Electric Charges and Fields: Class 12 Physics
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Explain the fundamental difference in how electrostatics and electrodynamics approach the study of electric charges, focusing on the condition of the charges themselves.

Electrostatics studies charges at rest, examining their properties and interactions without movement. Electrodynamics, on the other hand, studies charges in motion, dealing with electric current and electromagnetic induction.

Describe the relationship between electric charge, electric current, and time, expressing it in terms of an equation and briefly explaining each component.

The relationship is expressed as $Q = I \cdot t$, where Q represents electric charge in Coulombs, I is the electric current in Amperes, and t is the time in seconds.

Outline three key differences between electric charge and mass, emphasizing how they fundamentally differ as properties of matter.

  1. Charge can be positive or negative while mass is only positive.
  2. Charges experience attractive and repulsive forces; masses only experience attraction.
  3. Charge is quantized; mass quantization is not established.

A neutral object is brought into contact with a positively charged object. Explain what happens to the charge distribution within the neutral object and why.

<p>The neutral object will experience a separation of charge, with negative charges being attracted towards the positively charged object and positive charges repelled away. This occurs due to the electrostatic forces exerted by the charged object on the mobile charges within the neutral object.</p> Signup and view all the answers

Describe what it means for electric charge to be an 'intrinsic property' of matter, and name at least one other intrinsic property of matter.

<p>Electric charge being an 'intrinsic property' means it is a fundamental, unexplainable characteristic of matter. Another intrinsic property of matter is mass.</p> Signup and view all the answers

If an object has a deficiency of electrons, what type of charge does it possess, and how does this relate to the object's overall electric potential?

<p>An object with a deficiency of electrons possesses a positive charge. This indicates that the object has a higher electric potential compared to a neutral or negatively charged object.</p> Signup and view all the answers

Explain how an electroscope is used to detect the presence of electric charge on a body, detailing the basic principle of its operation.

<p>An electroscope detects charge by the movement of its leaves. When a charged object is brought near or in contact with the electroscope, the leaves either diverge (repel each other) if they acquire the same charge, or converge if the charge is opposite, indicating the presence of electric charge.</p> Signup and view all the answers

If two objects are rubbed together, and one gains electrons while the other loses electrons, describe the net charge of the system comprising both objects. Explain the underlying principle.

<p>The net charge of the system remains zero (neutral). This is due to the principle of conservation of charge, which states that the total charge in an isolated system remains constant. Electrons are simply transferred from one object to the other, not created or destroyed.</p> Signup and view all the answers

Explain how the principle of conservation of charge applies to a scenario where a neutral object becomes charged through contact with a charged object.

<p>When a neutral object comes into contact with a charged object, charge is transferred until both objects reach the same charge density. The total charge of the isolated system (both objects) remains constant throughout the charging process.</p> Signup and view all the answers

A small metallic sphere has a charge of -4.8 x 10^-19 C. How many excess electrons are present on the sphere?

<p>3 excess electrons. Since $Q = ne$, $n = Q/e = (-4.8 \times 10^{-19} C) / (-1.6 \times 10^{-19} C) = 3$.</p> Signup and view all the answers

Why is it safer to be inside a car during a lightning storm, considering the car's metal body acts as a conductor?

<p>The car's metal body acts as a Faraday cage, distributing the charge across the surface of the car. The electric field inside the car is close to zero, protecting the occupants.</p> Signup and view all the answers

Describe the differences in how charge distributes itself on a conductor versus an insulator when an external charge is brought near.

<p>In a conductor, charges redistribute freely across the surface. In an insulator, charges remain localized and do not move easily.</p> Signup and view all the answers

Explain the process of charging an object by induction, including the necessary steps of polarization and grounding.

<p>Charging by induction involves bringing a charged object near a neutral conductor. This causes polarization. Grounding the conductor allows charge to flow to or from the ground. Removing the ground while the charged object is still nearby leaves the conductor with a net charge.</p> Signup and view all the answers

Two point charges, +2q and -q, are separated by a distance r. Describe how the magnitude and direction of the electrostatic force on each charge compare to each other, according to Coulomb's Law and Newton's Third Law.

