9. Electric Field, Superposition, Continuous Charge Distribution A disk of negli
ID: 1791320 • Letter: 9
Question
9. Electric Field, Superposition, Continuous Charge Distribution A disk of negligible thickness has a uniform surface charge distribution and can be thought of as lying in the xy - plane. The total charge on the disk is +20AC. The disk has a radius of 2 cm. Surrounding the disk is a thin concentric ring of uniform charge distribution. Neglect any thickness of the ring. The total charge on the ring is (-)5uC. A very thin insulating material exists between the disk and ring so that charge doesn't flow. Find the magnitude and direction of the electric force on a point charge q1 = (-)2pC located 2 m directly above the midpoint of the charge distribution (along the z axis, say). (Ans: 0.067 N pointing vertically down)Explanation / Answer
Since the radius of disc and ring are very small as compared with distance of the point charge q1, the rings and the disc can be treated as point.
F = qE
= - 2e-6 *[ kQ/z^2] where Q = 20-5 uC = 15uC = 15e-6 C and z = 2m,
F = -2e-6*[9e9*15e-6/2^2]
= - 0.067 N negative sign shows it is attractive, or pointing down
F = 0.067 N downward.
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