SPH 4U: Unit 3 - Electric and Magnetic Fields

Transcription

1 Name: Class: _ Date: _ SPH 4U: Unit 3 - Electric and Magnetic Fields Modified True/False (1 point each) Indicate whether the statement is true or false. If false, change the identified word or phrase to make the statement true. 1. Coulomb s law is extremely accurate under the conditions that the spheres are small and that the spheres are also small compared to the distance between them. 2. When the distance between two charged spheres is decreased by a factor of 2, then the electric force decreases by a factor of When the charge on both spheres is doubled, the electric force is increased by a factor of four. _ 4. One of the ways in which Newton s law of universal gravitation differs from Coulomb s law is that gravitational force can only attract, whereas the electric force can only repel. 5. By convention, in electrostatic representations, electric fields start on negative charges and end on positive charges. _ 6. When the net force on an oil drop in a Millikan apparatus is zero, the gravity downward cancels the product of the mass and the electric field on the oil drop. 7. The magnitude of the magnetic force on a charged particle is directly proportional to the magnitude of the magnetic field and the speed and charge of the particle. _ 8. In a loop of wire with a current, the field lines inside the loop are closer together, indicating a stronger magnetic field than on the outside of the loop. _ 9. The SI definition of the ampere is a magnetic one and depends on an understanding of the force between two perpendicular current-carrying conductors. _ 10. In two-dimensional diagrams of magnetic fields Xs are drawn to represent field lines pointing out of and perpendicular to the page. _ 1

2 Name: Multiple Choice (2 points each) Identify the choice that best completes the statement or answers the question. 11. The leaves of a neutral electroscope separate when a positively charged rod is brought close to, but not touching the knob of the electroscope because a. the leaves are positive and the knob is neutral b. electrons are attracted to the knob causing both leaves to be positive and thus repel one another c. the leaves are negative and the knob is neutral d. electrons are repelled from the knob, causing both leaves to be negative and thus repel one another e. one leaf is positive, one leaf is negative, and the knob is neutral 12. When a negatively charged object makes contact with a neutral object, the negative charge is shared between the two objects, and the objects both become negatively charged. This is an example of a. charging by friction d. Coulomb s law b. charging by contact e. charging by induction c. induced charge separation 13. Two charged spheres are 5.00 cm apart. One sphere has a charge of 2.50! 10 "4 C and the other sphere has a charge of 3.20! 10 "5 C. Assuming k = 9.00! 10 9 N # m 2 /C 2, the electric force between the two spheres is a. 1.44! 10 1 N d. 2.88! 10 4 N b. 4.44! N e. 4.44! N c. 2.25! N 14. The electrostatic force between two point charges is 3.4!10 "7 N. If the distance between the charges is tripled but the size of the charges remains the same, the force between them will be a. 3.1!10 "6 N d. 3.8!10 "8 N b. 3.8!10 8 N e. 1.1!10 "7 N c. 2.6!10 7 N 15. The electrostatic force between two point charges is 5.7!10 "3 N. If the distance between the charges is tripled while the charge on one of them triples and the charge on the other doubles, the force between them will be a. 1.3!10 "4 N d. 2.6!10 7 N b. 3.1!10 "6 N e. 3.8!10 "3 N c. 8.6!10 3 N 2

3 Name: 16. Which of the following is NOT a similarity or difference between Coulomb s law and Newton s law of universal gravitation? a. The forces act along the line joining the centres of the masses or charges. b. The electric force can attract or repel, depending on the charges involved, whereas the gravitational force can only attract. c. The universal constant G is very small and in many cases the gravitational force can be ignored. Coulomb s constant k is very large, so that even small charges can result in noticeable forces. d. Coulomb s law is the product of two masses, whereas Newton s law of universal gravitation is the product of one mass. e. The size of the force is the same as the force that would be measured if all the mass or charge is concentrated at a point at the centre of the sphere. 17. Four charged spheres, A, D, P, and T are arranged as shown below. Sphere A has a charge of +4.5!10 "4 C, sphere D has a charge of "6.2!10 "4 C, sphere P has a charge of +7.5!10 "4 C, and sphere T has a charge of "1.5!10 "4 C. Which two spheres exert the smallest force on each other? a. A and D d. P and D b. A and T e. P and T c. A and P 18. Two isolated spheres, A and B, are 2 m apart. Sphere A has charge q and sphere B has charge 2q. The electrostatic force on sphere A is a. the same as on sphere B d. four times that on sphere B b. two times that on sphere B e. not enough information c. half that on sphere B 3