<p>The magnitude of the force on each charge is the same, but the directions are opposite. The force on +2q is attractive and directed towards -q, while the force on -q is attractive and directed towards +2q. This is consistent with Newton's Third Law.</p> Signup and view all the answers

State three limitations of Coulomb's Law regarding its applicability to different charge configurations and conditions.

<p>Coulomb's Law is only valid for point charges, for distances greater than 10^-15 meters but less than 10^15 meters, and when charges are at rest (or at least not accelerating rapidly).</p> Signup and view all the answers

Explain how the principle of superposition is used to determine the net electrostatic force on a charge due to multiple surrounding charges.

<p>The net force is the vector sum of the individual electrostatic forces exerted on that charge by all other charges present. Each individual force is calculated using Coulomb's Law, and then these forces are added as vectors.</p> Signup and view all the answers

Two identical positive charges (+q) are placed a distance 2a apart. A negative charge (-q) is placed midway between them. Describe the stability of the negative charge's equilibrium along the line joining the positive charges.

<p>The equilibrium is unstable. If the negative charge is displaced slightly along the line, the net force will push it further away from the center.</p> Signup and view all the answers

Two positive charges, +4q and +q, are separated by a distance d. Determine the location along the line connecting the two charges where a third charge, +q, would experience zero net force.

<p>The third charge should be placed a distance <em>d/3</em> from the +q charge (or 2d/3 from the +4q charge).</p> Signup and view all the answers

Distinguish between conductors and insulators based on their electrical properties and give an example of each.

<p>Conductors allow electric charge to move freely through them (e.g. copper), while insulators impede the flow of electric charge (e.g. rubber).</p> Signup and view all the answers

Two charges of equal magnitude but opposite signs are initially placed 10 cm apart. If the distance between them is doubled without altering the magnitude of the charges, by what factor does the electrostatic force change?

<p>The electrostatic force decreases by a factor of 4. Coulomb's Law states that force is inversely proportional to the square of the distance.</p> Signup and view all the answers

Imagine three identical positive charges are placed at the vertices of an equilateral triangle. Qualitatively describe the electric field at the exact center of the triangle.

<p>The electric field at the center of the triangle is zero. The electric field vectors due to each charge cancel each other out due to symmetry.</p> Signup and view all the answers

Explain how the increase of mass at a higher speed affects the magnitude of charge.

<p>The magnitude of electric charge is invariant and does not increase with speed. Only mass increases with speed.</p> Signup and view all the answers

If objects touch each other, what determines whether they stick together or repel each other?

<p>The net charge determines whether objects stick together or repel each other. If the net charge is negative, the test charge will be attracted to the reference charge and will stick together. If the net charge is positive, the test charge will be repulsed to the reference charge and will repel each other.</p> Signup and view all the answers

Flashcards

Electromagnetism

Study of charges at rest (electrostatics) and in motion (electrodynamics).

Electrostatics

Deals with properties and effects of charges at rest.

Electrodynamics

Deals with electric current, electromagnetic induction, and alternating current when charges are in motion.

Electric Charge

Fundamental property of matter causing electric and magnetic effects.

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Types of Charges

Positive, negative, or neutral.

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Electroscope

Detects electric charge on a body.

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SI Unit of Charge

Coulomb (C).

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Charge vs. Mass

Charge can be positive or negative; mass is only positive. Charges attract/repel; masses only attract.

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Conservation of Charge

The total electric charge in an isolated system remains constant.

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Quantization of Charge

Electric charge exists in integer multiples of the elementary charge (e). Q = ±ne

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Conductors

Materials that allow electric current to flow easily. Examples: metals, graphite.

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Insulators

Materials that resist the flow of electric current. Examples: plastic, wood, glass.

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Charging by Conduction

Charging by direct contact.

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Charging by Induction

Charging without direct contact.

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Charging by Friction

Transfer of charge by rubbing objects together.

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Coulomb's Law

The electrostatic force is proportional to the product of charges and inversely proportional to the square of the distance.

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Coulomb's Law Formula

F = k * (q1 * q2) / r^2, where k is Coulomb's constant.

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Electrostatic Force Direction

Electrostatic forces can either attract or repel.

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Superposition Principle

The net force on a charge is the vector sum of individual forces from other charges.

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F12 = -F21

The electric force between 2 charges.

