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Chapter 21

# Chapter 21 - Chapter 21 The Electric Field I Discrete...

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Chapter 21 The Electric Field I: Discrete Charge Distributions Conceptual Problems 1 Objects are composed of atoms which are composed of charged particles (protons and electrons); however, we rarely observe the effects of the electrostatic force. Explain why we do not observe these effects. Determine the Concept The net charge on large objects is always very close to zero. Hence the most obvious force is the gravitational force. 2 A carbon atom can become a carbon ion if it has one or more of its electrons removed during a process called ionization . What is the net charge on a carbon atom that has had two of its electrons removed? ( a ) + e , ( b ) – e , ( c ) +2 e , ( d ) –2 e Determine the Concept If two electrons are removed from a carbon atom, it will have a net positive charge of +2 e. ( ) c is correct. 3 •• You do a simple demonstration for your high school physics teacher in which you claim to disprove Coulomb’s law. You first run a rubber comb through your dry hair, then use it to attract tiny neutral pieces of paper on the desk. You then say Coulomb’s law states that for there to be electrostatic forces of attraction between two objects, both objects need to be charged. However, the paper was not charged. So according to Coulomb’s law, there should be no electrostatic forces of attraction between them, yet there clearly was. You rest your case. ( a ) What is wrong with your assumptions? ( b ) Does attraction between the paper and the comb require that the net charge on the comb be negative? Explain. Determine the Concept ( a ) Coulomb’s law is only valid for point particles. The paper bits cannot be modeled as point particles because the paper bits become polarized. ( b ) No, the attraction does not depend on the sign of the charge on the comb. The induced charge on the paper that is closest to the comb is always opposite in sign to of the charge on the comb, and thus the net force on the paper is always attractive. 4 •• You have a positively charged insulating rod and two metal spheres on insulating stands. Give step-by-step directions of how the rod, without actually touching either sphere, can be used to give one of the spheres ( a ) a negative charge, and ( b ) a positive charge. 1997

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Chapter 21 1998 Determine the Concept ( a ) Connect the metal sphere to ground; bring the insulating rod near the metal sphere and disconnect the sphere from ground; then remove the insulating rod. The sphere will be negatively charged. ( b ) Bring the insulating rod in contact with the metal sphere; some of the positive charge on the rod will be transferred to the metal sphere. 5 •• ( a ) Two point particles that have charges of +4 q and –3 q are separated by distance d . Use field lines to draw a visualization of the electric field in the neighborhood of this system. ( b ) Draw the field lines at distances much greater than d from the charges.
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