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Electrostatic Forces

Electrostatic Forces. Gravitational vs. Electrostatic Coulomb’s Law Coulomb’s Law Examples The Electric Field Electric Field Examples Continuous/constant Electric Fields Constant Field Examples. Gravitational vs. Electrostatic. Examples of Coulomb’s Law I. Example 16-1

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Electrostatic Forces

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  1. Electrostatic Forces • Gravitational vs. Electrostatic • Coulomb’s Law • Coulomb’s Law Examples • The Electric Field • Electric Field Examples • Continuous/constant Electric Fields • Constant Field Examples

  2. Gravitational vs. Electrostatic

  3. Examples of Coulomb’s Law I • Example 16-1 • Example 16-3 (1D vector addition)

  4. Examples of Coulomb’s Law II • Example 16-4 (2D vector addition) • Problem 13 (Charge Triangle)

  5. A Gravitational “Field” • All falling objects have a = 9.8 m/s2 down (early Sep) • Plug into velocity and displacement equations • All objects have “weight” mg down (late Sep) • Plug into F= ma, get a = 9.8 m/s2 down, v, x • Two objects attract according to F=GMm/r2 (early Oct) • Near Earth’s surface GM/r2 = 9.8 m/s2 down • Just multiply by mass • Most of the problem calculated “ahead of time” • GM/r2 can be considered “field” (now) • GM/r2 varies with altitude (< 9.8 m/s2) • Direction still down • Units acceleration (m/s2) or Force/mass (N/kg) • Multiply mass by “Vector field”

  6. Electric Field • In gravitation we calculated • Bracket part usually became “g” • Since a = 9.8 m/s2 => F = ma = mg • Also interpret g = Force/mass units N/kg • Gravitational “field” • Allows us to pre-calculate most of problem • Same with electrostatics

  7. Electric Field

  8. Examples of Electric Field • Example 16-8 • Example 16-9

  9. Continuous charge distributions • Field due to discrete charge • Field due to line of charge • Field due to ring of charge • Field due to sheet of charge • Coulomb’s Law plus a lot of integral calculus! (and we’ll just give you results)

  10. Line of Charge

  11. Ring of Charge

  12. Sheet of Charge

  13. Continuous charge distributions - Summary

  14. Continuous charge - Constant Electric Field These problems all assume a constant electric field in some region (like between two conducting sheets of charge). • Example 16-6 • Problem 23 • Problem 24 • Problem 27 • Problem 31

  15. Field Lines • Point in direction of electric field – Direction positive charge will move, opposite direction negative charge will move. • F = qE • Density is proportional to field strength - E proportional to lines/area. • Begin on positive, end on negative – In between field lines continuous. • Examples page 455.

  16. Gauss’s Law • Net number of field lines entering or leaving proportional to charge enclosed. ∫E dA = Qenc/εo εo = 1/4πk = 8.85 * 10-12 C2/Nm2 • Note • Surface must be closed surface • Field must be perpendicular to surface • Need symmetry to evaluate

  17. Gauss’s Law Examples • Point charge • Line of charge • Sheet of charge

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