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Electric Potential

Electric potential is the work energy needed to move a unit of electric charge from a reference paint to a specific point in an electric field

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Electrostatic Interactions

The force always points away from the reference Test charge's like charge. Or towards, if they are opposite.

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Equilibrium Point

When the net force on a test charge is zero.

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Study Notes

Introduction to Electric Charges and Fields

  • The lecture aims to solve doubts related to electric shocks, static electricity, and related phenomena using the principles of physics.
  • The chapter being started is "Electric Charges and Fields," a crucial chapter in Class 12th Physics.
  • Class 12th physics primarily covers electromagnetism, broken into electrostatics and electrodynamics.

Electromagnetism

  • Electromagnetism involves studying electrostatics and electrodynamics.
  • Electrostatics deals with charges at rest.
  • Electrodynamics deals with charges in motion.

Electrostatics vs. Electrodynamics

  • Electrostatics focuses on charge properties, potential, Gauss's law (with applications), and capacitors when charges are at rest.
  • Electrodynamics focuses on electric current, electromagnetic induction, and alternating current when charges are in motion.

Chapter Topics: Electric Charges and Fields

  • Topics covered in the chapter include electric charge, conservation, Coulomb's law, electric field & lines, electric dipoles, torque, continuous charge distribution, electric flux, Gauss's law and its applications.

Electric Charge: The Basics

  • Electric charge is an intrinsic property, meaning it's a fundamental, unexplainable property of matter.
  • Electric charge is a property associated with matter that can produce and experience electric and magnetic effects.
  • Matter has two fundamental properties: charge and mass.
  • Types of charges: positive (protons), negative (electrons), and neutral (neutrons).

Types of Charges and Properties

  • Positive charge is associated with protons.
  • Negative charge with electrons.
  • Neutrons have a neutral charge.
  • Mass of a proton (1.67 x 10^-27 kg) is greater than mass of an electron (9.11 x 10^-31 kg).

Detecting Electric Charge

  • An electroscope is a device used to detect electric charge on a body.

Key Quantities and Units

  • Symbol for charge: q
  • SI unit of charge: Coulomb (C)
  • Dimensional formula of charge: AT (Ampere * Time)
  • Charge is defined as current multiplied by time (Q = I * t).

Charge vs. Mass: Key Differences

  • Charge can be positive or negative, mass is only positive.
  • Charges experience attractive and repulsive forces, while masses only experience attraction.
  • Charge is quantized, mass quantization is not established.
  • Charge has SI unit Coulombs, mass has SI unit kilograms.
  • Charge is conserved, mass alone is not conserved (mass+energy conserved).
  • Charge cannot exist without mass, but mass can exist without charge.
  • Charge is independent of speed, mass is not. Mass increases with speed according to: m = m0 / sqrt(1 - v^2/c^2)

Conservation of Charge

  • The total electric charge in an isolated system never changes.
  • In an isolated system, the total amount of positive charge minus the amount of negative charge is always conserved.
  • Conservation of charge can be applied to solving problems by summing charges before and after a process and equating them.

Quantization of Charge

  • Electric charge exists only in integral multiples of the charge of an electron.
  • Formula: Q = ±ne, where n is an integer, and e is the elementary charge (1.6 x 10^-19 C).
  • Charge values such as 3.5e or -4.3e are not possible.

Conductors

  • Materials that allow electricity to flow easily.
  • Good conductors include graphite, human body, Earth, and metals like copper, gold, iron.
  • When charge is transferred to a conductor, it distributes across the entire surface.
  • Earth is considered an "ocean of electrons".

Insulators

  • Materials that impede the flow of electricity.
  • When a charge is applied to an insulator, it remains at the initial location and doesn't distribute.
  • Common insulators: plastic, wood, glass.

Methods of Charging

  • Charging by Conduction: Direct contact between charged and uncharged bodies.
  • Charging by Induction: Charging a body without direct contact through polarization and grounding.
  • Charging by Friction: Transfer of charge by rubbing two objects together.

