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Chapter 17

Chapter 17 . Electric Forces & Fields. Terms to Know. Electrostatics: Study of electric charge Charged object: has unequal #s of protons and electrons positively charged object negatively charged object Opposite charges: attract + and – Like charges: repel

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Chapter 17

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  1. Chapter 17 Electric Forces & Fields

  2. Terms to Know • Electrostatics: Study of electric charge • Charged object: has unequal #s of protons and electrons • positively charged object • negatively charged object • Opposite charges: attract • + and – • Like charges: repel • the same charge: + and +, or – and –

  3. Conductor: material in which a charge can move easily

  4. Insulator: material in which a charge cannot move easily

  5. Induction: to charge an object without touching it • Inducing by a Positive Charge • Induction by a Negative Object

  6. Induction is in… (1) Electroscope (2) Grounding

  7. (3) Polarization = separation of positive and negative charges

  8. Coulomb • Unit of charge: C = coulomb • 1 proton = 1.602×10-19 C; 1 electron = ‒1.602×10-19 C • 1 C = charge of 6.24×1018 electrons or protons

  9. Coulomb’s Law • Coulomb's Law (4 min) • the force, F, between two charged particles, qAand qB over a distance r • Called electrostatic force • Mutual attraction/repulsion • Notations: FAB= force of A exerting on B FBA = force of B exerting on A • |FAB | = | FBA | • A vector: has a magnitude and a direction

  10. Electrostatic Force = Coulomb’s Law • Magnitude: F = electrostatic force kc = Coulomb constant = 8.99×109 N· m2/C2 qA = the charge of particle A qB = the charge of particle B r = the distance between A and B • Analogous to Universal Law of Gravitation: G = 6.67×10‒11 N·m2/kg2

  11. Know that.... • Coulomb’s law allows us to • determine the magnitude of the force between two charged particles • determine if the two charges are attracting (positive F value) or repelling (negative F value) • Coulomb’s law doesn’t tell to which direction a particle is moving • Must be done “manually”

  12. Example • qA = +3 C located at +1 m from the origin and qB = -2 C located at -2m. • The negative F = Particles A and B are attracting • The particle A moves to left; B moves to right • How do we express the directions? • Don’t use a sign for attraction or repulsion • Use the signs to indicate the directions of force (N=+; S= ‒, etc)

  13. Problem-Solving Strategy • Drawing always helps • Indicate the direction of force • Use the signs for the directions of F, not for the charges of particles • Vectors are algebraically additive if they lie in the same dimension (Principal of superposition) • Use Trig functions to break down a vector to x- and y- component • Use the Coulomb’s law to get the magnitude of the force • Consider if the answer is reasonable

  14. Sample Problem, Pg 17A, Pg 635 The electron and proton of a hydrogen atom are separated, on average, by a distance of 5.3×10−11 m. Find the magnitude of the electric force and the gravitational force that each particle exerts on the other.

  15. To get the resultant force on a particle • Identify all forces acting on the particle • Don’t include the forces the particle exerting on other particles (Ex) • Resolve each force into x- and y-component • Get the sum of each component • Using the Pythagorean theorem, get the hypotenuse, which is the resultant force

  16. Example 17B, Pg 638 Consider three point charges at the corners of a triangle, as shown right, where q1=6.00×10-9 C, q2=-2.00×10-9 C, and q3=5.00×10-9 C. Find the magnitude and direction of the resultant force on q3.

  17. Example Problem 2 • A charged particle, A (+6.0 μC) is located near another charged particle B (-3.0 μC) and is located 4.0 cm from the right of A. • What is the force of B on A?

  18. (b) A third particle (+1.5 μC) is added to the configuration. If it is located 3.0 cm directly beneath A, what is the new net force on A?

  19. Equilibrium • the state in which the net force = 0 • Must have at least three charged particles • Must have two opposite forces with the same magnitude for the center particle (Ex) (1) + - + (2) - + - (3) - - - (4) + + + • is seen as objects at rest or at constant velocity

  20. Sample Problem 17C, Pg 640 Three charges lie along the x-axis. One positive charge, q1 = 15 µC, is at x = 2.0 m, and another positive charge, q2=6.0 µC, is at the origin. At what point on the x-axis must a negative charge, q3, be placed so that the resultant force on it (q3) is zero.

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