Two identical conducting spheres A and B carry equal charge. They are ...
Given:
- Two identical conducting spheres A and B carry equal charge.
- They are separated by a distance much larger than their diameters.
- The force between them is F.
- A third identical conducting sphere C is uncharged.
To find:
The force between A and B after sphere C is touched to A then to B and then removed.
Solution:
When sphere C is touched to sphere A, it acquires some charge by induction. The charge distribution on sphere A is redistributed due to the presence of the grounded sphere C. As a result, sphere A loses some charge to the ground and gains an equal amount of charge of opposite polarity from sphere C.
Similarly, when sphere C is touched to sphere B, it acquires some charge by induction. The charge distribution on sphere B is redistributed due to the presence of the grounded sphere C. As a result, sphere B loses some charge to the ground and gains an equal amount of charge of opposite polarity from sphere C.
After sphere C is removed, spheres A and B have acquired opposite charges due to the transfer of charge from sphere C. The magnitude of charges on spheres A and B remains the same as they were initially.
Now, let's analyze the forces between the spheres:
- Initially, when spheres A and B carry equal charge, the force between them is F.
- After sphere C is touched to sphere A, the force between A and B decreases because the induced charges on A and B are of opposite polarity, leading to a partial cancellation of the forces between them. Let this force be F1.
- After sphere C is touched to sphere B, the force between A and B decreases further due to the additional partial cancellation of forces between them by the induced charges. Let this force be F2.
The force between A and B after sphere C is removed is given by:
Force between A and B = F - F1 - F2
Since F1 and F2 are forces of attraction between the induced charges on spheres A and B, they are always positive. Therefore:
Force between A and B < />
Hence, the correct option is (c) 3F/8.
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