10th Class Physics Notes 2026 — Chapter 15 – Federal Board
Class 10 Physics – Chapter 15: Electrostatics
Board: Federal Board (FBISE)
Multiple Choice Questions (MCQs)
1. A rubber rod is rubbed with fur. The fur is then quickly brought near the bulb of an uncharged electroscope. The sign of the charge on the leaves of the electroscope is:
A. Positive
B. Negative
C. Neutral
D. Zero
Correct Answer: A. Positive
2. A negatively charged object is brought close to the surface of a conductor, whose opposite side is then grounded. What kind of charge is left on the conductor’s surface?
A. Positive
B. Negative
C. Neutral
D. Zero
Correct Answer: A. Positive
3. A small metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positively charged rod held near the ball. The ball must be:
A. Positive
B. Negative
C. Positive or neutral
D. Negative or neutral
Correct Answer: D. Negative or neutral
4. A positive ion is formed when:
A. A neutral atom loses an electron
B. A neutral atom gains an electron
C. A negatively charged atom loses an electron
D. All these cases can be true
Correct Answer: A. A neutral atom loses an electron
5. A charged rod is brought near a suspended object, which is repelled by the rod. We can conclude that the suspended object is:
A. Uncharged
B. Charged with a different sign
C. Charged with the same sign
D. All these cases can be true
Correct Answer: C. Charged with the same sign
6. You have two lightweight metal spheres, each hanging from an insulating nylon thread. One sphere has a net negative charge, while the other sphere has no net charge. You allow the two spheres to touch. Once they have touched, will the two spheres:
A. Attract each other
B. Repel each other
C. Exert no force on each other
D. Neutralize each other
Correct Answer: B. Repel each other
7. A negative point charge moves along a straight-line path directly toward a stationary positive point charge. Which aspect of the electric force on the negative point charge will remain constant as it moves?
A. Magnitude
B. Direction
C. Both magnitude and direction
D. Neither magnitude nor direction
Correct Answer: B. Direction
8. To be safe during a lightning storm, it is best to be:
A. In the middle of a grassy meadow
B. Inside a metal car
C. Next to a tall tree in a forest
D. On a metal observation tower
Correct Answer: B. Inside a metal car
9. What causes the leaves of an electroscope to move apart when charged?
A. Attraction of unlike charges
B. Repulsion of like charges
C. Neutral nature of leaves
D. Air pressure
Correct Answer: B. Repulsion of like charges
10. Corona discharge is most likely to occur at which of the following locations on a conductor?
A. Flat surfaces
B. Sharp points
C. Smooth spheres
D. Insulated areas
Correct Answer: B. Sharp points
11. What do Lichtenberg figures typically look like?
A. Straight lines
B. Circular rings
C. Branching tree-like patterns
D. Rectangular shapes
Correct Answer: C. Branching tree-like patterns
Constructed Response Questions
Question 1
(a) In your notebook, draw two parallel lines representing metal plates, one positive and one negative.
Diagram instructions:
Draw two parallel horizontal or vertical lines representing the metal plates.
Label the top plate as positively charged and the bottom plate as negatively charged.
Draw uniform, parallel electric field arrows pointing straight down from the positive plate toward the negative plate.
(b) Eight negative point charges of equal magnitude are distributed evenly around a circle. Sketch the electric field in the region around and within this charge distribution. Explain how this charge distribution can be used to model the electric field inside a coaxial cable.
Explanation:
– Inside the circle: By symmetry, the electric field contributions from equal negative charges positioned radially opposite each other cancel at the exact centre. Thus,
– Analogy to a coaxial cable: In a coaxial cable, the outer conductor is a cylinder carrying a uniform charge or acting as a grounded shield. The symmetric ring of negative charges models the cross-section of the outer shield. It illustrates how a symmetric charge distribution produces a predictable electric field and can result in cancellation at the centre or in certain external regions.
