⚡ Chapter 15: Electrostatics – Long Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This chapter covers gold leaf electroscope, Coulomb's law and factors affecting electrostatic force, electric field intensity and field lines representation, conductors vs insulators with free electron model, and electrostatic hazards in real life. Each long question is presented with the detailed exam-ready answer as per the official PECTAA 2026 Physics curriculum. Perfect for Punjab Boards (Lahore, Gujranwala, Multan, etc.) and all BISE boards across Pakistan.
📚 Related Resources – Chapter 15: Electrostatics
Electrostatics covers Coulomb's law, electric field, potential, capacitance, charging by induction, electroscope, lightning, corona discharge, and applications. Includes solved examples and numerical problems.
📖 Long Questions & Answers (PECTAA 2026)
15.1 Discuss the working of a gold leaf electroscope. How can it be used to determine the nature and magnitude of charge on a body?
Working of a Gold Leaf Electroscope: The gold leaf electroscope is a sensitive device used to detect electric charges.
It consists of a brass rod with a brass disk at the top and two thin gold leaves suspended at the bottom. The rod passes through an insulating material, allowing charges to move freely from the disk to the leaves. A thin aluminium foil is attached inside the jar and is grounded to protect the gold leaves from external electrical interference.
Determining the Nature of Charge: To detect the presence of charge, a body is brought near the disk of an uncharged electroscope. If the body is neutral, there is no deflection of the leaves. If the body is charged, the leaves diverge due to electrostatic induction. For example, when a negatively charged object is brought near the disk, a positive charge appears on the disk and a negative charge accumulates on the leaves. Since both leaves carry the same charge, they repel each other and move apart.
Determining the Magnitude of Charge: The extent of divergence of the gold leaves depends on the amount of charge present on the body. Thus, the electroscope helps in detecting the presence and estimating the magnitude of electric charge.
Working of a Gold Leaf Electroscope: The gold leaf electroscope is a sensitive device used to detect electric charges.
It consists of a brass rod with a brass disk at the top and two thin gold leaves suspended at the bottom. The rod passes through an insulating material, allowing charges to move freely from the disk to the leaves. A thin aluminium foil is attached inside the jar and is grounded to protect the gold leaves from external electrical interference.
Determining the Nature of Charge: To detect the presence of charge, a body is brought near the disk of an uncharged electroscope. If the body is neutral, there is no deflection of the leaves. If the body is charged, the leaves diverge due to electrostatic induction. For example, when a negatively charged object is brought near the disk, a positive charge appears on the disk and a negative charge accumulates on the leaves. Since both leaves carry the same charge, they repel each other and move apart.
Determining the Magnitude of Charge: The extent of divergence of the gold leaves depends on the amount of charge present on the body. Thus, the electroscope helps in detecting the presence and estimating the magnitude of electric charge.
15.2 How does Coulomb's law help in understanding the interaction between charged particles? What factors affect the force between them?
Coulomb's law states that; The magnitude of the electrostatic force between two point charges is directly proportional to the product of magnitude of these charges and inversely proportional to the square of the distance between them.
Consider two charges \(q_{1}\) and \(q_{2}\) separated by distance \(r\). Mathematically:
The value of \(k\) is determined by the medium between the charges. In vacuum or air, \(k\) is approximately \(9 \times 10^{9} Nm^{2}C^{-2}\).
Coulomb's law is applicable primarily to point charges whose sizes are negligible compared to the distance separating them.
If the charges are opposite in sign, they attract each other. If both charges are positive or both are negative, they repel each other. The magnitude of the force remains the same, but the direction of the force changes.
Factors Affecting the Electrostatic Force: The electrostatic force between two charged particles depends on:
i. The magnitudes of the charges (\(q_{1}\) and \(q_{2}\)): The force is directly proportional to the product of the charges.
ii. The distance between the charges (r): The force is inversely proportional to the square of the distance between them.
iii. The nature of the medium: The value of the electrostatic constant \(k\) depends on the medium between the charges, so the force changes with the medium.
Coulomb's law states that; The magnitude of the electrostatic force between two point charges is directly proportional to the product of magnitude of these charges and inversely proportional to the square of the distance between them.
Consider two charges \(q_{1}\) and \(q_{2}\) separated by distance \(r\). Mathematically:
\[F \propto q_{1}q_{2}\]\[\text{and } F \propto \frac{1}{r^{2}}\]\[\Rightarrow F \propto \frac{q_{1}q_{2}}{r^{2}}\]\[F = k\frac{q_{1}q_{2}}{r^{2}}\]
where \(k\) is a constant of proportionality which quantifies the strength of the electrostatic force between two point charges.
