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# db_lec02_post - Lecture 2 The Electric Field Summary from...

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Lecture 2: The Electric Field Summary from Last Class Electric Field 1/10/12 1 D. Bortoletto PHYS221

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Coulomb s Law of Electrostatic Force + + q 1 q 2 F r r q q k F ˆ 2 2 1 = Principle of Superposition: When several point charges are put together, the total force on any one charge is the vector sum of the each of the separate forces acting on that charge . 1/10/12 2 D. Bortoletto PHYS221
Coulomb s and Newton s Law F = k q 1 q 2 r 2 F = G m 1 m 2 r 2 * k electro-static constant * Inverse Square Law * Charge *Attractive/repulsive depending on sign of charges *Two kinds of charges *Dominates on small scale * G gravitational constant * Inverse Square Law * Mass *Always attractive *One kind of mass *Dominates on large scales DIFFERS Analogous 1/10/12 3 D. Bortoletto PHYS221

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Principle of Superposition When several point charges are put together, the total force on any one charge is the vector sum of the each of the separate forces acting on that charge . F = F 21 y + F 31 y = 2 F 21 y F = 2 × k Q 1 Q 2 r 2 cos30 0 F = 2 × 9 × 10 9 N m 2 C 2 × (10 6 C ) 2 × 0.866 1 m ( ) 2 F = 15.59 × 10 3 N F F 21 F 31 1/10/12 4 D. Bortoletto PHYS221 Exercise: Determine force on Q 1 Q 2 Q 1 =Q 2 =Q 3 =1 μ C Q 3 R=1m 60 0 Q 1 y x
Problem Solving Strategies: Draw a clear FORCE diagram Use consistent units (meter, Coulomb, Newton) Remember that the force is a vector Look for symmetry Check your result 1/10/12 5 D. Bortoletto PHYS221

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r = 1x10 -10 m Q p =1.6x10 -19 C Q e = -1.6x10 -19 C + - *What is the direction of the force on the proton due to the electron? (A) Left
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db_lec02_post - Lecture 2 The Electric Field Summary from...

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