Question

A negatively charged ebonite rod is made to touch an insulated brass sphere. The brass sphere gets electrically charged by the process of
A) Conduction
B) Induction
C) Radiation
D) All the above

Answer 1

Hint: A negatively charged material has excess number of electrons inside the material. When such a negatively charged material touches a metal, some of the electrons in the charged material get transferred to the metal. An insulated metal sphere cannot transfer charges to its surroundings.

Complete step by step answer:
We know that the number of electrons in a negatively charged material is higher than the number of positive charges inside the material. It can be said that a negatively charged material has an excess number of electrons. Suppose we bring such a negatively charged material close to a metal in such a manner that both the negatively charged material and the metal touch each other, some of the electrons from the negatively charged material get transferred to the metal. If an insulated metal is used in the process, charges inside the metal are restricted from further transfer to the surroundings.
Let us take an example, in which a negatively charged ebonite rod is made to touch an insulated brass sphere. We know that the given ebonite rod has an excess number of electrons. When this negatively charged ebonite rod is made to touch an insulated brass sphere, it is seen that some of the electrons inside the ebonite rod get transferred to the brass sphere. These electrons are free to move inside the brass sphere and tend to repel with each other, causing them to move to the surface of the brass sphere. Since the brass sphere is insulated, these electrons cannot move further to the surroundings or the ground, if connected. At this stage, the brass sphere is said to have electrically charged. This process of transfer of excess charges from a material to another material with the help of a contact is known as electric conduction. The following diagram clarifies the above explanation.

As clear from the diagram, the brass sphere gets negatively charged due to conduction. The brass sphere remains electrically charged even when the ebonite rod is taken away. Therefore, the correct answer is option A.

Additional information:
In electrostatics, induction refers to the attraction of unlike charges. For example, if a negatively charged polythene rod is brought close to an insulated conductor, the insulated conductor gets positively charged by the process of induction. Note that in the case of induction, the materials do not touch each other. They are only brought close to each other.

Note:
Electric conduction can also happen the other way round. When a positively charged particle is made to touch a metal, the metal gets positively charged. Here, the positively charged material has an excess number of positive charges. These positive charges, upon contact with the metal, attract electrons from the metal. The metal loses electrons in bulk, leaving only positive charges on it. Thus, the metal gets positively charged.