Electrostatics- Electric Charges and Fields

  Chapter 1: Electric Charges and Fields

1.1 INTRODUCTION (Electrostatics)

Meaning of Electrostatics

Electrostatics is the branch of physics that deals with the study of electric charges at rest and the forces, fields, and potentials associated with them.

The word static means unchanging with time. Hence, electrostatics concerns itself with situations where charges are not in continuous motion, unlike current electricity.

• Everyday Experiences of Electrostatics
• Electrostatic effects are commonly observed in daily life, such as: crackling sound while removing synthetic or woollen clothes
• Attraction of small paper bits by a rubbed comb
• Electric shock while touching a metal door after walking on carpet
• Lightning during thunderstorms

These phenomena occur due to accumulation and sudden discharge of electric charges.

Cause of Electrostatic Phenomena

Electrostatic effects arise mainly due to

a. Rubbing of insulating surfaces

b. Transfer of electrons

c. Imbalance of electric charges

No new charge is created in the process — charges are only transferred from one body to another.

Scope and Importance

Electrostatics forms the foundation of:

Electric field and potential

Capacitors

Lightning protection systems

Electrostatic precipitators

Photocopiers and laser printers

Entire theory of electromagnetism

Electrostatic force is a fundamental force of nature and is much stronger than gravitational force at atomic and molecular scales.

1.2 ELECTRIC CHARGE

Historical Background

Around 600 BC, Thales of Miletus observed that amber (elektron in Greek) rubbed with wool or silk attracts light objects like straw or feathers.

From this observation, the term electricity originated.

What is Electric Charge?

Electric charge is a fundamental property of matter due to which it experiences electric force in the presence of other charges or electric fields.

It is responsible for all electrostatic interactions.

Electrification

When a body acquires electric charge, it is said to be electrified or charged.

A body is:

Neutral → Equal positive and negative charges

Charged → Excess or deficiency of electrons

Types of Electric Charge

Experiments show that there are only two kinds of electric charges:

1. Positive charge (+)

2. Negative charge (−)

Fundamental Laws of Charges

Like charges repel each other

Unlike charges attract each other

These laws were established using experiments with glass rods, plastic rods, silk, fur, and pith balls.

Convention of Charges

According to Benjamin Franklin:

Glass rod rubbed with silk → Positively charged

Plastic/ebonite rod rubbed with fur → Negatively charged

This convention is universally accepted.

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Neutralisation of Charge

When two oppositely charged bodies are brought in contact:

Their charges cancel each other

Both bodies become electrically neutral

This shows that charges of opposite nature neutralise each other’s effect.

Atomic Explanation of Charging

Matter is made up of atoms, which consist of:

Protons (positive charge)

Electrons (negative charge)

Neutrons (no charge)

Normally, atoms are electrically neutral.

Key Points

• Only electrons are transferred during charging
• Protons remain fixed inside the nucleus
• A body becomes: Positively charged by losing electrons and Negatively charged by gaining electrons

 No new charge is created during rubbing

Transfer of Charge

When a charged body touches another body:

Charge flows from one body to another

The second body gets charged

This is called charging by contact

This explains charging of pith balls and light objects.

Polarity of Charge

The property that distinguishes the two types of charges is called polarity.

Charges acquired by glass and silk are of opposite polarity

Charges acquired by plastic and fur are also of opposite polarity

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ELECTROSCOPE

Purpose of an Electroscope

An electroscope is a device used to: Detect the presence of electric charge. Determine the nature of charge (positive or negative). Give a rough idea of the magnitude of charge

Gold Leaf Electroscope

• It consists of: A metal rod passing through an insulating stopper. A metal knob at the top. Two thin gold leaves attached at the bottom. A protective glass enclosure

Working Principle

When a charged body touches the metal knob: Charge spreads throughout the rod

Both gold leaves acquire same type of charge. Due to repulsion between like charges, the leaves diverge.

Greater divergence → Greater charge

Reason for Divergence

Both leaves carry similar charges

Like charges repel

Repulsion causes leaves to separate

Key Observations

Divergence indicates presence of charge

Leaves collapse when charge is removed or neutralised

Works on principles of:

Charge transfer

Repulsion of like charges

Significance of Electroscope

Simple and effective charge detector

Demonstrates fundamental laws of electrostatics

Helps verify charging by contact and induction

Paper Strip Experiment (Demonstration of Electric Charge)

Aim of the Experiment

To demonstrate the presence and nature of electric charge and to show that like charges repel while unlike charges attract.

Apparatus Required

Thin strips of paper (or aluminium foil / pith balls)

A plastic or ebonite rod

Wool / fur / dry cloth

Stand or thread for suspension

Procedure

Cut two identical thin paper strips.

Suspend them freely from a stand or hold them close together.

Rub a plastic (or ebonite) rod with dry wool or fur to charge it.

Bring the charged rod near the paper strips without touching them.

Now touch both paper strips with the charged rod and then remove the rod.

Observations

Initially, the paper strips are attracted towards the charged rod.

