JEE Main & Advanced JEE Main Paper (Held On 16 April 2018)

  • question_answer
    Two identical conducting spheres A and B, carry equal charge. They are separated by a distance much larger than their diameter, and the force between them is F. A third identical conducting sphere C is uncharged. Sphere C is first touched to A then to B, and the removed. As a result, the force between A and B would be equal to [JEE Main 16-4-2018]

    A) \[\frac{3F}{4}\]                                 

    B) \[\frac{F}{2}\]

    C)   F                               

    D)  \[\frac{3F}{8}\]

    Correct Answer: D

    Solution :

     the charge on A will be reduced to 3/4th of its original value, & on B it will be halved. den as we know the formula for the electrostatic force between 2 charges                 \[F=\frac{k{{q}_{1}}{{q}_{2}}}{{{r}^{2}}}\]                 \[{{F}_{f}}\] will become \[3/{{8}^{th}}\]of \[{{F}_{i}}.\] Initially charges on both sphere, q \[F=\frac{K{{q}^{2}}}{{{d}^{2}}}\]                                                .............(1) When sphere C will get touched with sphere A, then final charges on both will become \[\frac{q}{2}.\] Now, when this sphere C will get touched to sphere B, then final charges on both of them will be \[{{q}_{c}}=qd=\frac{q/2+q}{2}=\frac{3q}{4}\] Now force between A and B will be \[F'=\frac{k\times \frac{q}{2}\times \frac{3q}{4}}{{{d}^{2}}}\] \[F'=\frac{3F}{8}\]


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