Understanding Charging by Friction

Charging by friction, also known as triboelectric charging, occurs when two different materials are rubbed against each other. This rubbing causes electrons to transfer from one material to the other. One material gains electrons and becomes negatively charged, while the other loses electrons and becomes positively charged.

Conductors vs. Insulators and Charging by Friction

Materials can be broadly classified into conductors and insulators based on how easily electric charge can move through them.

• Conductors: These materials allow electric charge to move freely through them. Metals like copper are excellent conductors. If a conductor gains or loses charge at one point, the charge quickly distributes itself across the entire material. If the conductor is connected to the ground (like through a person holding it), any acquired charge can quickly flow away.
• Insulators: These materials do not allow electric charge to move freely. Materials like plastic, rubber, wool, and cloth are good insulators. If an insulator gains or loses charge at one point, the charge tends to stay localized in that area. This localization makes it easier to build up and retain a static electric charge on the surface of an insulator.

Charging Different Materials by Friction

Let's consider the options provided in the question:

• A woolen cloth: Wool is a good insulator. It is commonly used in experiments to demonstrate charging by friction (e.g., rubbing a balloon or a plastic rod with a woolen cloth). Wool easily gains or loses electrons when rubbed, building up a static charge.
• An inflated balloon: Balloons are typically made of rubber, which is an insulator. Balloons can be easily charged by rubbing them against hair or a woolen cloth. The static charge allows them to stick to walls or attract small pieces of paper.
• A plastic scale: Plastic is also an insulator. Rubbing a plastic scale against dry hair or a piece of cloth can easily charge it, enabling it to attract small objects like paper bits.
• A copper rod: Copper is a metal and an excellent conductor. While charge transfer does happen when a copper rod is rubbed against another material, any charge acquired will quickly spread throughout the rod. Furthermore, if the copper rod is held by a person (who is also a conductor and likely in contact with the ground), the charge will quickly dissipate into the ground. Therefore, it is very difficult to build up a significant, sustained static charge on a conductor like a copper rod by simple friction, especially compared to insulating materials.

The question asks which material cannot be charged easily by friction. Based on the properties of conductors and insulators, a copper rod, being a conductor, is significantly harder to charge and retain a static charge compared to the insulating materials listed.

Conclusion

Insulators like a woolen cloth, an inflated balloon, and a plastic scale can be easily charged by friction because the acquired charge remains localized on their surface. Conductors like a copper rod are difficult to charge easily by friction because the charge disperses rapidly and can dissipate if the object is not perfectly insulated from the ground.

Therefore, a copper rod cannot be charged easily by friction.