A copper ball of mass 81 g is positively charged to +423 C. It is attached to a
ID: 1655282 • Letter: A
Question
A copper ball of mass 81 g is positively charged to +423 C. It is attached to a metal plate by a plastic, insulating spring of equilibrium length 29 cm and strength 5.45 N/m.
(1) If the plate is not charged, what is the extended length of the spring? ______cm
(2) Using standard data for copper, estimate the number of atoms there are in the ball per excess electron that charges it. ______atoms/e-
(3) If the plate is attached to a power supply and the ball raises back to its equilibrium length, what is the magnitude of the electric field it puts out? ______N/C
Explanation / Answer
(1) Fnet = kx - m g = 0
5.45 x = (81 x 10^-3) (9.8)
x = 0.146 m or 14.6 cm
extended length = L0 + x = 43.6 cm
(2) Q = n e
n = (423 x 10^-6) / (1.602 x 10^-19)
n = 2.64 x 10^15 electrons
atoms = mole NA = (81 / 63.5) (6.022 x 10^23)
= 7.68 x 10^23 atoms
ratio = (2.64 x 10^15) / (7.68 x 10^23)
= 3.44 x 10^-9
(3) Fnet = q E - m g = 0
(423 x 10^-6) E = (81 x 10^-3) (9.8)
E = 1877 N/C