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Unformatted text preview: Physics C Electric Fields Name:_________________ AP Review Charge Net charge: excess electrons Net + charge: excess holes SI unit: coulomb (C) Quantum of charge: the proton charge (e) the electron charge (-e) e = 6.02 10-19 Coulombs Conservation of Charge The net charge in any process or reaction remains unchanged. Coulombs Law Calculates magnitude of force between charges Force is repulsive if charges have the same sign. Force is attractive if charges have opposite signs. F = kq 1 q 2 /r 2 F: force (N) k: constant 9.0 10 9 N m 2 /C 2 q 1 , q 2 : charges (C) r: distance between charge centers (m) NOTE: k = 1/(4 o ) where o = 8.85 10-12 C 2 / N m 2 Multiple Forces on a Charges F tot = F i Problems: Electric Force (1998) Questions 39- 40 As shown above, two particles, each of charge +Q, are fixed at opposite corners of a square that lies in the plane of the page. A positive test charge +q is placed at a third corner. 39. What is the direction of the force on the test charge due to the two other charges? (A) (B) (C) (D) (E) Explain your reasoning 40. If F is the magnitude of the force on the test charge due to only one of the other charges, what is the magnitude of the net force acting on the test charge due to both of these charges? (A) Zero (B) 2 F (C) F (D) F 2 (E) 2 Show your work Problem: Electric Force (1998) 55. Suppose that an electron (charge -e) could orbit a proton (charge +e) in a circular orbit of constant radius R. Assuming that the proton is stationary and only electrostatic forces act on the particles, which of the following represents the kinetic energy of the two-particle system? (A) R e 4 1 (B) R e 2 8 1 (C) R e 2 8 1 - (D) 2 2 4 1 R e (E) 2 2 4 1 R e - Show your work 2/26/2012 1 Problem: Electric Force (1988) 36 Two small spheres have equal charges q and are separated by a distance d. The force exerted on each sphere by the other has magnitude F. If the charge on each sphere is doubled and d is halved, the force on each sphere has magnitude (A) F (B) 2F (C) 4F (D) 8F (E) 16F Show your work Electric Field Exists in space due to the presence of charge. Predicts what will happen to a charged particle put in that location in space. Field is directed outward from the positive charges creating it. Field is directed inward toward the negative charge creating it. This equation calculates magnitude of field at point in space near one spherically symmetric charge: E = kq/r 2 (magnitude calculation) E: field (N/C) k: constant 9.0 10 9 N m 2 /C 2 q: charge (C) r: distance between center of charge and point in space (m) The equation above works only for spherically symmetric charge distributions....
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