Grade 7 Chapter 8: Static Electricity

Introduction to Electric Charge

Have you ever felt a mild shock after touching a doorknob, or seen your hair stand on end after combing it? These phenomena are due to static electricity. Static electricity deals with electric charges at rest. This chapter will explore the nature of electric charges, how they are produced, their interactions, and their applications, including natural phenomena like lightning.

Electric Charge

All matter is made up of tiny particles called atoms. Atoms contain even smaller particles: protons (positively charged), electrons (negatively charged), and neutrons (no charge).

• Electric Charge: An intrinsic property of matter that causes it to experience a force when placed in an electromagnetic field.
• Neutral Object: An object that has an equal number of protons and electrons, resulting in no net charge.

Types of Electric Charge

• Positive Charge (+): Created when an object loses electrons.
• Negative Charge (-): Created when an object gains electrons.

Interaction Between Charges

• Like charges repel each other: Positive repels positive, and negative repels negative.
• Unlike charges attract each other: Positive attracts negative.

Methods of Charging Objects

1. Charging by Friction (Rubbing)

• Process: When two different neutral objects are rubbed against each other, electrons are transferred from one object to the other. The object that gains electrons becomes negatively charged, and the object that loses electrons becomes positively charged.
• Examples:
  • Rubbing a plastic comb through dry hair: The comb gains electrons and becomes negatively charged, while the hair loses electrons and becomes positively charged, causing hair to stand on end.
  • Rubbing a glass rod with silk cloth: The glass rod becomes positively charged, and the silk cloth becomes negatively charged.
  • Rubbing an ebonite rod with wool: The ebonite rod becomes negatively charged, and the wool becomes positively charged.

2. Charging by Induction

• Process: Charging an object without direct contact with a charged object. When a charged object is brought near a neutral object, it causes a redistribution of charges within the neutral object.
• Example: If a negatively charged rod is brought near a neutral metal sphere, the electrons in the sphere are repelled to the far side, leaving the near side positively charged. If the sphere is then grounded (connected to Earth), the electrons escape to the Earth, leaving the sphere positively charged.

Electric Discharge

Electric discharge is the sudden flow of electric charge through a medium, such as air, that is normally an insulator. It occurs when a large amount of static charge builds up.

Lightning

• Cause: During thunderstorms, air currents move upwards, and water droplets move downwards. This vigorous movement causes friction between water particles and ice crystals in the clouds, leading to the accumulation of electric charges.
• Charge Separation: Positive charges accumulate near the top of the clouds, and negative charges accumulate near the bottom.
• Discharge: When the accumulated charges become very large, the air (an insulator) can no longer withstand the electric pressure. This leads to a sudden, massive flow of charge, which we see as a bright flash of light (lightning) and hear as thunder.
• Types of Lightning:
  • Cloud-to-cloud lightning.
  • Cloud-to-ground lightning (most dangerous).

Lightning Conductor

A lightning conductor (or lightning rod) is a metal rod or strip installed on the highest point of a building, connected to a thick copper wire that runs down the side of the building and is buried deep into the ground.

• Principle: It provides an easy path for the electric charge from lightning to flow safely into the Earth, preventing damage to the building.
• Function: When lightning strikes the building, it strikes the metal rod (being the highest point and a good conductor). The charge then flows harmlessly through the copper wire into the ground, protecting the building and its occupants.

Safety Precautions During a Thunderstorm

• Do not stand under isolated trees.
• Avoid open fields and high ground.
• Stay away from metal objects and electrical appliances.
• Do not use mobile phones or landlines.
• Seek shelter in a sturdy building or a car (metal body acts as a Faraday cage).
• If outdoors and no shelter is available, squat low to the ground with your head between your knees, minimizing your contact with the ground.

Summary

• Electric Charge: Property of matter (protons +, electrons -). Neutral objects have equal charges.
• Charge Interaction: Like charges repel, unlike charges attract.
• Charging Methods:
  • Friction: Transfer of electrons by rubbing (e.g., comb and hair).
  • Induction: Redistribution of charges without contact.
• Electric Discharge: Sudden flow of charge through an insulator.
• Lightning: Large-scale electric discharge in atmosphere due to charge buildup in clouds.
• Lightning Conductor: Device to safely channel lightning charge into the ground, protecting buildings.
• Safety: Precautions to take during thunderstorms to avoid lightning strike.