Question 2
A figure shows a source consisting of two charged particles labelled 1 and 2, with surrounding electric field lines.
(a) What is the sign of the charge on each particle?
Answer: Both particles 1 and 2 are positive charges.
Reason: The electric field lines are directed radially outward from both particles. This indicates that both are positive sources of electric fields.
(b) In which region (A, B, C or D) is the electric field the weakest?
Answer: Region B, the region between the two charges.
Reason: The electric field lines produced by two like positive charges oppose each other in the central region. Their contributions can cancel, producing a zero-field point.
(c) In which region (A, B, C or D) is the electric field the strongest?
Answer: Region A (or Region D immediately adjacent to particle 1).
Reason: The electric field lines are drawn closest together near the point charge. Greater field-line density represents a stronger electric field.
Short Response Questions
1. What is the relationship between charge and mass when a metal becomes charged, and assess whether the mass change is significant?
When a metal gains electrons, its mass slightly increases. When it loses electrons and becomes positively charged, its mass decreases.
The mass of an electron is extremely small:
Therefore, the change in mass is negligible in practical situations.
2. How do grounding systems prevent static buildup in fuel trucks? What are the dangers without grounding?
Grounding provides a low-resistance path for accumulated static charges to discharge safely into the Earth before fueling begins. Without grounding, electrostatic sparks may ignite flammable fuel vapours, causing serious fires or explosions during fuel transfer.
3. Why do electrons transfer charge instead of protons or neutrons, and how does this affect circuit and device design?
Electrons are present in the outer atomic shells and are comparatively loosely bound. Protons and neutrons, however, are tightly held inside the nucleus by strong nuclear forces. Consequently, electronic circuits and semiconductor devices are designed around controlling electron flow or hole mobility rather than moving nuclear particles.
4. Why does a charged balloon stick to a neutral wall due to polarization? Compare the interaction with a conductor versus an insulator.
A charged balloon causes polarization in the neutral wall. Opposite bound charges shift closer to the balloon, producing a net attractive electrostatic force.
In conductors, free electrons can move relatively freely through the material. In insulators, charges cannot move freely, but they can shift slightly within individual atoms or molecules.
5. How do positive and negative charges behave differently in an electric field?
Positive charges experience a force in the direction of the electric field lines, moving from higher potential to lower potential. Negative charges experience a force in the opposite direction to the electric field lines, moving from lower potential to higher potential.
6. What are the effects of changing electric field direction or strength in applications such as capacitors?
Increasing the electric field strength increases the charge stored on capacitor plates according to:
This continues up to the dielectric breakdown limit. Changing the field direction reverses the plate polarization and changes the direction of current during charging and discharging cycles.
7. Compare electric and gravitational fields in terms of strength, range, and forces. Evaluate situations where both fields affect objects.
Electric fields are immensely stronger than gravitational fields:
Electric forces can be attractive or repulsive, whereas gravitational forces are always attractive. Both follow inverse-square laws and have an infinite range.
Suspended charged oil droplets in Millikan’s experiment and charged dust particles in the atmosphere are examples in which both electric and gravitational forces act simultaneously.
8. Can insulators be charged by induction? Under what conditions does this happen, and what are alternative methods for charging insulators?
Insulators cannot be globally charged by induction in the same way as conductors because they lack freely moving electrons. However, local surface polarization can occur near an external charge.
Alternative methods of charging insulators include:
Friction or triboelectric charging.
Direct contact charging.
9. What is corona discharge around conductors, especially sharp points?
Corona discharge is an electrical discharge caused by the ionization of the air surrounding a high-voltage conductor. It occurs prominently around sharp points because the surface charge density and electric field strength are highest at these locations.
10. How does electric field concentration cause ionization, and what are its practical uses or problems?
A high electric field accelerates nearby free electrons to high speeds. These electrons can collide with air molecules and remove their electrons, causing ionization.
Practical applications include electrostatic precipitators and photocopiers. However, corona discharge can also cause energy losses and acoustic noise in high-voltage transmission lines.