4 Name: 19. Which of the following diagrams best illustrates the electric field in the area around two identical charges? a. d. b. e. c. 20. If point charge q was absent, the electric field at point B would be E. What is the electric field between the two point charges, q and q, at point B which lies at the midpoint between the two charges? a. 2E [right] d. b. 0 e. c. 2E [left] E 2 [left] E 2 [right] 21. How much work is done to move an electron with a charge of 1.6! C from the positive terminal of a 1.5-V battery to the negative terminal? a. 2.4! J b. 9.4! J c. 1.0! J d. 9.6! J e. unable to determine without the distance between the plates 22. The charge, in coulombs, required to suspend a 1.7! kg oil drop in an electric field with intensity 2.5! 10 5 N/C is a. 6.8! C d. 6.7! C b. 4.3! C e. 1.1! C c. 6.1! C 23. A sphere of charge +q is in a fixed position. A smaller sphere +q is placed near the larger sphere and released from rest. Which one of the following best describes its motion? a. decreasing velocity and increasing acceleration b. decreasing velocity and constant acceleration c. increasing velocity and decreasing acceleration d. increasing velocity and increasing acceleration e. decreasing velocity and decreasing acceleration 4

5 Name: 24. Three spheres are equal distances apart as shown below. Spheres X and Y are held in fixed positions but sphere Z is free to move. Which path will sphere Z follow? a. A d. D b. B e. E c. C 25. The potential difference between two parallel plates is 1.7! 10 4 V. To move a small charge from one plate to another opposing the electric field requires 0.56 J of work. The magnitude of the charge is a. 3.0! 10 4 C d. 3.3! 10 5 C b. 9.5! 10 3 C e. 4.8! C c. 1.5! C 26. The direction of a magnetic field B ä is from right to left as shown below. A proton travels into and perpendicular to the plane of the page. The direction of the magnetic force acting on the proton is a. down b. up c. right d. left e. directly out of the page and perpendicular to the paper 27. A positively charged particle of mass 3.0! kg and charge +6.4! 10 5 C is perpendicular to a 1.5-T magnetic field. If the particle has a speed of 4.8! 10 3 m/s, the acceleration of this particle is a. 3.4! 10 4 m/s 2 d. 1.5! 10 4 m/s 2 b. 2.6! m/s 2 e. 1.7! m/s 2 c. 1.5! m/s A 5.0-cm straight conductor carries a current of 12 A through a uniform 0.85-T magnetic field. When the angle between the current and the magnetic field is 25, the magnitude of the force on the conductor is a. 120 N d N b N e. 22 N c. 86 N 5

6 Name: 29. A conductor is located between the poles of a horseshoe magnet. Current flows in the direction indicated by the arrow on the diagram. In which direction will the conductor move? a. upward d. right b. left e. out of the page c. downward 30. An object with charge +q experiences an electric force F E when put in a particular location in the electric field $. The positive charge +q is removed and an object with charge 2q is placed in the same location in the electric field. This charge would feel an electric force of a. 2F E d. 4F E b. "F E 2 "2F E e. "F E 4 c. q Problem (5 points each) Complete 4 of the following 5 problems 31. The electrostatic force between two point charges is 4.7!10 "5 N. If the distance between them is tripled, the charge of one of the points triples and the charge of the other point halves. What will be the force between them? 32. Point charges W, X, Y, and Z each have the same magnitude charge of 4.46! 10 "6 C. Two of the charges are positive as shown and two are negative. The corners of the square are 10.0 cm long. Calculate the net force on each of the point charges. Draw vector diagrams for each point charge. 6

7 Name: 33. (a) What is the electric potential from a charge of 3.0! 10 5 C at 15 cm and 65 cm? (b) How much work must be done to carry a +2.3! 10 3 C charge from 65 cm to 15 cm? 34. An electron accelerates from rest through an electric field and into a magnetic field as shown in the diagram below. The plates have a potential difference of 25 V, and the magnetic field has a magnitude of 0.50 T. (Remember: m e = 9.1! kg and e = 1.6! C.) (a) Calculate the initial speed of the electron upon entering the magnetic field. (b) Calculate the magnitude and direction of the magnetic force on the electron. (c) Calculate the radius of the electron s circular path. 35. At what distance from a straight conductor, carrying a current of 15.0 A, is the magnitude of the magnetic field 9.4! 10 6 T? 7