Electrostatics Review - Answer Key

7.5 - Electrostatics Review - KEY I. 1. General Knowledge A proton has a positive charge; a neutron has a neutral charge; an electron has a negative charge. Electrons are the particles that are free to move. 2. To become positively charged, the object must lose electrons. 3. To become negatively charged, the object must gain electrons. 4. Describe how an object can become charged through the process of friction. Two neutrally charged objects of different affinity levels are rubbed against each other. The object with the higher affinity will gain electrons and become negatively charged. The object with the lower affinity will lose electrons and become positively charged. The net charge for both objects is zero. 5. Negative charge 6. Positive charge 7. Charge is transferred by contact. If a neutral object is touched to a positively charged, electrons will flow from the neutral object to the positively charged object. Since the neutral object loses electrons, it will become positively charged. If a neutral object is touched to a negatively charged, electrons will flow to the neutral object from the negatively charged object. Since the neutral object gains electrons, it will become negatively charged. 8. A separation of charge is created by bringing a charged object near a neutral object, but not making contact. If a positively charged object is brought near a neutral object, electrons will move from the side farthest away from the charged object to the side closest to the charged object.. This creates a separation of charge where the side closest to the charged object is negatively charged and the opposite side is positively charged. If a negatively charged object is brought near a neutral object, electrons will move away from the side closest to the charged object to the side farthest away from the charged object.. This creates a separation of charge where the side closest to the charged object is positively charged and the opposite side is negatively charged. 9. Separation of positive and negative charges on a single object. 10. Like charges will repel each other; opposite charges will attract each other. 11. Grounding is the process of discharging an object by giving the charges a path to ground. 12. a. Both forces have an inverse square relationship to distance; both forces have a force constant. b. The gravitational force is created by masses, the electrostatic force is created by charges. The gravitational force is always attractive while the electrostatic force can be either attractive or repulsive. The gravitational constant is very small compared to the electrostatic constant; this makes gravity is a much weaker force than the electrostatic force. 13. In order to increase the electrostatic force, the charges should be increased and the distance between the charges should be decreased. 14. In order to decrease the electrostatic force, the charges should be decreased and the distance between the charges should be increased. 15. Identify the direction that a charged particle would want to move when it is placed in an electric field (space where electrostatic forces occur). To give an object electrical potential energy, the object should be moved opposite to the direction it would want to move. II. Practice Question 16. a. Two times as much b. Four times as much (2 x 2) c. ¼ as much ( 1 / (2^2) d. 16 times as much ( (2 x 2)/(0.5^2) 17. A. Since the distance between the charged objects is decreased, the electrostatic force is increased. B. Since the objects have moved the direction they wanted to go, the electrical potential energy. 18. A. Point 1 (opposite the direction it would want to move) B. Point 3 (opposite the direction it would want to move) 19. Neutral - there was no net charge (total charge) prior to friction so there cannot be any net charge afterwards. Even though the individual objects have a charge, the net charge is zero. III. Multiple Choice (Answers are in bold and underlined). 20. Use the model to answer the following question. Select one strength and one limitation of the model comparing the Gravitational Force and the Electrostatic Force. Part A: A strength of the model (Choose one of the following): A. The gravitational force can only attract but the electrostatic force can attract or repel. B. The vectors are all the same length indicating that the electrostatic force and the gravitational force are equal in magnitude. C. Distance between the charges or masses does not have an effect on the electrostatic or gravitational forces. Part B: A limitation of the model (Choose one of the following): A. Both the gravitational force and the electrostatic force can only attract. B. The vectors are all the same length indicating that the electrostatic force and the gravitational force are equal in magnitude. C. Distance between the masses does not have an effect on the gravitational force but the distance between the charges does have an effect on the electrostatic force. 21. During a teacher-led demonstration, an inflated balloon that has been rubbed against a person’s hair is touched to a neutral wall and remains attracted to it. She draws four pictures on the board to represent the charge distribution on the balloon and the wall. Which of the representations best illustrates the charge distribution? A. Image A, because the top and bottom of the balloon have less charge. B. Image B, because the negatively charged balloon attracts positive charges in the wall’s surface and repels the negative charges away from the wall’s surface. C. Image C, because there is not enough charge in the balloon to completely repel all of the negative charges on the wall’s surface. D. Image D, because the negative charges in the wall will be attracted to the positive charges on the other side of the balloon. 22. During a laboratory investigation, students observe that the foil leaf inside a neutral electroscope deflects when a positively charged rod is brought near, but not touching it. Within their lab group, students are trying to explain what is happening to cause the movement of the leaf of the electroscope. Which of these answers would best explain the observation? A. Positive charges jump from the rod to the electroscope because charges want to be balanced. The rod becomes less charged and the electroscope becomes more charged causing the leaf of the electroscope to move. B. Negative charges are the only ones that can move so they must jump from the neutral electroscope to the positive rod in order to neutralize it. That causes the electroscope to have a net charge and it moves. C. Positive charges in the electroscope move to be closer to the positively charged rod. The bottom of the electroscope becomes negatively charged and the leaf deflects away. D. Negative charges in the electroscope are attracted to the top of the positively charged rod. The electroscope becomes polarized, resulting in positive charges which repel, causing the leaf to deflect away. 23. Two charged objects are held 1 meter apart from each other. Then they are moved 2 meters apart. How will the electrostatic force and gravitational forces between them be affected by this move? A. Both forces will be ¼ as strong after the move. B. Both forces will be ½ as strong after the move. C. The gravitational force will be ½ as strong and the electrostatic force will be ¼ strong. D. The gravitational force will be ¼ as strong and the electrostatic force will be ½ as strong. 24. During an investigation of methods of charge transfer, students are asked to make an electroscope using a flask. They insert a metal rod through a stopper and bend a hook on the end that will be inside the flask. They have a bent piece of metal on the hook to create foil leaves. Once the stopper has been pushed into place, they attach a metal ball to the end of the metal rod sticking out of the flask. The students are asked to use an acrylic rod to charge the electroscope by conduction. They begin to develop the following procedure: ● Step 1 - Touch the rod to the metal ball. ● Step 2 - Observe any movement of the foil leaves inside the flask. ● Step 3 - Charge the rod. ● Step 4 - Ground the electroscope by touching a conductive material to the metal ball. The students agree that these steps would allow them to observe charging by conduction but they disagree on the order in which they should be completed. What is the correct order? A. 1, 2, 3, 4 B. 1, 3, 2, 4 C. 2, 3, 4, 1 D. 4, 3, 1, 2