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Coulomb’s Law and Electric Fields

Coulomb’s Law and Electric Fields. Q p =1.6x10 -19 C. Q e = -1.6x10 -19 C. +. -. r = 1x10 -10 m. Recall Coulomb’s Law. Magnitude of the force between charges q 1 and q 2 separated a distance r: F = k q 1 q 2 /r 2 k = 9x10 9 Nm 2 /C 2 Force on nucleus of Hydrogen from e -.

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Coulomb’s Law and Electric Fields

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  1. Coulomb’s Lawand Electric Fields

  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) 5

  3. Qp=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 • E = (9109)(1.610-19)/(10-10)2 N = 1.41011 N/C (to the right) 21

  4. What is the direction of the electric field at point B? • Left • Right • Zero Since charges have equal magnitude, and point B is closer to the negative charge net electric field is to the left y A B x 23

  5. 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 25

  6. 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 = Eq • Both are vectors, with magnitude and direction. Add x & y components. Direction determines sign. 26

  7. 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 37

  8. 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 y A B x 39

  9. Electric Field Lines • Closeness of lines shows field strength • - lines never cross • # lines at surface  Q • Arrow gives direction of E • - Start on +, end on - 42

  10. X A B Y Charge A is 1) positive 2) negative 3) unknown Field lines start on positive charge, end on negative. 44

  11. 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 45

  12. X A B Y 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 46

  13. 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 47

  14. E inside of conductor • Conductor  electrons free to move • Electrons feels electric force - will move until they feel no more force (F=0) • F=Eq: if F=0 then E=0 • E=0 inside a conductor (Always!) 48

  15. X A B Y "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 50

  16. 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

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