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Coulomb’s Law and Electric Fields

Learn about Coulomb’s Law, electric fields, forces on charges, and field directions through examples and preflight questions in this lecture.

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Coulomb’s Law and Electric Fields

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  1. Physics 102:Lecture 02 Coulomb’s Lawand Electric Fields • Today’s lecture will cover Textbook Sections 16.3-6 Physics 102: Lecture 2, Slide 1

  2. Qp=1.6x10-19 C Qe = -1.6x10-19 C + - r = 1x10-10 m Recall Coulomb’s Law • Magnitude of the force between charges q1 and q2 separated a distance r: F = k q1q2/r2 k = 9x109 Nm2/C2 • Force on nucleus of Hydrogen from e- Example F • F = (9x109)(1.6x10-19)(1.6x10-19)/(10-10)2 N • = 2.3x10-8 N (to the right) Physics 102: Lecture 2, Slide 2

  3. Example Three Charges • Calculate force on +2mC charge due to other two charges • Calculate force from +7mC charge • Calculate force from –3.5mC charge • Add (VECTORS!) Q=+2.0mC 4 m 6 m Q=+7.0mC Q=-3.5 mC Physics 102: Lecture 2, Slide 3

  4. q=1.6x10-19 C + r = 1x10-10 m Electric Field • Charged particles create electric fields. • Direction is the same as for the force that a + charge would feel at that location. • Magnitude given by: E  F/q = kq/r2 Example • E = Direction? Physics 102: Lecture 2, Slide 4

  5. Preflight 2.3 • What is the direction of the electric field at point B? • Left • Right • Zero y A B x Physics 102: Lecture 2, Slide 5

  6. ACT: E Field • What is the direction of the electric field at point C? • Left • Right • Zero y C x Physics 102: Lecture 2, Slide 6

  7. ACT: E Field • What is the direction of the electric field at point C? • Left • Right • Zero Red is negative Blue is positive Away from positive charge (right) Towards negative charge (right) Net E field is to right. y C x Physics 102: Lecture 2, Slide 7

  8. Comparison:Electric Force vs. Electric Field • Electric Force (F) - the actual force felt by a charge at some location. • Electric Field (E) - found for a location only – tells what the electric force would be if a charge were located there: F = qE • Both are vectors, with magnitude and direction. Add x & y components. Physics 102: Lecture 2, Slide 8

  9. Preflight 2.2 • What is the direction of the electric field at point A? • Up • Down • Left • Right • Zero y A B x Physics 102: Lecture 2, Slide 9

  10. Preflight 2.2 • What is the direction of the electric field at point A? • Up • Down • Left • Right • Zero Red is negative Blue is positive y A B x Physics 102: Lecture 2, Slide 10

  11. ACT: E Field II • What is the direction of the electric field at point A, if the two positive charges have equal magnitude? • Up • Down • Right • Left • Zero y A B x Physics 102: Lecture 2, Slide 11

  12. ACT: E Field II • What is the direction of the electric field at point A, if the two positive charges have equal magnitude? • Up • Down • Right • Left • Zero Red is negative Blue is positive A y B x Physics 102: Lecture 2, Slide 12

  13. X A B Y Preflight 2.5 Charge A is 1) positive 2) negative 3) unknown Field lines start on positive charge, end on negative. Physics 102: Lecture 2, Slide 13

  14. X A B Y Preflight 2.5 Charge A is 1) positive 2) negative 3) unknown Field lines start on positive charge, end on negative. Physics 102: Lecture 2, Slide 14

  15. Preflight 2.6 X A B Y Compare the ratio of charges QA/ QB 1) QA= 0.5QB 2) QA= QB 3) QA= 2 QB # lines proportional to |Q| Physics 102: Lecture 2, Slide 15

  16. Preflight 2.6 X A B Y Compare the ratio of charges QA/ QB 1) |QA|= |0.5QB| 2) |QA|= |QB| 3) |QA|= |2 QB| # lines proportional to Q Physics 102: Lecture 2, Slide 16

  17. X A B Y Preflight 2.8 The magnitude of the electric field at point X is greater than at point Y 1) True 2) False Physics 102: Lecture 2, Slide 17

  18. X A B Y Preflight 2.8 The electric field is stronger when the lines are located closer to one another. The magnitude of the electric field at point X is greater than at point Y 1) True 2) False Density of field lines gives E Physics 102: Lecture 2, Slide 18

  19. ACT: E Field Lines B A Compare the magnitude of the electric field at point A and B 1) EA>EB 2) EA=EB 3) EA<EB Physics 102: Lecture 2, Slide 19

  20. ACT: E Field Lines B A Compare the magnitude of the electric field at point A and B 1) EA>EB 2) EA=EB 3) EA<EB Physics 102: Lecture 2, Slide 20

  21. E inside of conductor • Conductor  electrons free to move • Electrons feels electric force - will move until they feel no more force (F=0) • F=qE: if F=0 then E=0 • E=0 inside a conductor (Always!) Physics 102: Lecture 2, Slide 21

  22. E inside of conductor • Conductor  electrons free to move • Electrons feels electric force - will move until they feel no more force (F=0) • F=qE: if F=0 then E=0 • E=0 inside a conductor (Always!) Physics 102: Lecture 2, Slide 22

  23. X A B Y Preflight 2.10 "Charge A" is actually a small, charged metal ball (a conductor). The magnitude of the electric field inside the ball is: (1) Negative (2) Zero (3) Positive Physics 102: Lecture 2, Slide 23

  24. X A B Y Preflight 2.10 "Charge A" is actually a small, charged metal ball (a conductor). The magnitude of the electric field inside the ball is: (1) Negative (2) Zero (3) Positive Physics 102: Lecture 2, Slide 24

  25. Recap • E Field has magnitude and direction: • EF/q • Calculate just like Coulomb’s law • Careful when adding vectors • Electric Field Lines • Density gives strength (# proportional to charge.) • Arrow gives direction (Start + end on -) • Conductors • Electrons free to move  E=0 Physics 102: Lecture 2, Slide 25

  26. To Do • Read Sections 17.1-17.3 • Extra problems from book Ch 16: • Concepts 12-18 • Problems 11, 13, 25, 31 • Do your preflight. See you next time! Physics 102: Lecture 2, Slide 26

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