The value of \(k\) is determined by the medium between the charges. In vacuum or air, \(k\) is approximately \(9 \times 10^{9} Nm^{2}C^{-2}\).
Coulomb's law is applicable primarily to point charges whose sizes are negligible compared to the distance separating them.
If the charges are opposite in sign, they attract each other. If both charges are positive or both are negative, they repel each other. The magnitude of the force remains the same, but the direction of the force changes.
Factors Affecting the Electrostatic Force: The electrostatic force between two charged particles depends on:
i. The magnitudes of the charges (\(q_{1}\) and \(q_{2}\)): The force is directly proportional to the product of the charges.
ii. The distance between the charges (r): The force is inversely proportional to the square of the distance between them.
iii. The nature of the medium: The value of the electrostatic constant \(k\) depends on the medium between the charges, so the force changes with the medium.
15.3 Explain the concept of electric field intensity. How can the electric field of a point charge and a parallel plate capacitor be represented using field lines?
Electric Field: An electric field is the region around a charged body where it exerts an electrostatic force on another charged body.
Electric Field Intensity: The electrostatic force per test charge (unit positive charge) when it is brought to the electric field of a source charge is called electric field intensity.
Explanation: Consider a test charge \(q_{0}\) is brought into the electric field of a source charge \(Q\). The source charge will exert an electrostatic force \(F\) on the test charge. Mathematically:
Electric field intensity is a vector quantity because it has both magnitude and direction. Its direction is the same as the direction of the force on a positive test charge.
Representation of Electric Field Lines:
(a) Around a Point Charge: Around a point charge, the electric field is radial, extending outward in all directions.
For a positive point charge, the electric field lines radiate outward, indicating that a positive test charge placed nearby would be repelled.
In contrast, for a negative point charge, the electric field lines point inward, showing that a positive test charge would be attracted towards it.
(b) Parallel Plate Capacitor: When two large parallel conducting plates are given equal and opposite charges, a uniform electric field is created between them, commonly used in capacitors.
The electric field lines point from the positively charged plate to the negatively charged plate, and they are parallel and equally spaced, showing a uniform electric field.
This uniformity means the electric field has the same magnitude and direction at all points between the plates, which is due to the plates being large and close together, minimizing edge effects.
Electric Field: An electric field is the region around a charged body where it exerts an electrostatic force on another charged body.
Electric Field Intensity: The electrostatic force per test charge (unit positive charge) when it is brought to the electric field of a source charge is called electric field intensity.
Explanation: Consider a test charge \(q_{0}\) is brought into the electric field of a source charge \(Q\). The source charge will exert an electrostatic force \(F\) on the test charge. Mathematically:
\[E = \frac{F}{q_{0}}\]
The SI unit of electric field intensity is newton per coulomb \((NC^{-1})\).
Electric field intensity is a vector quantity because it has both magnitude and direction. Its direction is the same as the direction of the force on a positive test charge.
Representation of Electric Field Lines:
(a) Around a Point Charge: Around a point charge, the electric field is radial, extending outward in all directions.
For a positive point charge, the electric field lines radiate outward, indicating that a positive test charge placed nearby would be repelled.
In contrast, for a negative point charge, the electric field lines point inward, showing that a positive test charge would be attracted towards it.
(b) Parallel Plate Capacitor: When two large parallel conducting plates are given equal and opposite charges, a uniform electric field is created between them, commonly used in capacitors.
The electric field lines point from the positively charged plate to the negatively charged plate, and they are parallel and equally spaced, showing a uniform electric field.
This uniformity means the electric field has the same magnitude and direction at all points between the plates, which is due to the plates being large and close together, minimizing edge effects.
15.4 Explain how and why electrical conductors and insulators behave differently in terms of charge transfer and movement of electrons.
Electrical Conductors: Electrical conductors are materials that allow electric charge to flow freely through them.
Mechanism: They have loosely bound electrons in their atomic structure, which can flow easily when a voltage is applied, creating an electric current.
Examples: Examples of electrical conductors include metals, such as copper, aluminum, and silver, as well as materials like graphite and electrolytes.
Electrical Insulators: Electrical insulators are materials that resist or prevent the flow of electric charge. As a result, they do not allow electric current to pass through them.
