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Understanding Electric Fields in AP Physics I

Learn the relationship between electric fields and forces in this lecture. Discover how charges create fields and affect objects. Explore concepts through demos and examples to understand electric field strength and forces on charges. Practice calculations to enhance your understanding.

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Understanding Electric Fields in AP Physics I

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  1. Plan for Today (AP Physics I) • Lecture/Notes on Electric Fields

  2. Remember • Relationship between FE and FG

  3. We have g, which is an acceleration of a mass in a gravitational field • If we put a mass in the “field”, what happens? • It falls/goes towards Earth • Equation F = mg

  4. We need a similar concept in electrostatics • We have charged objects, which cause electric fields around them • If you held the Vandergraph generator, you could feel the field

  5. Example/Demo • If I put a charge on a balloon on a string, • Then put it near the Vandergraph • Watch the balloon react when it gets near • We can see how the space around the Vandergraph is effected

  6. Something like “g” • We have • Something similar • What is E? • E = electric field strength

  7. What this means • If E = 200 N/C, this means if I put a 1 C charge in this field, it will experience a 200 N force

  8. Tie in to Newton’s 2nd Law

  9. Direction is a problem • With g, the direction was always the same (down) • Earth only “sucks” • With E, it can repel or attract

  10. Direction Definition • The electric field points in the direction a positive test charge would flow • Example • Put a positive charge in the field

  11. Example • How does the electric field look?

  12. If we put a 2 test charge at A and then at B, can we find the force acting on it. • We don’t have to use F = kq1q2/r^2 because of Newton’s 2nd law • F = ma or F = qE

  13. Ea = 180000 N/C • Fa = .36 N • Eb = 45000 N/C • Fb = .0899 N

  14. Example • Find the electric field at point C from the two charges

  15. Now what is the force on a charge of +2 put here?

  16. Assignment • Work on the Coulomb’s worksheet • Find the electric field at the points assigned and the force on a test charge there

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