When the scientist wipes the rubber rod with a wool cloth, a phenomenon known as triboelectric charging occurs. In this process, electrons are transferred from the wool to the rubber rod, giving the rod a negative charge.
When this negatively charged rubber rod touches a neutral metal sphere, the following sequence of events takes place:
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Induction: The negative charge on the rubber rod repels the electrons in the metal sphere. Since the sphere is neutral, it has an equal number of positive and negative charges. The presence of the negatively charged rod causes the electrons in the sphere to move away from the rod, creating a positive area near the rod and a negative area farther away.
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Charge Transfer: When the scientist touches the negatively charged rubber rod to the metal sphere, electrons from the rod are able to move onto the sphere. This is because metals allow free movement of charges. As a result, some of the excess electrons on the rod will transfer to the neutral sphere.
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Final Charge of the Sphere: After the contact, the sphere becomes negatively charged because it has gained excess electrons from the rod. The rubber rod, having lost some of its electrons, will have a lesser negative charge than it did initially.
In summary, the scientist's actions cause a transfer of negative charge from the rubber rod to the neutral metal sphere, resulting in the sphere becoming negatively charged. This is a demonstration of charge transfer via conduction due to contact between a charged object and a neutral conductor.