Mechanism: In insulators, electrons are tightly bound to their atoms and cannot move freely. These materials are used to block the flow of electricity and provide safety in electrical systems.
Examples: Examples of electrical insulators include rubber, plastic, glass, wood, and ceramics.
Free Electron Model Comparison: Electrical conductors and insulators can be explained using a simple electron model:
In Conductors: The outermost electrons of atoms are loosely bound and can move freely throughout the material. These "free electrons" act like a sea of mobile charges, allowing electric current to flow easily when a voltage is applied.
In Insulators: Electrons are tightly bound to their atoms and cannot move freely. When a voltage is applied, the electrons remain fixed, preventing the flow of electric current.
This difference in electron behaviour explains why conductors allow electricity to flow while insulators block it.
Electrical Conductors: Electrical conductors are materials that allow electric charge to flow freely through them.
Mechanism: They have loosely bound electrons in their atomic structure, which can flow easily when a voltage is applied, creating an electric current.
Examples: Examples of electrical conductors include metals, such as copper, aluminum, and silver, as well as materials like graphite and electrolytes.
Electrical Insulators: Electrical insulators are materials that resist or prevent the flow of electric charge. As a result, they do not allow electric current to pass through them.
Mechanism: In insulators, electrons are tightly bound to their atoms and cannot move freely. These materials are used to block the flow of electricity and provide safety in electrical systems.
Examples: Examples of electrical insulators include rubber, plastic, glass, wood, and ceramics.
Free Electron Model Comparison: Electrical conductors and insulators can be explained using a simple electron model:
In Conductors: The outermost electrons of atoms are loosely bound and can move freely throughout the material. These "free electrons" act like a sea of mobile charges, allowing electric current to flow easily when a voltage is applied.
In Insulators: Electrons are tightly bound to their atoms and cannot move freely. When a voltage is applied, the electrons remain fixed, preventing the flow of electric current.
This difference in electron behaviour explains why conductors allow electricity to flow while insulators block it.
15.5 Discuss how electrostatics plays a role in real-life hazards, such as dust explosions in factories and static shocks from synthetic clothing. How can these hazards be minimized?
Accumulation of Charges: The accumulation of charges, often referred to as static electricity, occurs when electrons are transferred from one object to another, leading to an imbalance of positive and negative charges.
Real-Life Hazards: This buildup can have several consequences:
In Everyday Life: It can cause minor shocks when touching metal objects or make clothes cling together.
In Industrial Settings: Accumulated charges can be more serious, potentially damaging electronic components, igniting flammable substances, or disrupting manufacturing processes.
Minimizing the Hazards: These hazards can be minimized by using proper grounding (earthing), anti-static measures, and humidity control, which safely remove accumulated charges and reduce the risk of electrical discharge.
Accumulation of Charges: The accumulation of charges, often referred to as static electricity, occurs when electrons are transferred from one object to another, leading to an imbalance of positive and negative charges.
Real-Life Hazards: This buildup can have several consequences:
In Everyday Life: It can cause minor shocks when touching metal objects or make clothes cling together.
In Industrial Settings: Accumulated charges can be more serious, potentially damaging electronic components, igniting flammable substances, or disrupting manufacturing processes.
Minimizing the Hazards: These hazards can be minimized by using proper grounding (earthing), anti-static measures, and humidity control, which safely remove accumulated charges and reduce the risk of electrical discharge.
📐 Key Formulas – Electrostatics
Coulomb's Law: \( F = k \frac{q_1 q_2}{r^2} \)
Electric Field Intensity: \( E = \frac{F}{q_0} \)
Electric Field (Point Charge): \( E = k \frac{q}{r^2} \)
Electric Potential: \( V = \frac{W}{q} \)
Coulomb's Constant: \( k = 9 \times 10^9 \, \text{Nm}^2\text{C}^{-2} \)
Quantization of Charge: \( q = ne \)
Unit 10Thermal Physics
Unit 11Transfer of Thermal Energy
Unit 12Waves
Unit 13Sound
Unit 14Light
Unit 15✨ Electrostatics
Unit 16Electricity
Unit 17Electromagnetism
Unit 18Electromagnetic Induction & EM Waves
Unit 19Electronics
Unit 20Atomic and Nuclear Physics
Unit 21Space and Environment
📖 Complete syllabus coverage for Class 10 Physics (PECTAA 2026) – Units 10 to 21
💡 Exam Tip:
For board exams, define key terms precisely, mention formulas with units, draw diagrams where required, and relate to real-life examples. These long questions follow the PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus