When two different materials are rubbed against each other, a transfer of electric charge can occur. This process is known as charging by friction or triboelectric effect. One material gains electrons and becomes negatively charged, while the other loses electrons and becomes positively charged.

Understanding Charge Transfer During Rubbing

Electric charges come in two types: positive charges (carried by protons) and negative charges (carried by electrons).

• Protons are located in the nucleus of atoms and are generally not easily moved from one material to another during simple friction.
• Electrons are orbiting the nucleus and, in many materials, especially conductors or insulators with loose outer electrons, can be transferred from one atom or material to another.

Therefore, when materials are rubbed together, it is the transfer of negative charges (electrons) that is responsible for the net charge acquired by each material.

Analyzing the Rod and Wool Scenario

In this specific case, a rod is rubbed with wool, and the rod becomes negatively charged. For the rod to become negatively charged, it must have gained a surplus of electrons. These extra electrons must have come from the other material involved in the rubbing, which is the wool.

So, the negative charges (electrons) are transferred from the wool to the rod.

Evaluating the Given Statements

Let's examine each statement based on our understanding of charging by friction:

1. The positive charges are transferred from rod to wool.

   This statement is incorrect. Positive charges (protons) are not typically transferred during rubbing. Charge transfer involves electrons.

2. The positive charges are transferred from wool to rod.

   This statement is also incorrect. Again, positive charges are not transferred during rubbing. Electron transfer causes charging.

3. The negative charges are transferred from rod to wool.

   This statement suggests the rod lost negative charges and gave them to the wool. If the rod lost negative charges, it would become positively charged, or less negatively charged if it was initially neutral. However, the question states the rod becomes negatively charged, meaning it gained negative charges.

4. The negative charges are transferred from wool to rod.

   This statement suggests the wool lost negative charges (electrons) and the rod gained them. If the rod gains negative charges, it becomes negatively charged. This aligns perfectly with the information given in the question that the rod becomes negatively charged.

Conclusion on Charge Transfer

When a rod is rubbed with wool and becomes negatively charged, it signifies that the rod has acquired electrons. These electrons originated from the wool. Thus, the transfer of negative charges occurred from the wool to the rod.

Therefore, the correct statement is that the negative charges are transferred from wool to rod.

Revision Table: Charge Transfer by Friction

Additional Information on Electrostatic Charging

Charging an object means giving it a net electric charge, either positive or negative. Besides friction, objects can be charged by other methods:

• Charging by Conduction: This happens when a charged object touches an uncharged object. Some charge is transferred through direct contact.
• Charging by Induction: This happens when a charged object is brought near an uncharged object without touching it. The presence of the charged object causes a separation of charges within the uncharged object, creating an induced charge.

The triboelectric series is a list that ranks materials based on their tendency to gain or lose electrons when rubbed together. A material higher on the list tends to lose electrons (become positive), while a material lower on the list tends to gain electrons (become negative).