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Questions 729 At a distance r from a point charge q, the electric field due to the charge is given by (23.9) where rˆ is a unit vector directed from the charge toward the point in question. The electric field is directed radially outward from a positive charge and radially inward The electric field due to a group of point charges can be obtained by using the su- perposition principle. That is, the total electric field at some point equals the vector (23.10) The electric field at some point due to a continuous charge distribution is (23.11) where dq is the charge on one element of the charge distribution and r is the distance Electric field lines describe an electric field in any region of space. The number of lines per unit area through a surface perpendicular to the lines is proportional to E in that region. A charged particle of mass m and charge q moving in an electric field E has an acceleration (23.12) a ! q
E m E ! k e
%
dq
2
rˆ E ! k e
$
i q i r
i 2
rˆ i E ! k e
q r
2
rˆ 1. Explain what is meant by the term “a neutral atom.’’ Explain what “a negatively charged atom’’ means. 2. A charged comb often attracts small bits of dry paper that then fly away when they touch the comb. Explain. 3. Sparks are often seen or heard on a dry day when fabrics are removed from a clothes dryer in dim light. Explain. 4. Hospital personnel must wear special conducting shoes while working around oxygen in an operating room. Why? 5. Explain from an atomic viewpoint why charge is usually transferred by electrons. 6. A light, uncharged metallic sphere suspended from a thread is attracted to a charged rubber rod. After it 7. A foreign student who grew up in a tropical country but is studying in the United States may have had no experience 8. Explain the similarities and differences between Newton’s law of universal gravitation and Coulomb’s law. 9. 10. A light strip of aluminum foil is draped over a horizontal wooden pencil. When a rod carrying a positive charge is 11. When defining the electric field, why is it necessary to specify that the magnitude of the test charge be very small? 12. How could you experimentally distinguish an electric field from a gravitational field? 13. A large metallic sphere insulated from ground is charged with an electrostatic generator while a student standing on 14. Is it possible for an electric field to exist in empty space? Explain. Consider point A in Figure 23.23(a). Does charge 15. When is it valid to approximate a charge distribution by a point charge? 16. Explain why electric field lines never cross. Suggestion: Begin by explaining why the electric field at a particular 17. Figures 23.14 and 23.15 show three electric field vectors at the same point. With a little extrapolation, Figure Q U E S T I O N S |