All matter contains positive charges (protons) and negative charges (electrons). In a neutral object, the positive and negative charges are equal. Objects become charged when electrons are transferred between them.
Charging by friction
When two insulating materials are rubbed together:
- Electrons transfer from one material to the other.
- The material that gains electrons becomes negatively charged.
- The material that loses electrons becomes positively charged.
- No charge is created or destroyed; it is transferred.
Example: rubbing a polythene rod with a cloth. Electrons transfer from the cloth to the rod. The rod becomes negatively charged; the cloth becomes positively charged.
Laws of electrostatics
- Like charges repel (positive repels positive; negative repels negative).
- Unlike charges attract (positive attracts negative).
Why charged objects attract uncharged objects
A charged object brought near a neutral object induces a charge separation. The near side of the neutral object develops an opposite charge (by attraction of electrons or repulsion of electrons within the material). The attractive force between the closer opposite charges is stronger than the repulsive force between the further like charges, so there is a net attraction.
Conductors and insulators
- Conductors (metals): electrons are free to move, so charge flows easily. Conductors cannot hold a static charge unless insulated from their surroundings.
- Insulators (plastic, glass, rubber): electrons are not free to move, so charge stays where it is placed. Insulators can hold a static charge.
Common errors and how to correct them
Saying “protons are transferred.” Only electrons move. Protons are fixed in the nucleus and do not transfer between materials.
Stating “friction creates charge.” Friction transfers charge from one material to another. The total charge is conserved.
How to apply this in an exam
Always describe charging in terms of electron transfer. State which material gains electrons (becomes negative) and which loses electrons (becomes positive). For attraction of neutral objects, explain induced charge separation.
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