Coulomb's Law: Quantifying Electrostatic Force

  • Coulomb's Law: the electrostatic force between two stationary point charges is directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the separation distance between the charges.
  • Formula: F = k * (q1 * q2) / r^2, where k is Coulomb's constant.
  • Coulomb's constant (k) value: 9 x 10^9 Nm²/C².
  • The formula doesn't require the sign of charge.
  • This is an Inverse Square law; the graph of F vs r would be similar to gravitation (F decreases with distance).
  • Electrostatic Force may be repulsive or attractive. In contrast, the Gravitational force is only attractive.
  • Coulomb's Law depends on Medium unlike Gravitational Force.
  • Coulomb's Force is greater in magnitude than Gravitational Force given the much larger k as compared to G.

Coulomb's Law in Vector Form

  • F12 = -F21; The forces are equal and opposite.
  • F12 = k * q1 * q2 / |r1 - r2|^3 ) * (r1 - r2)
  • F21 = k * q1 * q2 / |r2 - r1|^3 ) * (r2 - r1)

Coulomb's Law: Limitations

  • Only valid for point charges, not extended charges.
  • Valid for distances greater than 10^-15 meters and less than 10^15 meters.
  • Valid only when charges at rest, not true for Charges in Motion. Valid, if one charge is moving or both charges are in motion.

Important Points About Coulomb's Law

  • Coulomb's Law is based completely on experiments.
  • The presence of other charges does not affect the force between two charges.
  • The force always acts along the line joining the charges (Central Force).
  • Coulomb’s force is a Conserative force.
  • Obeys Newton's Third Law of Motion (equal and opposite forces).
  • The force depends on the medium.

Calculating Net Force: Superposition Principle

  • The net electric force on a charge due to multiple charges is the vector sum of individual forces.
  • For example, with three charges, the net force on charge 3 would be F3 = F31 + F32.

Article Problem

Two +q charges are at points A and B of an equilateral triangle of sides length l. Find the net force on a +q charge at point C.

  • First find the magnitude of the Force F exerted on C by A and force exerted on C by B; these will be equal.
  • Net force = F√3

Article Problem

A +q charge at points A and B of an equilateral triangle of sides length l, and -q at C. What is the net Force at C?

Article Problem: Symmetrical Charge Distributions

  • When charges are symmetrically arranged the net Force at the center will be zero, if all charges at the perimeter are equal.
  • If they are not equal you need to calculate it.

Article Problem: Finding Equilibrium Points

  • When the force at q, the test charge, is at zero there is equilibrium.
  • When Charges are the same: The position of the +q charge will fall between the two fundamental charges.
  • The values then are calculated.

Article Problem: Finding Equilibrium with Unequal and Opposite Charges

  • With two charges with opposite signs, the third charge can be placed only to the left of the smallest charge for a positive signed test charge to be in equilibrium.
  • In these cases, it’s the charge away from which you must measure, just like gravity.
  • With these considerations, the values may then be calculated.

Potential Difference

  • A high voltage of 13,600V can result in lethal shock, and touching an automobile after stepping out can also result in a shock.
  • Lightning involves the phenomena of electric charges.
  • Dielectrics are insulating substances with no free electrons; thus, charges can’t move freely.
  • Conductors have a conductivity of 10^2 to 10^8, while insulators are far weaker.
  • In Class 12 Physics, the emphasis is on charge.
  • Electric potential, or electrostatic potential, is the work energy needed to move a unit of electric charge from a reference paint to a specific point in an electric field. Unit is in Coulomms.
  • If your Q charge were to be moved from point Q -> P, the difference in potential then is (Work needed to be done on Test Charge from Q -> P)

Force

  • The two types of Electrostatic Interactions were noted and shown by diagrams.
  • When a test charge brought into the proximity of a reference charge, there will be 2 types.
  • If they are of like terms, the vector will point away. Otherwise Vector direction in towards.

Charge Convention

  • As a test; where +e and -e are located from each other.
  • A +ve charge is to the left side of the center. And on the left -Ve would be the diagram.

Key Differences of Field

  • In a 3D diagram, the field is proportional. Positive or negative has been checked.

Gravitational Force is the Basic Force

  • Gravitational Mass: A constant that is used to calculate the gravitational force between them.
  • Electrostatic Force: Electric force is only the measure of a force.

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Solve your doubts related to electric shocks and static electricity with physics principles. This lecture introduces 'Electric Charges and Fields,' a crucial chapter in Class 12th Physics, covering electromagnetism, electrostatics, and electrodynamics.

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