Explain that charging of solids by friction involves only a transfer of negative charge (electrons)

4.2.1 Electric Charge – Charging by Friction

When two solids rub together, they can become electrically charged. The key idea is that **only electrons (negative charge) are transferred** between the materials. This is why we say that charging by friction involves a *transfer of negative charge*.

What is Electric Charge?

Electric charge is a property of particles that causes them to experience a force in an electric field. Charges can be positive (+) or negative (−).

In atoms, the number of protons (positive) and electrons (negative) are normally equal, so the atom is neutral.

Frictional Charging

When two different solids are rubbed together, electrons are transferred from one to the other. This happens because the surfaces have different tendencies to hold onto electrons.

• Material A loses electrons → becomes positively charged.
• Material B gains electrons → becomes negatively charged.

⚡️ Analogy: Think of electrons as tiny “water droplets” that can hop from one surface to another when you rub them together. The surface that gives away the droplets becomes “dry” (positive), while the one that receives them becomes “wet” (negative).

Why Only Electrons Move

Protons are fixed inside the atomic nucleus and cannot move easily. Electrons, however, are loosely bound and can be transferred during friction.

Mathematically, the charge transferred can be expressed as:

$Q = n \times e$

where $n$ is the number of electrons transferred and $e$ is the elementary charge ($1.602\times10^{-19}\,\text{C}$).

Common Examples

1. Rubbing a balloon on your hair → balloon becomes negatively charged, hair becomes positively charged.
2. Wearing a wool sweater and touching a metal doorknob → you feel a shock because electrons move to the doorknob.
3. Using a plastic comb on dry hair → comb becomes negatively charged, hair becomes positively charged.

Material Tendencies – A Quick Reference

Quick Summary

• Charging by friction is all about electrons moving from one solid to another. • The solid that loses electrons becomes positive; the one that gains electrons becomes negative. • Protons stay put, so only electrons are involved. • Remember the key phrase: “Electrons move, charges change.”