A copper ball of mass 144 g is positively charged to +281 C. It is attached to a

Question

A copper ball of mass 144 g is positively charged to +281 C. It is attached to a metal plate by a plastic, insulating spring of equilibrium length 32 cm and strength 5.79 N/m.

If the plate is not charged, what is the extended length of the spring?

Using standard data for copper (molar mass Cu), estimate the number of atoms there are in the ball per excess electron that charges it.

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?

Explanation / Answer

Fnet = kx - m g = 0

(5.79)x = (0.144)(9.8)

x = 0.244 m = 24.4 cm

L = x + 32 = 56.4 cm

---------------

n = (281 x 10^-6) / (1.6 x 10^-19)

n = 1.756 x 10^15

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then Fe = m g

q E = m g

E = (0.144)(9.8) / (281 x 10^-6)

E = 5022 N/C

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