Explain the concept and principles of static electricity (detection of static charges, types of materials, capacitors, charge distributions and lightning conductor)

Static charges can be detected using simple tools:

• Gold-leaf electroscope: A metal rod with a thin gold leaf inside a glass container. When a charged object touches the metal cap, the gold leaf repels from the rod and moves away, showing that charge is present.
• Pith ball electroscope: Two small pith balls suspended from a thread. When a charged object is brought near, the pith balls either attract or repel each other.
• Attraction of light objects: A charged comb or plastic rod can attract small pieces of paper due to the induced charge.

Materials are classified based on their ability to allow electric charge to move through them:

Charging by induction occurs when a charged object is brought near a conductor without touching it. The charges in the conductor rearrange: negative charges are repelled, leaving positive charges near the charged object.

A capacitor is a device that stores electric charge. It consists of two conductive plates separated by an insulating material (dielectric).

How it works:

• When a voltage is applied across the plates, electrons accumulate on one plate, making it negative
• The opposite plate loses electrons, becoming positive
• The charge remains stored even when the voltage source is removed

Simple capacitor example: A parallel-plate capacitor with plate area $0.02\ \text{m}^2$ and plate separation $0.005\ \text{m}$ has capacitance calculated by:

$C = \frac{\varepsilon_0 A}{d}$

Where $\varepsilon_0 = 8.85 \times 10^{-12}\ \text{F/m}$. Substituting values:

$C = \frac{(8.85 \times 10^{-12}) \times 0.02}{0.005} = 3.54 \times 10^{-11}\ \text{F}$

This shows capacitors can store very small amounts of charge.

On a charged conductor, charge distributes unevenly:

• On pointed surfaces: Charge concentrates at sharp points, creating strong electric fields
• On curved surfaces: Charge is greater on more curved (pointed) areas
• Inside a hollow conductor: No charge resides on the inner surface; all charge appears on the outer surface

This principle explains why lightning rods have sharp points.

A lightning conductor (lightning rod) protects buildings from lightning strikes.

How it works:

1. The pointed metal rod at the top of the building is connected to a thick copper wire
2. The wire runs down the building and is buried deep in the ground
3. When a negatively charged cloud passes overhead, the sharp point induces a positive charge on the ground below
4. The strong electric field at the sharp point ionizes the air, allowing a safe path for lightning to travel to the ground
5. The charge flows safely through the wire into the earth, preventing damage to the building

Key parts: The lightning conductor has a sharp metal rod (air terminal), a connecting wire, and a ground electrode (earth plate).

A plastic comb is rubbed on hair and becomes negatively charged. When the comb is brought near small pieces of paper:

a) Why do the pieces of paper move toward the comb? b) What type of charging is this?

Solution:

a) The negatively charged comb induces a positive charge on the near side of each paper piece. Since opposite charges attract, the papers move toward the comb.

b) This is charging by induction. The comb does not touch the papers, yet the papers become temporarily polarized (positive on one side, negative on the other), causing attraction.

In Tanzania, lightning strikes are common during the rainy season, especially in areas like Mbeya, Morogoro, and Dar es Salaam. Buildings such as schools, markets, and homes use lightning conductors to protect people and property. For example, a shop in Arusha with a metal roof should have a properly installed lightning conductor connected to a copper plate buried in the ground — this prevents fire hazards and keeps electrical equipment safe during thunderstorms.