Charging by Friction

During ordinary electrostatic charging, the protons do not move between objects: they are held inside atomic nuclei. Some outer electrons can be removed from one material and transferred to another. Therefore, explanations of charging by friction must describe the movement of electrons.

What rubbing does

When two insulating materials are rubbed together, their surfaces repeatedly make very close contact. The two materials hold surface electrons with different strengths, so some electrons can be pulled from one surface and transferred to the other. Rubbing provides the energy and repeated contact needed for this transfer.

It is important that friction does not create electrons or manufacture charge. The electrons were already present: rubbing simply transfers them, separating charge between the two materials.

Imagine two neutral insulating strips. If several electrons move from strip A to strip B, strip A has an electron shortage while strip B has an electron excess. Both strips have therefore become charged, although the total charge of the pair has not been created or destroyed.

Why the charge remains on an insulator

An insulator has almost no charges that are free to move through it. Electrons transferred onto or away from one part of its surface therefore cannot easily spread through the material or flow to Earth. The resulting charge remains in place and is described as static electricity.

Examples of insulating materials include polythene, Perspex or acetate, rubber and nylon. A dry insulating rod can consequently remain charged after being rubbed with a cloth. Moisture makes surfaces more conducting, so static charge tends to leak away more quickly in damp conditions.

This differs from a metal conductor held in a bare hand. In a conductor, charge can move freely and may pass through the hand and body to Earth, so it does not remain built up on the metal in the same way.

A concrete example

When a dry polythene rod is rubbed with a woollen cloth, electrons are transferred from the cloth to the rod. The essential mechanism is the transfer itself: the cloth loses exactly the electrons that the rod receives. Once the materials are separated, the transferred charge remains on their surfaces because they are insulators.

Which way electrons move depends on the particular pair of materials. You cannot decide the direction merely from the fact that they were rubbed; a question must identify the materials or provide information about the resulting charge.

The complete causal chain is therefore:

rubbing and repeated surface contact → electrons transfer between the materials → an imbalance of charge is produced → the charge remains because the materials are insulators.