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Question

# Two identical spheres having charges 8μC and −4μC are kept at a certain distance apart. Now they are brought into contact, after that again they are kept at the same distance. Compare the forces in the two cases

A
4:1
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B
12:1
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C
8:1
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D
16:1
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Solution

## The correct option is C 8:1If two identical sphere having charge 8μC and −4μC are kept a certain distance apart, then force between them is F.Let the distance between charged sphere is d. then, F=k×8×(−4)×10−12d2 [attractive]d2=32k×1012F ....(i)Now, when both the identical sphere touch, then charge on each sphere is average of initial charge on sphere. i.e, Q=(8−4)μC2=2μC on each sphere.Now, Force between them is:F′=k×(2×10−6)2d2From equation (i), we getF′=k×4×10−12×F32k×10−12F′=F8so, the ratio is 8:1

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