When a plastic comb is moved in dry hair, it starts to attract small pieces of paper. Similarly, when a glass rod is rubbed with silk cloth or an ebonite rod is rubbed with a woolen cloth, these rods also begin to attract small particles. We say that these substances have developed the property of attraction because they have acquired electric charge. The following experiment shows that electric charges are of two types:
i. Rub two glass rods with silk and suspend them horizontally in a line close to each other (fig. 15.1-a). You would see that they repel each other.
ii. If you repeat this experiment with two ebonite rods after rubbing them with a woolen cloth, you would again see that these two rods repel each other (fig.15. 1-b)
In the first part of the experiment, as both the rods are of glass and both of them have been rubbed with silk, so we are justified to assume that the charge on both the rods would be of the same kind. Similarly, in the second part of the experiment, both the ebonite rods would have the same kind of charge. Hence, we conclude from this experiment that like charges repels each other.
Two ebonite rods rubbed with wool repel each otheriii. Now suspend the glass rod rubbed with silk near an ebonite rod rubbed with wool, this time the two rods attract each other (fig. 15.1-c). this shows that the charge on the two rods is not of the same kind and their nature is opposite. Therefore, in order to differentiate between the two kinds, we call the charge on glass rod positive and that on an ebonite rod negative. From this part of the experiment, we conclude that unlike charges attract each other.
The charge on glass and ebonite rod is in fact due to the presence of positive and negative charged particles. The glass rod rubbed with silk has a large number of positive particles, whereas an ebonite rod rubbed with wool has a large number of negative particles. Now question arises, from where these positive and negative particles have appeared?
The answer to this question can be given by considering the structure of atom, the fundamental particle of matter. The atom of an element consists of two parts one is its central part, known as nucleus. It consists of protons and neutrons. The charge on a proton is positive, while the neutron has no charge. The second part of the atom consists of electrons which are moving outside the nucleus in various orbits.
The electron has a negative charge. The charge on a proton and that on an electron are equal in magnitude but opposite in kind. The atom as a whole is neutral. This would be possible only when the number of protons and electrons present in an atom are equal we know that there is a force of attraction between positive and negative charges.
So there exists a force of attraction between the protons present in the nucleus and the outside orbiting electrons. Due to this force of attraction the electrons are constrained to move in their respective orbits which they cannot leave. But as the distance of orbits from the nucleus increases the force of attraction between the nucleus and the electron decreases.
Hence the electrons in the outermost orbit are bound in the orbit with a very weak force. Such electrons can be separated from their respective atoms by a small externally applied force. When an ebonite rod is provided an external force by rubbing with wool, the loosely bound electrons in the wool are transferred to ebonite rod. As electrons carry negative charge, therefore, a negative charge is developed on the ebonite rod. When glass rod is rubbed with silk, the loosely bound electrons of glass are transferred to silk, the due to which number of electrons in the glass gets deficient, so it shows a positive charge.
Solid bodies are charged due to the transfer of electrons. In general a solid body is neutral. If electrons are transferred to it, it gets negatively charged. If electrons leave it, it gets positively charged. In metallic substances, the electrons in the last orbit are so loosely bound that their motion is no longer confined to its orbit but instead they can move freely through the whole body of the metal. These are known as free electrons and they can be easily transferred to other bodies.