Topic 2 of 5
Charging by rubbing
Rubbing can transfer electrons from one material to another. It separates charge rather than creating it.
An electron carries negative charge. A neutral object has balanced total positive and negative charge. During the ordinary charging processes here, protons remain bound in their atomic nuclei; they do not move from the cloth to the rod.
Follow the electron transfer
Contact and separation as two materials are rubbed can move electrons between them. The final sign depends on which material gains electrons. Use the stated direction of transfer; do not assume every kind of plastic rod always gains or always loses electrons.
| Change to an initially neutral object | Result |
|---|---|
| Gains electrons | Excess negative charge: the object becomes negative. |
| Loses electrons | Electron deficit: the object becomes positive. |
Count the electrons before and after transfer
Each material starts with four positive charge symbols and four electron symbols in this small model. The symbols are not a measured particle count. Positive charges remain in their original material.
The cloth loses electrons and the rod gains them. With no other charge transfer, the pair's total charge remains zero.
Charge accounting
Electrons move from cloth to rod
In the drawing, each material begins with four positive-charge symbols and four electrons. Each positive symbol balances one electron. After two electrons transfer:
- Rod: four positive symbols and six electrons give a negative excess.
- Cloth: four positive symbols and two electrons give a positive excess.
- Whole pair: there are still eight positive symbols and eight electrons. Its total charge has not changed.
These are schematic charge counts, not measured particle counts in real materials. If both objects began neutral and no charge entered or left the pair, their final net charges are equal in magnitude and opposite in sign.
A positive rod would instead indicate that it had lost electrons in the stated process. Becoming positive does not require positive particles to arrive from the cloth.
Why a charged object may need insulation
In a conductor, some electrons can move through the material. In an insulator, excess charge is much less free to move through the material, so it can remain near the region that was rubbed.
Holding a conductor directly can provide a path through the hand to Earth, allowing charge to leak away. An insulating handle or support helps separate the charged conductor from that path. Insulation does not mean the material contains no charged particles.
Separate the observation from its explanation
Compare a rod's effect before and after rubbing, using similar distances and the same small test objects. A rubbed rod may attract small neutral paper pieces, or repel an appropriately charged suspended object.
The movement is the observation. Electron transfer is the explanation for the rod's changed net charge. Attraction of neutral paper alone does not identify whether the rod is positive or negative; the charge balance inside a neutral object can respond to a nearby charged object.
Damp surfaces and unintended contact can allow charge to leak away. A weak or short-lived effect does not establish that the material contains no charge. Keep the support, surroundings and comparison distance similar when comparing trials.
Check the boundary of the charge account. Equal opposite final charges follow for an initially neutral pair with no other charge transfer. Charge lost to the hand or surroundings must also be included in a wider account.