20. A thin plastic rod has uniform linear positive charge density λ distributed along a semicircular arc of radius R. What is the magnitude of the electric field at the center of the arc? (A)(B)(C)(D)(E)

8. A uniform soda can of base area A = 12 cm², mass m = 10 g and height h = 20 cm is filled with soda of density ρ = 1 g/cm³. Use x to denote the height of the remaining soda; find the x that makes the center of mass of the whole system the lowest. It occurs when the soda is (A) half-filled (B) less than half, but more than a quarter filled (C) less than a quarter filled (D) completely empty (E) none of the above.

20. A thin plastic rod has uniform linear positive charge density λ distributed along a semicircular arc of radius R. What is the magnitude of the electric field at the center of the arc?

(A)
(B)
(C)
(D)
(E)

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