After touching the rod, both strips acquire the same kind of charge.

The two strips repel each other and move apart.

When left undisturbed, the strips may slowly come back together due to discharge.

Explanation

Rubbing the rod transfers electrons to or from it, charging it.

When the charged rod touches the paper strips, charge is transferred to them.

Both strips acquire charges of the same nature.

Since like charges repel, the strips move apart.

This experiment clearly demonstrates:

Charging by contact

Repulsion between like charges

Conclusion

Electric charge can be transferred from one body to another.

Like charges repel each other.

Paper strips act as light test bodies for detecting electric charge

1.3 Electric Charge and it's Properties 

> Electric charge is a fundamental physical property of matter due to which it experiences electric force in an electric field or produces an electric field around it.

Symbol: q

SI unit: Coulomb (C)

Dimension: [A T]

Charge is associated with electrons and protons

Nature of Electric Charge

There are two types of electric charges:

a. Positive charge (+)

b. Negative charge (–)

Interaction rule: Like charges repel & Unlike charges attract

 This interaction is governed by Coulomb’s law.

 Origin of Charge

All matter is made of atoms

Atoms contain: Protons (+e),  Electrons (–e) &  Neutrons (0)

> Charging of a body occurs due to transfer of electrons only.

(Protons are bound inside the nucleus.)

Loss of electrons → Positive charge

Gain of electrons → Negative charge

Basic Properties of Electric Charge 

(i) Additivity of Charge (Scalar Nature)

> The total charge of a system is the algebraic sum of individual charges.

If a system has charges

q₁, q₂, q₃, … qₙ

Q =  q₁+  q₂ +  q₃+ ..... + qₙ

Charge is a scalar quantity

Proper signs (+ / –) must be used

 Difference from mass:

Mass is always positive, but charge can be positive or negative.

(ii) Conservation of Charge

> The total electric charge of an isolated system always remains constant.

Charge can neither be created nor destroyed

It can only be transferred or redistributed

 Example:

Rubbing glass rod with silk

Rod becomes +ve

Silk becomes –ve

Net charge = 0

 Even in nuclear reactions:

∆{Total charge before} = ∆{Total charge after}

(iii) Quantisation of Charge

> Electric charge exists in discrete packets and not continuously.

Experimentally it is established that all free charges are integral multiples

of a basic unit of charge denoted by e. Thus charge q on a body is always

given by

q = ne

where: n = integer (positive or negative)

e = fundamental charge

|e| = 1.602 × 10 -¹⁹ C

Smallest free charge = charge of electron or proton

At macroscopic scale, charge appears continuous, but microscopically it is quantised.

Although quarks possess fractional electric charges, they are permanently confined and cannot exist as free particles. Quantisation of charge is an experimentally verified property of free observable charges, which always occur as integral multiples of e. Hence, quarks are not considered fundamental in the quantisation of charge.

(iv) Invariance of Charge (Non-relativistic)

> Electric charge of a particle is independent of its speed and frame of reference (at non-relativistic speeds).

Charge does not change due to:

Motion

Temperature

Chemical reaction

Charge of electron = same everywhere

5. Unit of Charge

SI unit: Coulomb (C)

1 Coulomb = charge flowing in 1 second through a conductor carrying 1 ampere current

1C = 1A × 1s

Other units:

1 mC = 10-⁶ C

1 µC = 10-⁹ C

6. Charge of Fundamental Particles

Particle Charge

Electron –e

Proton +e

Neutron 0

7. Neutral Body

> A body is electrically neutral if total positive charge = total negative charge.

Neutral body still contains charges, but net charge is zero.

1.4 CONDUCTORS AND INSULATORS

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📘 1.3 Conductors and Insulators, written exactly at the Class XI / higher-physics level, matching the page you shared.

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1.3 Conductors and Insulators

> Materials are classified on the basis of how easily electric charges can move through them.

Depending on the mobility of electric charges (electrons), substances are divided into:

 Conductors, Insulators   & Semiconductors.

Conductors

> Conductors are substances which allow electric charge to flow easily through them.

Reason

They contain free or loosely bound electrons.

These electrons can move under the action of an electric field

Examples Metals (copper, aluminium, iron)

Human body

Earth Graphite

Important Properties

When charge is supplied to a conductor:

• It redistributes itself over the entire surface
• Excess charge does not remain localized
• Conductors cannot easily retain static charge

Insulators

> Insulators are substances which do not allow electric charge to flow easily through them.

Reason

Electrons are tightly bound to atoms.

There are no free charge carriers

Examples

Glass

Plastic

Rubber

Nylon

Wood

Porcelain

Important Properties

When charge is placed on an insulator:

• It remains localized
• It does not spread over the surface
• Insulators can retain static charge

 Example from daily life:

Plastic or nylon comb gets charged when rubbed

Metal spoon does not show charging when held in hand

Semiconductors 

> Semiconductors have electrical resistance intermediate between conductors and insulators.

Examples Silicon, Germanium etc

Important

Their conductivity can be controlled

Widely used in electronic