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

4.2.1 Electric Charge

Objective

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

1. What is electric charge?

Electric charge is a fundamental property of matter. It exists in two types:

• Positive charge
• Negative charge

Like charges repel and opposite charges attract. The magnitude of charge is quantised in multiples of the elementary charge \$e\$:

\$Q = n e\$

where \$n\$ is an integer (positive, negative or zero) and \$e = 1.6\times10^{-19}\,\text{C}\$.

2. Conservation of charge

The total charge in an isolated system remains constant. Charge can be transferred from one object to another, but it is never created or destroyed.

3. Methods of charging a solid

• Charging by friction (rubbing)
• Charging by conduction
• Charging by induction

4. Charging by friction – the electron‑transfer model

When two different solids are rubbed together, electrons are transferred from one material to the other. The key points are:

1. Electrons are the only particles that move during frictional charging.
2. The material that loses electrons becomes positively charged.
3. The material that gains electrons becomes negatively charged.
4. No protons are transferred because they are bound in the nuclei.

5. Why only electrons move

• Electrons are located in the outer electron shells and are loosely bound compared with protons.
• During rubbing, mechanical energy can overcome the binding energy of outer‑shell electrons, allowing them to jump to the other surface.
• Protons reside in the nucleus and are bound by the strong nuclear force; the energy required to remove a proton is many orders of magnitude larger than that supplied by friction.

6. Evidence for electron transfer

Experiments that support the electron‑transfer model include:

1. Using a electroscope, a rubber rod rubbed with wool makes the electroscope’s leaves diverge (negative charge), indicating an excess of electrons on the rod.
2. Measuring the charge on two rubbed objects shows equal magnitude and opposite sign, consistent with \$Q{\text{lost}} = -Q{\text{gained}}\$.
3. Materials known to have a high electron affinity (e.g., glass) consistently become positively charged when rubbed with materials of lower electron affinity (e.g., silk).

7. Typical material pairings

Materials can be classified as “electron donors” (tend to lose electrons) or “electron acceptors” (tend to gain electrons). The table below summarises common pairings used in the IGCSE curriculum.

8. Practical demonstration – rubbing a glass rod with silk

1. Clamp a clean glass rod horizontally.
2. Take a dry piece of silk and rub it firmly along the length of the rod (≈ 10 s).
3. Immediately bring the rod near a small piece of dry paper.
4. The paper is attracted to the rod, indicating that the glass has acquired a negative charge (excess electrons).
5. Touch the rod with a metal probe connected to an electroscope; the leaves diverge, confirming the presence of charge.

9. Summary of key points

• Frictional charging involves only the transfer of electrons.
• The object that loses electrons becomes positively charged; the object that gains electrons becomes negatively charged.
• Protons remain in the nuclei; they do not move during friction.
• Charge is conserved: \$Q{\text{lost}} + Q{\text{gained}} = 0\$.
• Material properties (electron affinity) determine which object will gain or lose electrons.