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Chapter Analysis
Intermediate44 pages • EnglishQuick Summary
Chapter 1 of Class 12 Physics Part 1 introduces students to the fundamental concepts of electric charges and fields. It covers the basic properties of electric charge such as quantisation, additivity, and conservation. The chapter explains Coulomb’s Law which describes the force between two point charges and introduces the concept of the electric field as a way to understand force interaction at a distance. It also discusses applications like electric dipoles and details on field lines and the superposition principle.
Key Topics
- •Properties of Electric Charge
- •Coulomb's Law and its Applications
- •Electric Field Concept and Calculations
- •Superposition Principle
- •Electric Field Lines and their Properties
- •Electric Dipoles and their Behavior
- •Gauss's Law
- •Quantization and Conservation of Charge
Learning Objectives
- ✓Understand the basic properties of electric charge.
- ✓Apply Coulomb’s Law to calculate forces between point charges.
- ✓Describe the concept of an electric field and how it is related to force.
- ✓Utilize the superposition principle to find net electric fields.
- ✓Interpret electric field lines and visualize electric field configurations.
- ✓Explain the behavior and significance of electric dipoles.
Questions in Chapter
1.6 Four point charges qA = 2 mC, qB = –5 mC, qC = 2 mC, and qD = –5 mC are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 mC placed at the centre of the square?
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1.7 (a) An electrostatic field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not?
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1.7 (b) Explain why two field lines never cross each other at any point?
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1.8 Two point charges qA = 3 mC and qB = –3 mC are located 20 cm apart in vacuum. (a) What is the electric field at the midpoint O of the line AB joining the two charges? (b) If a negative test charge of magnitude 1.5 × 10–9 C is placed at this point, what is the force experienced by the test charge?
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1.9 A system has two charges qA = 2.5 × 10–7 C and qB = –2.5 × 10–7 C located at points A: (0, 0, –15 cm) and B: (0,0, +15 cm), respectively. What are the total charge and electric dipole moment of the system?
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1.10 An electric dipole with dipole moment 4 × 10–9 C m is aligned at 30° with the direction of a uniform electric field of magnitude 5 × 104 NC–1. Calculate the magnitude of the torque acting on the dipole.
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1.11 A polythene piece rubbed with wool is found to have a negative charge of 3 × 10–7 C. (a) Estimate the number of electrons transferred (from which to which?) (b) Is there a transfer of mass from wool to polythene?
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1.12 (a) Two insulated charged copper spheres A and B have their centres separated by a distance of 50 cm. What is the mutual force of electrostatic repulsion if the charge on each is 6.5 × 10–7 C? The radii of A and B are negligible compared to the distance of separation. (b) What is the force of repulsion if each sphere is charged double the above amount, and the distance between them is halved?
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1.13 Figure 1.30 shows tracks of three charged particles in a uniform electrostatic field. Give the signs of the three charges. Which particle has the highest charge to mass ratio?
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1.14 Consider a uniform electric field E = 3 × 103 î N/C. (a) What is the flux of this field through a square of 10 cm on a side whose plane is parallel to the yz plane? (b) What is the flux through the same square if the normal to its plane makes a 60° angle with the x-axis?
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1.15 What is the net flux of the uniform electric field of Exercise 1.14 through a cube of side 20 cm oriented so that its faces are parallel to the coordinate planes?
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Additional Practice Questions
Explain what is meant by the quantisation of electric charge.
easyAnswer: Quantisation of electric charge means that the total charge can only be an integral multiple of the elementary charge, which is approximately 1.6 x 10^-19 coulombs. This implies that electric charge is discrete and not continuous.
Derive the expression for the electric field due to a point charge.
mediumAnswer: Using Coulomb’s law, the force between two point charges is given by F = k*q1*q2 / r^2. When considering the electric field (E) created by a point charge (Q) at a distance (r), it is defined as the force felt per unit positive test charge (q0), E = F/q0 = k*Q/r^2. This shows the electric field decreases with the square of the distance from the charge.
Discuss the principle of superposition in the context of electric fields.
mediumAnswer: The superposition principle states that the net electric field resulting from multiple charges is the vector sum of the individual fields produced by each charge independently. This means that each charge creates its electric field as if the other charges are not present, and the resultant field is determined by adding these fields vectorially.
What would happen if electric charges were not conserved?
hardAnswer: If electric charges were not conserved, the foundational principle that the total charge in an isolated system remains constant would be violated. This would have significant implications for the law of conservation of energy and could potentially disrupt electrical processes and reactions, leading to instability in the nature of matter.
How does an electric dipole interact with a uniform electric field?
mediumAnswer: An electric dipole placed in a uniform electric field experiences a torque but no net force. The torque τ acts to align the dipole with the field and is given by τ = p × E, where p is the dipole moment. This causes the dipole to rotate until it is parallel to the field direction.
Explain the concept of the electric field line and its significance.
easyAnswer: Electric field lines are imaginary lines that represent the direction of the electric field. The density of these lines indicates the strength of the field; they emerge from positive charges and terminate on negative charges. Field lines never cross and provide a visual means to represent electric fields in space, helping to understand the field's behavior and direction at different points.
Discuss the significance of Coulomb’s constant in electric force calculations.
easyAnswer: Coulomb’s constant (k), approximately 9 × 10^9 N m^2 C^-2, is a proportionality factor in Coulomb's law that quantifies the amount of force between two point charges. It reflects the strength of the electrostatic interaction and allows for this force to be calculated in the International System of Units.
What is the physical interpretation of a dipole moment, and how is it calculated?
mediumAnswer: The dipole moment is a vector quantity that measures the separation of positive and negative charges within a system. It is calculated as the product of the charge magnitude and separation distance, p = q*d. It indicates the polarity of a molecule and its interaction with external electric fields.
Explain how electric field intensity varies with distance for a point charge.
easyAnswer: The electric field intensity for a point charge decreases inversely with the square of the distance from the charge, according to the formula E = k*Q/r^2. As the distance increases, the electric field strength diminishes rapidly due to the inverse squared relationship.
If multiple charges are placed at the vertices of a polygon, how would you determine the resultant electric field at the center?
hardAnswer: To calculate the resultant electric field at the center, determine the electric field contribution from each charge using E = k*Q/r^2, where Q is the charge and r is the distance from the charge to the center. Vectorially add all these field contributions to determine the net electric field at the center, considering both magnitudes and directions.