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

4.2.1 Electric Charge

Objective

Students will be able to:

• State that there are positive and negative charges and recall the attraction/repulsion rules.
• Describe and carry out simple experiments that show charge produced by friction and how it can be detected.
• Explain why charging by friction involves only the transfer of electrons (negative charge).
• Distinguish conductors from insulators by a practical activity.
• State the unit of charge (coulomb) and give a basic description of electric fields and their typical patterns.

1. What is electric charge?

• Two types: positive (+) and negative (‑).
• Like charges repel; opposite charges attract.
• Charge is a fundamental property of matter.

2. Quantisation of charge

The charge on any object is an integer multiple of the elementary charge \(e\):

\[ Q = n\,e \qquad (n = 0,\pm1,\pm2,\dots) \]

where \(e = 1.6\times10^{-19}\,\text{C}\).

3. Unit of charge

The SI unit is the coulomb (C). One coulomb equals the charge of \(6.25\times10^{18}\) electrons.

4. Conservation of charge

In an isolated system the total charge remains constant:

\[ Q{\text{lost}} + Q{\text{gained}} = 0 \]

Charge can be transferred between objects but is never created or destroyed.

5. Methods of charging a solid (Core requirement)

1. Charging by friction (rubbing)
2. Charging by conduction
3. Charging by induction

6. Charging by friction – the electron‑transfer model

When two different solids are rubbed together, only electrons move from one surface to the other.

• The material that loses electrons becomes positively charged.
• The material that gains electrons becomes negatively charged.
• Protons remain bound in atomic nuclei and do not move during friction.

7. Why only electrons move

• Electrons in the outer shells are loosely bound (binding energies of a few eV).
• The mechanical energy supplied by rubbing can overcome these binding energies, allowing electrons to jump to the other surface.
• Protons are confined to the nucleus by the strong nuclear force; removing a proton would require ≈ 10⁶ MeV, far beyond any ordinary frictional energy.

8. Experimental evidence for electron transfer

1. Electroscope test: Rub a rubber rod with wool, bring it near a gold‑leaf electroscope – the leaves diverge, showing an excess of electrons on the rod.
2. Charge balance: Measuring the charge on the two rubbed objects always gives equal magnitude and opposite sign, confirming \(Q{\text{lost}} = -Q{\text{gained}}\).
3. Material‑pair trend: Materials with higher electron affinity (e.g., glass) become negatively charged when rubbed with a lower‑affinity material (e.g., silk), indicating that electrons move from the donor to the acceptor.

9. Typical material pairings (IGCSE “electron donor / acceptor” table)

10. Practical: Producing charge by friction

Materials: rubber rod, wool cloth, small pieces of dry paper, gold‑leaf electroscope (or simple electroscope).

1. Clamp the rubber rod horizontally.
2. Rub the wool firmly along the length of the rod for about 10 s.
3. Observation: Bring a piece of paper close to the rod. The paper is attracted – the rod now carries a negative charge (excess electrons).
4. Touch the rod with the metal probe of the electroscope. The leaves diverge, confirming the presence of charge.

Link to syllabus: Demonstrates charge production by friction (Core AO 1) and detection with an electroscope (Core AO 3).

11. Quick‑check experiment – distinguishing conductors from insulators

Purpose: Show that only free electrons can move readily, fulfilling the syllabus point on comparing conductors and insulators.

Materials: metal rod, plastic (e.g., PVC) rod, wool cloth, electroscope.

1. Charge the metal rod by rubbing it with wool; repeat with the plastic rod.
2. One at a time, bring each rod close to the electroscope (without touching).
3. Observations:

   • Metal rod: The charge spreads quickly over the whole surface; the electroscope shows a strong, immediate leaf deflection.
   • Plastic rod: The charge remains localized; the electroscope shows a weaker, slower response.

4. Conclusion: Metals (conductors) allow electrons to move freely, whereas plastics (insulators) restrict electron motion.

12. Electric field – brief supplement (Supplementary requirement)

An electric field (\(\mathbf{E}\)) is a region of space around a charged object where another charge would experience an electrostatic force:

\[ \mathbf{F} = q\,\mathbf{E} \]

Field‑line conventions:

• Lines start on positive charges and end on negative charges.
• The density of lines indicates field strength.
• Field lines are always perpendicular to the surface of a conductor.

Suggested ready‑to‑use diagrams

1. Point charge: Radial lines outward for a positive charge, inward for a negative charge.
2. Charged conducting sphere: Lines emerge uniformly from the surface (positive) or converge onto it (negative) and are perpendicular to the surface.
3. Parallel‑plate capacitor: Uniform, parallel lines between the plates, pointing from the positive plate to the negative plate.

These sketches can be drawn on the board or inserted into slide decks; label the direction of the field (the direction a positive test charge would move).

13. Summary of key points

• Frictional charging involves the transfer of electrons only; protons stay in nuclei.
• The object that loses electrons becomes positively charged; the object that gains electrons becomes negatively charged.
• Charge is conserved: the total charge of the two objects after rubbing is zero.
• Charge is measured in coulombs (C) and occurs in integer multiples of the elementary charge \(e\).
• Material properties (electron‑affinity or “donor/acceptor” tendency) decide which solid gives up electrons.
• Conductors allow electrons to spread quickly; insulators keep the charge localized.
• Electric fields describe the force a charge would feel; standard field‑line patterns are point charge, charged sphere, and parallel‑plate capacitor.