Electric Force Definition, Examples & Electrostatic Force | Class 12 Physics

Electrostatics: Charges at Rest

• Electrostatics is the branch of physics that deals with the study of electric charges at rest and the interactions between them.

• Meaning of “charges at rest”
  • Charges are either stationary or moving so slowly that their motion can be neglected.
  • Only electric effects are considered; magnetic effects are ignored.
• Nature of electric charge in matter
  • Matter contains two kinds of charges: positive and negative.
  • Charge is an intrinsic property of particles such as protons and electrons.
  • When these charged particles are not in motion, the associated phenomena belong to electrostatics.
• Behaviour of charges at rest
  • Charges at rest are not inactive.
  • A charged particle influences the space around it.
  • It can exert force on other charges placed nearby without physical contact.
  • Electric interaction can occur even in vacuum.
• Importance of electrostatics
  • Helps us understand how charged bodies interact.
  • Forms the foundation for electric force, electric field, and electric potential.

Interaction Between Electric Charges

• The mutual influence between two charged particles placed near each other, without physical contact, is called electric interaction.

• Medium independence of electric interaction
  • Occurs in air as well as in vacuum.
  • No material medium is required for electric interaction.
  • Presence of a charge alters the physical condition of space around it.
• Effect of electric interaction
  • Observed in the form of a force acting on charges.
  • Force may cause attraction or repulsion.
• Microscopic significance
  • Electric interactions determine the structure of atoms and molecules.
  • At atomic scale, electric force dominates over gravitational force.

Electric Force: Meaning and Definition

• The force experienced by a charged particle due to the presence of another charged particle is called electric force.

• Nature of electric force
  • Acts between charges whether they are in contact or separated by a distance.
  • It is a non-contact force.
  • Acts across empty space.
• Fundamental importance
  • Electric force is one of the fundamental forces of nature.
  • Responsible for attraction between electrons and nuclei.
  • Governs interactions between atoms and molecules.

Nature of Electric Force

• Attractive and repulsive nature
  • Electric force may be attractive or repulsive.
  • Like charges repel each other.
  • Unlike charges attract each other.
• Direction of electric force
  • Electric force acts along the straight line joining the two charges.
  • Hence, electric force is a central force.
• Action-at-a-distance property
  • Electric force acts even when charges are separated by empty space.
  • No physical contact or material connection is required.
  • This leads to the introduction of the electric field concept.

Factors Affecting Electric Force

• The magnitude of electric force between two charged particles depends on the properties of the charges and the conditions of their surroundings.

• Dependence on magnitude of charges
  • Electric force depends on the magnitude of the charges involved.
  • Greater the magnitude of charges, stronger is the electric force between them.
• Dependence on distance between charges
  • Electric force depends on the distance between the charges.
  • As the separation between charges increases, the electric force decreases.
  • Thus, electric force becomes weaker as charges move farther apart.
• Dependence on nature of medium
  • Electric force depends on the nature of the medium between the charges.
  • Presence of a material medium can reduce the force compared to its value in vacuum.
  • Vacuum gives maximum electric force.

Electric Force in Different Media

• Behaviour of electric force in various media
  • Electric force between two charges is maximum in vacuum.
  • When a material medium (air, water, glass, etc.) is introduced, the force decreases.
• Reason for reduction of force in a medium
  • Molecules of the medium become electrically polarised in the presence of charges.
  • This polarisation partially reduces the effective interaction between the charges.
  • Medium weakens electric interaction.
• Dielectric constant / relative permittivity

  • The effect of a medium on electric force is described by a quantity called the dielectric constant or relative permittivity of the medium.

  • A higher dielectric constant means a greater reduction in electric force.
• Comparison of common media
  • For most practical purposes, air behaves almost like vacuum.
  • Water has a high dielectric constant.
  • Therefore, electric force between charges in water is much smaller than in air or vacuum.

Comparison with Gravitational Force

• Electric force and gravitational force are both fundamental non-contact forces that act between particles without any physical contact.

• Similarities between electric force and gravitational force
  • Both forces act along the line joining the interacting particles.
  • Both forces decrease as the distance between the particles increases.
  • Both follow an inverse-square dependence on distance (same mathematical nature).
• Differences in nature of the forces
  • Gravitational force is always attractive in nature.
  • Electric force can be attractive or repulsive, depending on whether the charges are unlike or like.
  • Gravitational force acts between all masses.
  • Electric force acts only between electric charges.
• Difference in strength
  • Electric force is enormously stronger than gravitational force.
  • At atomic and molecular scales, gravitational force is negligible.
  • For example, the electric repulsion between two protons is about $10^{35}$ times stronger than the gravitational attraction between them.
• Effect of medium
  • Electric force depends on the nature of the medium present between charges.
  • Gravitational force is independent of the surrounding medium.

