12.2 - Charging by Rubbing

Charging by rubbing is a method of transferring charge between two objects by rubbing them together. This process involves the transfer of electrons from one object to another. In this section, we will explore the process of charging by rubbing, the types of charges, and the results of such an interaction.

When two objects are rubbed against each other, one object may lose electrons and the other may gain them. This happens because the objects have different tendencies to attract electrons, a property known as electronegativity. The object that has a greater tendency to gain electrons will acquire a negative charge, while the object that loses electrons will develop a positive charge.

Example: Rubbing a Glass Rod with a Silk Cloth

One of the simplest examples of charging by rubbing is rubbing a glass rod with a silk cloth. When the glass rod is rubbed with the silk cloth, electrons are transferred from the glass to the silk. As a result, the glass rod becomes positively charged (because it loses electrons), and the silk cloth becomes negatively charged (because it gains electrons).

Nature of Charges

There are two types of electric charges: positive charge and negative charge. These charges behave in the following way:

• Like charges repel each other.
• Opposite charges attract each other.

Law of Conservation of Charge

According to the law of conservation of charge, the total charge in an isolated system remains constant. This means that when two objects are rubbed together and charge is transferred, the total amount of positive and negative charge remains the same. If one object gains an equal amount of negative charge, the other object must lose the same amount of positive charge.

Induced Charge

When a charged object is brought near a neutral object, it can induce a charge on the neutral object. The neutral object becomes polarized, meaning that the charges within the object rearrange. The side of the neutral object closest to the charged object will become oppositely charged, while the far side will acquire a charge of the same type as the charged object.

Illustration with a Plastic Rod and Wool

If a plastic rod is rubbed with wool, the plastic rod gains electrons and becomes negatively charged, while the wool loses electrons and becomes positively charged. If a neutral object, such as a small piece of paper, is brought near the charged plastic rod, the paper will be attracted to the rod. This happens because the electrons in the paper will be repelled by the negative charge on the plastic rod, causing the paper to become polarized and creating an attraction between the paper and the rod.

Charging by Contact

Charging by rubbing is often followed by charging by contact. In this case, when a charged object is brought into contact with a neutral object, the neutral object gains the same type of charge as the charged object. For example, if a negatively charged rod is touched to a neutral metal sphere, the metal sphere will become negatively charged as well, since electrons from the rod transfer to the sphere.

Electrostatic Series

Materials have different tendencies to lose or gain electrons when rubbed. The electrostatic series is a list of materials arranged according to their ability to either donate or accept electrons. Some materials, like glass, have a high tendency to lose electrons, while others, like rubber, are more likely to gain electrons. The electrostatic series helps to predict which object will become positively or negatively charged when rubbed together.

The electrostatic series helps explain why certain combinations of materials will always result in one object becoming positively charged and the other negatively charged. This series is important for understanding and predicting the behavior of materials in electrostatic experiments.