Problem 1. What is the force between two small charged spheres that have charges of C and C and are placed 30 cm apart in air?

Transcription

1 5. NAME: Problem 1. What is the force between two small charged spheres that have charges of C and C and are placed 30 cm apart in air? 2. What is the value of charge of a body that carries 20 excess electrons? \\ 3. Two charges +5.0 C and +2.0 C are placed at a distance of 3.0 m from each other as shown in the diagram. Where would you put a positive charge of +1.0 C in the diagram so that the net electrostatic force on it is zero? 4. Three charged particles are placed in a line, as shown in the figure below. a. Find the magnitude and direction of the force on particle A. b. Find the magnitude and direction of the force on particle B.

2 5. Three charged particles are arranged as shown in the figure below. Calculate the magnitude and direction of the force experienced by particle B. 6 Three point charges of 3.4 C each are arranged at the points of an equilateral triangle with sides of length 5.0 cm, as shown in the figure below. What is the magnitude of the force acting on one of the charges?

3 1. ANS: 10 3 N NOT: The force between the charges is calculated using Coulomb's law. 2. ANS: C NOT: The charge on an object is calculated using q = ne, where n is the number of electrons and e is the charge of one electron. 3. ANS: 1.2 m to the left of the 2.0 C charge NOT: The third charge is to be placed at a point where the force from the right charge is cancelled by the left charge. Since the forces are vectors, the charge is placed between the left and the right charges at a distance of x from the right charge. The net force is a vector addition.

4 4. ANS:

5 5.ANS: PTS: 1 7. ANS: Since the triangle is uniform, start with any charge (A) and calculate the forces of the other charges using this point as the origin.

6 The force of F C on A acts along the x-axis but the force of F B on A needs to be resolved into its x- and y- components to find the total force. F B on A, x = (F B on A ) cos = (42 N)(cos 60.0 ) = 21 N F B on A, y = (F B on A ) sin = (42 N)(sin 60.0 ) = 36 N PTS: 1

7 b. T y = mg = (5.0 g)( 9.80 m/s 2 ) = N F e = T x = T y tan 30 = ( N)(tan 30 ) = N Because the force is repulsive, the pith balls have like charges. The magnitude of the charges on the pith balls is C. c. If the charge doubles on one pith ball, then the electric force doubles to become N. T y does not change, so it still has a magnitude of N. The new angle is found as follows: F e = T y tan PTS: 1

8 TRUE/FALSE 14. ANS: T Materials that allow electrons to move freely are called conductors. PTS: 1 DIF: 1 REF: Page 544 Page 545 OBJ: Describe the differences between conductors and insulators. TOP: Describe the differences between conductors and insulators. KEY: Conductor MSC: 1 NOT: /T/ Correct! /F/ Materials that allow electrons to move freely are called conductors. MULTIPLE CHOICE 15. ANS: C Electric forces are repulsive between like charged objects and attractive between objects that possess opposite charges. A B C D Feedback Objects with opposite charges attract each other and objects with like charges repel each other. Objects attract or repel each other depending on the charge they possess. Correct! Electric forces cause objects to either repel or attract each other depending on the type of charge they possess. PTS: 1 DIF: 1 REF: Page 542 OBJ: Demonstrate that charged objects exert forces, both attractive and repulsive. TOP: Demonstrate that charged objects exert forces, both attractive and repulsive. KEY: Force between charged objects MSC: ANS: C If objects are similarly charged, the force between them is repulsive. However, if the objects are oppositely charged, the force between them is attractive. A B C D Feedback Rods P and Q cannot repel each other if they are oppositely charged. Rods P and R cannot attract each other if they are similarly charged. Correct! There is no force of attraction between two similarly charged objects. PTS: 1 DIF: 2 REF: Page 542 Page 543 OBJ: Demonstrate that charged objects exert forces, both attractive and repulsive. TOP: Demonstrate that charged objects exert forces, both attractive and repulsive. KEY: Charge MSC: ANS: A When a negatively charged balloon is brought near a wooden door, the negative charge of the balloon pushes the electrons of the wood molecules away from the location of contact. The charged balloon causes the wood molecules to polarize causing attraction between the two objects.

9 A B C D Feedback Correct! The wooden door is neutral because an uncharged balloon does not stick to it. Wood is an insulator and until charged, cannot attract or repel another object. Wood cannot change charge balance until it is polarized. PTS: 1 DIF: 2 REF: Page 543 Page 544 OBJ: Demonstrate that charged objects exert forces, both attractive and repulsive. TOP: Demonstrate that charged objects exert forces, both attractive and repulsive. KEY: Charge MSC: 2 COMPLETION 18. ANS: PTS: 1 DIF: 1 REF: Page 544 OBJ: Recognize that charging is the separation, not the creation, of electric charges. TOP: Recognize that charging is the separation, not the creation, of electric charges. KEY: Charging Conservation MSC: 1