Mathematical Expressions

• Gravitational force between two masses
  • $$ F_g = G \frac{m_1 m_2}{r^2} $$
• Electric force between two charges in vacuum
  • $$ F_e = \frac{1}{4\pi\varepsilon_0}\frac{q_1 q_2}{r^2} $$

Comparison Summary

Electric Force	Gravitational Force
Acts between electric charges	Acts between masses
May be attractive or repulsive	Always attractive
Depends on the surrounding medium	Independent of the surrounding medium
Extremely strong at atomic scale	Very weak at atomic scale
Can cancel due to presence of opposite charges	Cannot be cancelled

Need for a Mathematical Law

• Qualitative ideas help us understand the nature of electric force, but they are not sufficient to calculate its exact magnitude in a given physical situation.

• Limitations of qualitative description
  • Qualitative discussion explains how charges interact.
  • It identifies the factors affecting electric force.
  • However, it does not tell us how strong the force is for given values of charge and distance.
• Need for quantitative analysis
  • In practical problems, values of charges and distance are specified.
  • We need a precise relationship to calculate the magnitude of force.
  • This requires a mathematical law based on experiments.
• Requirement of a valid law
  • The law must be based on experimental observations.
  • It should correctly relate electric force with charge, distance, and medium.
  • This leads to a quantitative description of electric force.

Coulomb’s Law as a Quantitative Description

• Coulomb’s law gives the mathematical expression for the electric force between two point charges at rest.

• Nature of Coulomb’s law
  • It is an experimentally verified law.
  • It quantifies the dependence of electric force on charges and distance.
• Statement of proportionality
  • Electric force is directly proportional to the product of the magnitudes of charges.
  • Electric force is inversely proportional to the square of the distance between charges.
• Scope of discussion here
  • Coulomb’s law is introduced only as a quantitative support to conceptual ideas.
  • Detailed mathematical treatment and numerical problems are discussed separately.

Conditions and Limitations of Electric Force Description

• The simple description of electric force is valid only when certain physical conditions are satisfied.

• Condition related to motion of charges
  • The interacting charges must be at rest.
  • Only then is the force purely electrostatic in nature.
• Condition related to size of charges
  • Charges are assumed to be point charges.
  • Their physical size must be much smaller than the distance between them.
• Condition related to medium
  • The medium between charges is assumed to be uniform and isotropic.
  • If this condition is not satisfied, the simple description may not apply.

Common Misconceptions About Electric Force

• Misconceptions about electric force often arise due to incomplete understanding of its properties.

• Misconception about contact
  • It is often believed that electric force requires physical contact.
  • In reality, electric force is a non-contact force.
  • It can act across empty space.
• Misconception about nature of force
  • Electric force is sometimes assumed to be always repulsive.
  • In fact, it can be repulsive or attractive.
  • The nature depends on whether charges are like or unlike.
• Misconception about role of medium
  • It is sometimes assumed that the medium has no effect on electric force.
  • In practice, the nature of the medium affects the magnitude of force.
• Importance of clarification
  • Removing these misconceptions is essential for a correct understanding of electrostatics.
  • It helps build a consistent conceptual foundation for further study.

Next Topics in Electric Charges and Fields

• Electric Charge and Basic Properties
• Coulomb’s Law and Vector Form
• Electric Field and Calculation of Electric Field

Practice and Revision

• Electrostatics Important Questions for Class 12
• Electrostatics Numericals and Solved Problems
• Application-Based Electric Force Problems

Electrical Force – Handwritten Revision Notes

Electrical Force – Mind Map

Figure below shows a concept map explaining what are electrostatic forces

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• Notes
  • Electric Charge , Basic properties of electric charge and Frictional Electricity
  • Electrical and electrostatic force
  • Coulomb's Law (with vector form)
  • Principle Of Superposition
  • Electric Field & Calculation of Electric Field
  • Electric Field Due to Line Charge
  • Electric Field Lines
  • Electric Flux
  • Electric Dipole and Dipole Moment
  • Electric Dipole Field
• Assignment
  • Electrostatics Multiple Choice Questions
  • Electrostatics Problems
  • Electrostatics Questions and answers
  • Electrostatics Important Questions for Class 12

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