Describe an experiment to distinguish between electrical conductors and insulators

4.2.1 Electric Charge – Distinguishing Conductors from Insulators

Learning Objective

Describe a simple, controlled experiment that (a) shows how friction can produce a detectable static charge and (b) uses that charge to decide whether an unknown material behaves as an electrical conductor or an insulator.

Syllabus Alignment (Cambridge IGCSE 0625 – 4.2.1)

Key Concepts (AO1)

• Electric charge: two types – negative (excess electrons) and positive (deficit of electrons).
• Measured in coulombs (C); 1 C ≈ 6.24 × 10¹⁸ elementary charges.
• Frictional charging (triboelectric effect): rubbing two different materials transfers electrons from the material with lower electron affinity to the one with higher affinity.
  • Rubber (or plastic) rod with wool → rod becomes **negatively** charged.
  • Glass rod with silk → rod becomes **positively** charged.
• Electric field (E): a region of space in which a charged particle experiences a force.
  • Direction of E is the direction a **positive** test charge would move.
  • Field‑line patterns (illustrative):
    • Point charge – radial lines outward for a positive charge, inward for a negative charge.
    • Charged conducting sphere – radial lines, uniform magnitude outside the sphere, zero inside.
    • Parallel plates – straight, parallel lines from the positive to the negative plate; magnitude is uniform between the plates.

    

    

    

• Electron model of materials:
  • Conductors – outer‑valence electrons are loosely bound and can move freely throughout the material.
  • Insulators – electrons are tightly bound to their atoms; they cannot move appreciably.
• Electroscope (metal‑leaf or pith‑ball type): detects the presence of charge. The greater the charge transferred to the leaves, the larger the divergence.

Apparatus

1. Rubber (or plastic) rod – to be charged negatively.
2. Glass rod – optional, for producing a positive charge.
3. Wool cloth (for the rubber rod) and silk cloth (for the glass rod).
4. Two test objects of unknown nature (e.g., a metal nail and a dry piece of wood).
5. Electroscope (metal‑leaf or pith‑ball) with a non‑conductive stand.
6. Insulating holder for the test objects (e.g., a wooden block).
7. Grounding wire or metal plate for resetting the electroscope.

Experimental Procedure (AO2 – description & reasoning)

Part A – Producing a Detectable Static Charge

1. Place the electroscope on the insulating stand and ensure the leaves hang vertically.
2. Charge the rubber rod:
   • Rub the rod vigorously with the wool cloth for about 10 s. Electrons transfer from the wool to the rod, giving the rod a **negative** charge.
3. Bring the charged rod close (≈1 cm) to the metal knob of the electroscope without touching it.
4. Observe the leaves:
   • If the leaves diverge, the rod has transferred charge to the electroscope – the experiment successfully **produces** a detectable static charge.
5. Ground the electroscope (touch the metal knob to the earth or connect the grounding wire) to discharge it, then lift the ground connection.

Part B – Distinguishing Conductors from Insulators

6. Place the first test object on the insulating holder and ensure it does **not** touch the electroscope.
7. Bring the **same** negatively charged rubber rod close to the test object (again ≈1 cm, no contact).
8. Observe the electroscope:
   • Conductor: free electrons in the object are repelled, travel to the surface, and some flow onto the electroscope’s knob. The leaves spread apart.
   • Insulator: electrons are bound; the external field only polarises the material. No net charge reaches the electroscope, so the leaves stay unchanged.
9. Record the observation (see table below).
10. Ground the electroscope to reset it.
11. Repeat steps 6–10 with the second test object.

Observations Table

Explanation (AO2 – why the observations occur)

When the negatively charged rod is brought near a conductor, the free electrons inside the material experience a repulsive force and move toward the surface that is in electrical contact with the electroscope. Some of these electrons flow onto the electroscope’s metal knob, giving the leaves a net negative charge; like charges repel, so the leaves diverge.

In an insulator, the electrons are tightly bound to their parent atoms. The external electric field from the rod can only cause a slight shift of electron clouds (polarisation) but no net charge is transferred to the electroscope. Consequently the leaves remain in their original position.

The initial part of the experiment (rub‑rod → electroscope) demonstrates that friction can **produce** a static charge, satisfying the syllabus requirement to “describe a simple experiment to produce electrostatic charge by friction”. The second part uses that charge to differentiate conductors from insulators.

Safety Precautions

• Avoid touching the charged rod with bare hands after rubbing – a mild shock may be felt.
• Handle the electroscope gently; the metal leaves are delicate.
• Keep all apparatus away from moisture; damp conditions can make an insulator appear conductive.
• When grounding, ensure the earth point is a good conductor (e.g., a metal pipe or the building’s earth terminal).

Suggested Diagram

Extension Questions

• Why does grounding the electroscope reset it for the next test?
• How would the results differ if a positively charged glass rod were used instead of a negatively charged rubber rod?
• Can a material behave as a conductor in one condition and as an insulator in another? Give at least two examples (e.g., pure water vs. salty water; intrinsic silicon vs. doped silicon).
• Explain how the concept of an electric field helps you understand why the leaves of the electroscope move.
• Sketch the field‑line pattern for a positively charged rod and describe the direction of the field at a point to the right of the rod.