One very small uniformly charged plastic ball is located directly above another

Question

One very small uniformly charged plastic ball is located directly above another such charge in a test tube as shown in the figure. The balls are in equilibrium a distance d apart. If the charge on each ball is doubled, the distance between the balls in the test tube would become A) 2d. B) 8d. C) 4d. D). The figure shows two unequal point charges, q and Q, of opposite sign. Charge Q has greater magnitude than charge q. In which of the regions X, Y, Z will there be a point at which the net electric field due to these two charges is zero? A) only regions X and Y B) only region Y C) only region X D) only region Z E) all three regions

Explanation / Answer

4)
The upper ball will be balanced when its weight is equal to electrostatic force

So if charge is doubled, electrostatic force should still remain same

F1 = K*q*q/d^2
F2 = K*(2q)*(2q)/d'^2
use:
F1=F2=Mg
s,
F1=F2
K*q*q/d^2 = K*(2q)*(2q)/d'^2
d'^2 = 4*d^2
d' = 2*d
Answer: A

5)
E will be 0 on the side of lower charge value
Answer will be in region x
Answer: C

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