A cork ball of mass 1.65 g is placed between twovery large horizontal plates. Th

Question

A cork ball of mass 1.65 g is placed between twovery large horizontal plates. The bottom plate has a uniform chargedensity of +0.26 C/m2, whereasthe upper plate has a uniform charge density of -0.88C/m2. The cork ball, whichcarries an unknown charge, is placed between the plates and isobserved to float motionlessly. What is the charge on the ball?
only thing i have to go onis

Enet = (sigma top plate / epsilon) + (sigma botton plate/
epsilon)
and then q = m*g) / Enet  

but i don't know how to get Enetfirst.
only thing i have to go onis

Enet = (sigma top plate / epsilon) + (sigma botton plate/
epsilon)
and then q = m*g) / Enet  

but i don't know how to get Enetfirst.

Explanation / Answer

Aaarrgghh! I'm being stupid... very small mistake withthe signs. The E fields add together in the gap! So the negativesign on the negative charge must be ignored. AND you have to usethe factor of 2 in the denominator. Like this... . E =   0.88 x 10-6 / 2 * 8.85x 10-12   + 0.26 x 10-6 / 2 *8.85 x 10-12 = .          =    64407 N/C .    Then you have     upward force of E field on ball = downward pull ofgravity on ball     or .           qE = m g .     q = m g / E = 0.00165 * 9.8 / 64407 =    2.51 x10-7 Coloumb .          =    64407 N/C .    Then you have     upward force of E field on ball = downward pull ofgravity on ball     or .           qE = m g .     q = m g / E = 0.00165 * 9.8 / 64407 =    2.51 x10-7 Coloumb

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