Electric Charges and Fields

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• The chapter is titled "Electric Charges and Fields".

Introduction

• Seeing a spark or hearing a crackle when taking off synthetic clothes in dry weather, lightning, and electric shocks from cars or metal bars are due to the discharge of electric charges accumulated through rubbing of insulating surfaces.
• Static means not moving or changing with time.
• Electrostatics studies forces, fields, and potentials from static charges.

Electric Charge

• Thales of Miletus, Greece, around 600 BC, is credited with discovering that amber rubbed with wool or silk attracts light objects.
• The word electricity comes from the Greek word "elektron," meaning amber.
• Rubbing materials can attract light objects like straw and paper.
• Glass rods rubbed with wool/silk repel each other, but a glass rod and wool attract.
• Plastic rods rubbed with fur repel each other, but attract the fur.
• A plastic rod attracts a glass rod and repels the silk/wool used to rub the glass rod.
• There are two types of electric charge: like charges repel, unlike charges attract.
• The property differentiating charges is called polarity.
• When a glass rod is rubbed with silk, each acquires a different kind of charge.
• Electrified objects lose their charges upon contact, which means unlike charges neutralize each other.
• American scientist Benjamin Franklin named the charges positive and negative.
• The charge on a glass rod or cat's fur is positive, while on plastic or silk it is negative.
• An object having electric charge is described as electrified or charged.
• An object having no electric charge is electrically neutral.

Conductors and Insulators

• A gold-leaf electroscope detects charge: a metal rod in a box with gold leaves at the bottom.
• Charge flows to leaves when a charged object touches the knob, causing them to diverge, indicating the charge amount.
• Matter contains charges that are normally balanced, rendering materials electrically neutral.
• Forces holding molecules or atoms together are electrical, stemming from forces between charged particles.
• Electric force is pervasive. To electrify a neutral body, add or remove one kind of charge, which creates an excess or deficit of charge, which are less tightly bound electrons in the atoms.
• A body loses electrons becomes positively charged.
• A body gains electrons becomes negatively charged.
• Rubbing a glass rod with silk transfers electrons from the rod to the silk, charging them oppositely.
• No new charge is created, and transferred electrons are a small fraction of the total.
• Conductors allow electricity to pass through them easily because they have free-moving electric charges (electrons).
• Metals, human/animal bodies, and the earth are conductors.
• Insulators, like glass, plastic, and wood, resist electricity.
• Transferred charge distributes across a conductor's surface, but stays in place on an insulator.
• Nylon or plastic combs get electrified due to this, while metal articles don't.
• Metal rod with a wooden/plastic handle shows charging when rubbed without touching the metal part.

Basic Properties of Electric Charge

• There are two types of charges, positive and negative, which cancel each other's effects.
• Point charges are charged bodies that are very small compared to the distances between them.

Additivity of Charges

• The total charge of a system with point charges is the algebraic sum of individual charges.
• Charge has magnitude only, similar to mass, but can be positive or negative.
• The total charge of a system containing +1, +2, -3, +4, and -5 units is -1 unit.

Charge is Conserved

• When bodies are charged by rubbing, electrons are transferred, but not created or destroyed.
• In an isolated system, charges redistribute but the total charge remains conserved, this has been experimentally determined.
• Net charge cannot be created or destroyed in an isolated system, though charge-carrying particles may be.
• A neutron transforms into a proton and an electron that have equal and opposite charges, after the creation the total charge is zero.

Quantization of Charge

• Free charges are integral multiples of 'e'.
• Charge 'q' on a body is given by q = ne, where n is a positive or negative integer.
• The basic unit of charge, 'e', is the charge of an electron or proton.
• By convention, the charge of an electron was taken to be negative; therefore charge on an electron is written as -e and that on a proton as +e.
• Electric charge is always an integral multiple of e.
• Quantisation of charge was first suggested by Faraday's laws of electrolysis, then demonstrated by Millikan in 1912.
• The SI unit of charge is the coulomb (C).
• One coulomb equals the charge flowing through a wire in 1 second with a 1-ampere current.
• The value of the basic unit of charge is e = 1.602192 × 10-19 C.
• 1C contains about 6 × 10^18 electrons.
• In electrostatics, smaller units like microcoulombs (µC) or millicoulombs (mC) are common.
• Observable charges are integral multiples of e.
• A body with n₁ electrons and n₂ protons has a charge of (n₂ – n₁)e.
• Charge on any body is an integral multiple of e and can only increase/decrease in steps of e.
• Since 'e' is very small, charge seems continuous at the macroscopic level.

Coulomb's Law

• Coulomb's law quantifies the force between two point charges, in which their sizes can be ignored.
• Coulomb measured that the force varies inversely with the square of the distance between charges and is proportional to the product of the charges' magnitudes.
• The force acts along the line joining the charges.