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a A B C D The net force on that charge is zero

Question:. Four point charges, each of the same magnitude, with varying signs are arranged at the corners of a square as shown. Which of the arrows labeled A , B , C , and D gives the correct direction of the net force that acts on the charge at the upper right corner?. a A B C D

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a A B C D The net force on that charge is zero

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  1. Question: Four point charges, each of the same magnitude, with varying signs are arranged at the corners of a square as shown. Which of the arrows labeled A, B, C, and D gives the correct direction of the net force that acts on the charge at the upper right corner? • a A • B • C • D • The net force on that charge is zero Answer: b

  2. Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true:(a) FAB = –3FBA, (b) FAB = –FBA, or (c) 3FAB = –FBA Question Answer (b)

  3. Electric charge always occurs in multiples ofe e 19 = 1.60 10 C × Q = ne (n =±0、1、2、3…)

  4. Question: A conducting sphere has a net charge of -4.8x10-17 C. What is the approximate number of excess electrons on the sphere? (a) 100 (c) 300(e) 500 (b) 200 (d) 400 Answer: c (-4.8x10-17 C/-1.6x10-19 C=300 electrons)

  5. Question:Two charges of +Q are 1 cm apart. If one of the charges is replaced by a charge of –Q, the magnitude of the force between them is • zero • smaller • the same • larger Answer: c

  6. Question:A charge of +q is placed 2 cm from a charge of –Q. A second charge of +q is then placed next to the first. The force on the charge of –Q • decreases to half its former magnitude • remains the same • increases to twice its former magnitude • increases to four times its former magnitude Answer: c

  7. Question: Two positive point charges Q and 2Q are separated by a distance R. If the charge Q experiences a force of magnitude F when the separation is R, what is the magnitude of the force on the charge 2Q when the separation is 2R ? (a) F/4(c) F (e) 4F (b) F/2 (d) 2F Answer: a

  8. Question:Two charges, one positive and the other negative, are initially 2 cm apart and are then pulled away from each other until they are 6 cm apart. The force between them is now smaller by a factor of • 9 • 3 • 27 • 3 Answer: a

  9. Outline for Today • Electric Field • Electric Field Lines

  10. Electric Field • Maxwell developed an approach to discussing fields • An electric field is said to exist in the region of space around a charged object • When another charged object enters this electric field, the field exerts a force on the second charged object

  11. Electric Field, cont. • A charged particle, with charge Q, produces an electric field in the region of space around it • A small test charge, qo, placed in the field, will experience a force

  12. Electric Field • Mathematically, • Use this for the magnitude of the field • The electric field is a vector quantity • The direction of the field is defined to be the direction of the electric force that would be exerted on a small positive test charge placed at that point

  13. Direction of Electric Field • The electric field produced by a negative charge is directed toward the charge • A positive test charge would be attracted to the negative source charge

  14. Direction of Electric Field, cont • The electric field produced by a positive charge is directed away from the charge • A positive test charge would be repelled from the positive source charge

  15. Electric Field Lines • A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the field vector at any point • These are called electric field lines and were introduced by Michael Faraday

  16. Electric Field Lines, cont. • The field lines are related to the field by • The electric field vector, E, is tangent to the electric field lines at each point • The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region

  17. Electric Field Line Patterns • Point charge • The lines radiate equally in all directions • For a positive source charge, the lines will radiate outward

  18. Electric Field Line Patterns • For a negative source charge, the lines will point inward

  19. Electric Field Line Patterns • An electric dipole consists of two equal and opposite charges • The high density of lines between the charges indicates the strong electric field in this region

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