SM_chapter23 - 23 Electric Fields CHAPTER OUTLINE 23.1 23.2...

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23 Electric Fields CHAPTER OUTLINE 23.1 Properties of Electric Charges 23.2 Charging Objects by Induction 23.3 Coulomb’s Law 23.4 The Electric Field 23.5 Electric Field of a Continuous Charge Distribution 23.6 Electric Field Lines 23.7 Motion of a Charged Particle in a Uniform Electric Field ANSWERS TO QUESTIONS Q23.1 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. *Q23.2 (i) Suppose the positive charge has the large value 1 m C. The object has lost some of its conduction electrons, in number 10 6 C (1 e/1.60 × 10 19 C) = 6.25 × 10 12 and in mass 6.25 × 10 12 (9.11 × 10 31 kg) = 5.69 × 10 18 kg. This is on the order of 10 14 times smaller than the ~1g mass of the coin, so it is an immeasurably small change. Answer (d). (ii) The coin gains extra electrons, gaining mass on the order of 10 14 times its original mass for the charge 1 m C. Answer (b). Q23.3 All of the constituents of air are nonpolar except for water. The polar water molecules in the air quite readily “steal” charge from a charged object, as any physics teacher trying to perform electrostatics demonstrations in the summer well knows. As a result—it is difF cult to accumulate large amounts of excess charge on an object in a humid climate. During a North American winter, the cold, dry air allows accumulation of signiF cant excess charge, giving the potential (pun intended) for a shocking (pun also intended) introduction to static electricity sparks. Q23.4 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. Q23.5 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 ±igure 23.4a, 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 insufF cient to support the weight of the balloon.
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This note was uploaded on 01/19/2012 for the course PHY 232 taught by Professor Williams,frank during the Spring '11 term at Ohio State.

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SM_chapter23 - 23 Electric Fields CHAPTER OUTLINE 23.1 23.2...

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