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*.
Exam Tip: Remember that in frictional charging the *electron* moves from one solid to the other. The solid that loses electrons becomes positively charged, while the one that gains electrons becomes negatively charged.
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.
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.
⚡️ 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).
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}\$).
| Material | Tendency to Gain Electrons | Tendency to Lose Electrons |
|---|---|---|
| Rubber | High | Low |
| Glass | Low | High |
| Plastic | High | Low |
Exam Tip: When asked to explain why a particular material becomes positively or negatively charged, mention the transfer of electrons and the material’s tendency to either gain or lose electrons.
• 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.”