Describe an experiment to distinguish between electrical conductors and insulators

4.2.1 Electric charge

Objective: Describe an experiment to distinguish between electrical conductors and insulators

Imagine a river (electric current) flowing through different types of pipes. A conductor is like a smooth, wide pipe that lets water flow freely, while an insulator is like a narrow, rough pipe that blocks the flow. In this experiment we will use a simple “water‑flow” analogy with a static electricity setup to see which materials allow the “water” (charge) to pass through and which do not.

Materials Needed

• Two identical glass rods (≈10 cm long) – one rubbed with a wool cloth, the other left untouched.
• Two small metal spheres (≈2 cm diameter) – one made of aluminium, the other of plastic.
• Thin, non‑conductive thread (≈30 cm) – e.g., nylon or silk.
• Support stand with a clamp to hold the thread.
• Lightweight paper or a small piece of cardboard.
• Optional: a small LED light.

Experimental Procedure

1. Charge the glass rod. Rub the first rod vigorously with the wool cloth for about 30 seconds. This transfers electrons from the cloth to the rod, giving it a negative charge. ⚡️ Tip: The more friction, the stronger the charge.
2. Attach the thread. Tie one end of the thread to the charged rod and the other end to the aluminium sphere. 🔌 Note: The aluminium sphere is a good conductor and will share the charge.
3. Observe the effect. Hold the aluminium sphere near a small piece of paper.
   • If the paper lifts or the sphere attracts the paper, the aluminium sphere is conducting the charge.
   • Place the plastic sphere in the same position and observe that nothing happens – the plastic is an insulator.
4. Optional LED test. Connect the LED to the aluminium sphere and the other end to a ground point.
   • If the LED lights up, the aluminium is conducting.
   • If the LED remains off, the plastic is not conducting.

Why It Works – The Science Behind It

When the glass rod is rubbed, electrons are transferred from the wool to the rod, giving the rod a negative charge ($Q_{\text{rod}} < 0$). A conductor (aluminium) has free electrons that can move easily. The negative charge on the rod induces a positive charge on the aluminium sphere’s surface opposite the rod, creating an attractive force $F = k \frac{|Q_{\text{rod}} Q_{\text{sphere}}|}{r^2}$. An insulator (plastic) has tightly bound electrons; the charge cannot move, so no attraction occurs. This is analogous to water flowing through a pipe: smooth pipes (conductors) allow flow, rough pipes (insulators) block it.

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