B 11. Electrostatic Forces Three charges, A, B, and C are placed at the corners of a right angle triangle, as shown in the diagram. Using the coordinate system, the angle of the net electrostatic force on C is? (Record your three-digit answer on the answer sheet.) 5

6 12. Historical Theories of the Nature of Electric Charges Theory Proposed by: A. Objects contain positively charged protons in a 1. Ancient Greeks nucleus surrounded by negatively charged electrons. B. There are two types of electric charges because 2. Benjamin Franklin charged objects can exhibit attractive or repulsive effects. C. Electric charges could be positive or negative. 3. Rutherford and Bohr The correct match of theories A, B, and C, to the corresponding numbers, respectively, are,, and. (Record all three digits of your answer on the answer sheet.) 13. The direction of the electric field is defined as the direction of the force on a small test a. mass b. positive charge c. negative charge d. north magnetic pole 14. With reference to the directions above, given the number to match each of the descriptions below: a. The direction of the electric field between the plates. b. The direction of the electric field at point P due to q2 c. The direction of the net electric field at the point P. d. The direction of the electric force on q2 due to the plates. The values of a, b, c, and d, are,,, and. (Record all four digits of your answer in the numerical-response section on the answer sheet.) 6

7 15. The magnitude of the electric field at a distance "r" from a point charge is " point charge doubles, the magnitude of the electric field should be: E ". When the distance from the a. b. c. d A electric dipole is released from rest in a uniform electric field with the orientation shown. Which entry in the table below correctly describes the net torque and the net translational force on the dipole? net torque net force net torque net force a. zero zero b. clockwise zero c. counterclockwise zero d. clockwise non-zero e. counterclockwise non-zero 17. Which of the following best describes how electric potential varies with distance in the region around a point charge? a. V r b. V 1/r c. V r 2 d. V 1/r A proton is made to travel in a straight line near a fixed positively charged object as shown in the diagram below. What is happening to the proton s electric potential energy as it travels from A to B? a. It is increasing. b. It is decreasing. c. It is increasing then decreasing. d. It is decreasing then increasing. 19. Which physical quantity is correctly paired with its unit? a. power and W s b. energy and N s c. electric current and A/C d. electric potential difference and J/C 20. An object, which has a charge of 2 Coulombs, is moved through a potential difference of 10 Volts. The magnitude of the work done on the object by the electric forces is a. 2 Joules b. 5 Joules c. 10 Joules d. 20 Joules e. 40 Joules 7

8 21. Two metal parallel plates lie in the horizontal plane, 1.50 cm apart. The electric field between the plates is 16 N/C. The kinetic energy gained by an alpha particle as it moves from the positive plate to the negative plate is: a x 10-1 J b x J c x J d x J 22. An operating lamp draws a current of 0.50 A. The amount of charge passing through the lamp in 10 s is: a C b. 2.0 C c. 5.0 C d. 20 C 23. Two wires A and B are made of the same material and have the same diameter. Wire A is twice as long as wire B. If each wire has the same potential difference across its ends, which one of the following statements is true concerning the current in wire A? a. The current is one-fourth that in B. b. The current is four times that in B. c. The current is equal to the current in B. d. The current is half as much as that in B. e. It is twice as much as that in B. f. 24. A charged sphere, located in the space between two flat parallel plates connected to a battery, experiences an electrical force of 3.32 x 10-3 N. The plates are kept at the original separation, If the charge on the sphere is reduced to exactly one-half and the potential difference between the plates is exactly tripled, then the force on the charged sphere will be b x 10-3 N. The value of b is. (Round and record your answer to three digits.) 25. An object cannot have a charge of a C b C c C d C 8