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Phsyics 1B - Electric Forces and Electric Fields CHAPTER...

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519 Electric Forces and Electric Fields CHAPTER OUTLINE 19.1 Historical Overview 19.2 Properties of Electric Charges 19.3 Insulators and Conductors 19.4 Coulomb’s Law 19.5 Electric Fields 19.6 Electric Field Lines 19.7 Motion of Charged Particles in a Uniform Electric Field 19.8 Electric Flux 19.9 Gauss’s Law 19.10 Application of Gauss’s Law to Symmetric Charge Distributions 19.11 Conductors in Electrostatic Equilibrium 19.12 Context Connection The Atmospheric Electric Field ANSWERS TO QUESTIONS Q19.1 A neutral atom is one that has no net charge. This means that it has the same number of electrons orbiting the nucleus as it has protons in the nucleus. A negatively charged atom has one or more excess electrons. Q19.2 The clothes dryer rubs dissimilar materials together as it tumbles the clothes. Electrons are transferred from one kind of molecule to another. The charges on pieces of cloth, or on nearby objects charged by induction, can produce strong electric fields that promote the ionization process in the surrounding air that is necessary for a spark to occur. Then you hear or see the sparks. Q19.3 To avoid making a spark. Rubber-soled shoes acquire a charge by friction with the floor and could discharge with a spark, possibly causing an explosion of any flammable material in the oxygen- enriched atmosphere. Q19.4 Similarities: A force of gravity is proportional to the product of the intrinsic properties (masses) of two particles, and inversely proportional to the square of the separation distance. An electrical force exhibits the same proportionalities, with charge as the intrinsic property. Differences: The electrical force can either attract or repel, while the gravitational force as described by Newton’s law can only attract. The electrical force between elementary particles is vastly stronger than the gravitational force. Q19.5 No. The balloon induces polarization of the molecules in the wall, so that a layer of positive charge exists near the balloon. This is just like the situation in Figure 19.5a, except that the signs of the charges are reversed. The attraction between these charges and the negative charges on the balloon is stronger than the repulsion between the negative charges on the balloon and the negative charges in the polarized molecules (because they are farther from the balloon), so that there is a net attractive force toward the wall. Ionization processes in the air surrounding the balloon provide ions to which excess electrons in the balloon can transfer, reducing the charge on the balloon and eventually causing the attractive force to be insufficient to support the weight of the balloon.
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520 Electric Forces and Electric Fields Q19.6 An electric field once established by a positive or negative charge extends in all directions from the charge. Thus, it can exist in empty space if that is what surrounds the charge. There is no material at point A in Figure 19.18(a), so there is no charge, nor is there a force. There would be a force if a charge were present at point A , however. A field does exist at point A .
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