Learning Objectives
- Understand lightning and electric discharge
- Learn about static electricity and charging by rubbing
- Understand earthquakes, their causes, and safety measures
- Know about the construction of a lightning conductor
Key Concepts
Electric Charge
When certain objects are rubbed together, they acquire an electric charge. This is called static electricity (charges at rest). There are two types of charges: positive (+) and negative (-).
Like charges repel each other; unlike charges attract each other.
When a glass rod is rubbed with silk, the glass rod becomes positively charged (loses electrons) and the silk becomes negatively charged (gains electrons). When a plastic comb is rubbed with dry hair, the comb becomes negatively charged.
Transfer of Charge
Charges are transferred by the movement of electrons. The object that loses electrons becomes positively charged; the object that gains electrons becomes negatively charged. Charge is neither created nor destroyed, only transferred.
Electroscope
An electroscope is a device used to detect the presence of electric charge on a body. When a charged object touches the metal disc, the gold leaves diverge (spread apart) because they acquire the same charge and repel.
Lightning
Lightning is an electric discharge (spark) that occurs between clouds, or between a cloud and the earth. During a thunderstorm, air currents move upward and water droplets move downward. The collision of these particles causes separation of charges. The top of the cloud becomes positively charged and the bottom becomes negatively charged. When the charge buildup is large enough, it is discharged as lightning.
Lightning Safety
- Stay indoors during a thunderstorm
- Do not stand under tall trees, poles, or open areas
- Do not use telephones or electrical appliances
- If in a car, stay inside with windows closed
- If in open space, crouch low and keep feet together
Lightning Conductor
A lightning conductor is a metal rod installed on tall buildings. It is connected to a metal plate buried in the ground. When lightning strikes, the charge is safely conducted to the ground through the metal rod, protecting the building.
Earthquakes
An earthquake is a sudden shaking or trembling of the earth caused by disturbances deep inside the earth's crust. Earthquakes are caused by the movement of tectonic plates.
Seismic zones: Regions prone to earthquakes. Focus: The point inside the earth where the earthquake originates. Epicentre: The point on the surface directly above the focus.
Seismograph: An instrument used to record the intensity of earthquakes. Earthquake intensity is measured on the Richter scale.
Earthquake Safety
- Take shelter under a strong table or doorway
- Stay away from tall buildings, walls, electricity poles
- If outdoors, go to an open field away from buildings
- Buildings in seismic zones should be designed to withstand earthquakes
Summary
Static electricity is created by rubbing objects together, which transfers electrons. Lightning is a massive electric discharge between clouds or cloud and ground. Lightning conductors protect buildings. Earthquakes are caused by tectonic plate movements and are measured on the Richter scale using a seismograph.
Important Terms
- Static Electricity: Electric charge at rest, produced by friction
- Lightning: Electric discharge between clouds or cloud and ground
- Lightning Conductor: Metal rod on buildings that safely conducts lightning to the ground
- Earthquake: Sudden shaking of the earth due to tectonic plate movement
- Richter Scale: Scale used to measure earthquake intensity
- Seismograph: Instrument that records earthquake waves
- Epicentre: Point on earth's surface directly above the earthquake's focus
Quick Revision
- Like charges repel; unlike charges attract
- Rubbing transfers electrons → static charge
- Lightning = electric discharge; use lightning conductor for safety
- Earthquake: tectonic plates move; measured on Richter scale
- Focus = origin point inside earth; Epicentre = point above focus on surface
- Safety: stay indoors, avoid trees/poles, crouch in open areas