4.2.1 Electric charge – Distinguishing Conductors from Insulators
Learning Objective
Describe a simple experiment that can be used to tell whether a material is an electrical conductor or an insulator.
Key Concepts
Electrical charge can be transferred by friction (static electricity).
Conductors allow charge to move freely; insulators do not.
When a charged object is brought close to a material, the material’s ability to allow charge flow can be observed.
Apparatus
Rubber rod (or plastic rod) – to be charged by rubbing with wool.
Wool cloth – for charging the rod.
Two test objects of unknown nature (e.g., a metal nail and a piece of dry wood).
Electroscope (or simple pith‑ball electroscope).
Insulating stand or non‑conductive holder.
Experimental Procedure
Secure the electroscope on the insulating stand.
Rub the rubber rod vigorously with the wool cloth for about 10 s to give it a negative charge.
Bring the charged rod close to the first test object without touching it. Observe the electroscope:
If the electroscope’s leaves diverge, charge has been transferred to the object – it is a conductor.
If there is no noticeable change, the object is likely an insulator.
Remove the first object, discharge the electroscope (ground it), and repeat the test with the second object.
Record the observations for each material.
Observations Table
Test Object
Electroscope Response
Conclusion
Object A (e.g., metal nail)
Leaves spread apart
Conductor
Object B (e.g., dry wood)
No movement of leaves
Insulator
Explanation
When the negatively charged rod is brought near a conductor, free electrons in the conductor are repelled and travel to the surface, allowing charge to flow onto the electroscope and causing its leaves to diverge. In an insulator, electrons are tightly bound to atoms, so little or no charge moves, and the electroscope remains unchanged.
Safety Precautions
Do not touch the charged rod with bare hands after rubbing – it may give a mild shock.
Handle the electroscope gently to avoid breaking the delicate leaves.
Keep the experiment away from moisture, which can affect the insulating properties of materials.
Suggested Diagram
Suggested diagram: Schematic of the electroscope setup with a charged rod, test objects, and observation of leaf movement.
Extension Questions
Why does grounding the electroscope reset it for the next test?
How would the results differ if a positively charged rod were used?
Can a material behave as a conductor in one condition and as an insulator in another? Provide examples.