The electric field near the surface of the earth points downwardand has a magnit

Question

The electric field near the surface of the earth points downwardand has a magnitude of
120 N/C. Compare the upward electric force on an electron with thedownward gravitational
force; i.e., what is the ratio Fe/Fg? The elementary charge is 1.6× 1019 C and the mass
of an electron is 9.11 × 1031 kg.

What is the magnitude of the charge that
should be placed on a coin of mass 5.5 g so
that the electric force balances the weight of
the coin near the earth’s surface?
Answer in units of C.

Explanation / Answer

a) Fe / Fg = Eq / mg    = 120 * 1.6 x 10-19 / 9.1 x 10-31 *9.8                = 2.15 x 1012 b) Eq = mg       ==> q = mg / E                 = 5.5 x 10-3 * 9.8 / 120                 = 0.4499 x 10-3 C

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