11. What causes Lichtenberg figures to form after lightning strikes, and why do they not always appear?
Lichtenberg figures are branching electrical discharge patterns created by high-voltage dielectric breakdown across insulating surfaces or skin tissues. They appear only when the electric potential gradient exceeds the local breakdown threshold of the particular surface material.
12. How do lightning rods protect buildings, and what is the role of pointed rods in attracting discharge?
Lightning rods provide a safe, low-resistance path for lightning current to pass into the Earth without damaging the building structure.
Pointed ends concentrate the electric field and promote ionization, helping to influence the discharge and provide a conducting path to the ground.
13. How can lightning rod design and placement be optimized for better protection?
Lightning protection can be improved by:
Placing conductive rods at the highest points of a structure.
Connecting the rods with thick copper down-conductors.
Providing low-resistance grounding rods buried deeply in damp ground.
Using early streamer emission (ESE) terminals to widen the protective zone.
Long Response Questions
1. Explain electrification and how charge conservation and quantization govern particle behaviour, with real-world examples such as electronics.
Electrification
Electrification is the process of imparting an electric charge to a body by adding or removing electrons. Matter in its natural state is electrically neutral because the number of positive protons equals the number of negative electrons.
Law of Conservation of Charge
The net electric charge of an isolated system remains constant over time. Charge cannot be created or destroyed; it can only be transferred from one body to another.
Example: In pair production, a high-energy photon produces an electron and a positron. Their charges are:
The total initial charge is zero, and the total final charge is also zero:
Quantization of Charge
Electric charge exists in discrete packets. The total charge \(Q\) on any body is always an integral multiple of the elementary charge.
where \(n=1,2,3,\ldots\)
Real-World Application in Electronics
Integrated circuits (ICs), microprocessors and flash memory rely on controlling electron movement and stored charge. Binary bits (0 and 1) can be represented through different electrical states in electronic devices.
2. Describe how objects gain charge through triboelectric effects and how they behave when in contact with conductors.
Triboelectric Effect
When two different insulating materials are brought into close contact and rubbed together, electrons can transfer from one material to the other.
The material that loses electrons becomes positively charged, while the material that gains electrons becomes negatively charged.
Triboelectric Series
Materials are ranked according to their tendency to gain or lose electrons. For example:
Glass tends to lose electrons to silk.
Rubber tends to gain electrons from wool.
Interaction with Conductors
Direct Contact (Conduction):
When a triboelectrically charged object touches a neutral conductor, free electrons transfer between them until their electric potentials equalize. The conductor acquires the same polarity as the charging body.
Discharge:
Because conductors allow charges to move freely, touching a grounded conductor can quickly neutralize a charged object by transferring surplus electrons to or from the Earth.
3. Compare conductors and insulators at the atomic level, focusing on electron configurations and their practical uses.
Atomic-Level Comparison
Feature | Conductors (e.g., copper, aluminium) | Insulators (e.g., rubber, glass) |
|---|---|---|
Valence electrons | Usually 1, 2 or 3 valence electrons, comparatively weakly bound | Generally more tightly bound electrons |
Energy band gap | Overlapping conduction and valence bands; approximately \(0\text{ eV}\) | Large energy band gap, often greater than \(5\text{ eV}\) |
Free electrons | Abundant mobile electrons | Virtually no free electrons at room temperature |
Conductivity | High electrical conductivity, approximately \(10^7\text{ S/m}\) | Low electrical conductivity, approximately \(10^{-10}\) to \(10^{-16}\text{ S/m}\) |
Practical Uses
Conductors:
Electrical wiring.
Printed circuit board (PCB) tracks.
Heating elements.
Grounding systems.
Transformer windings.
Insulators:
Wire coatings and jackets.
High-voltage ceramic supports.
Dielectric layers in capacitors.
Protective rubber gloves for electricians.
4. Evaluate charging methods (friction, conduction and induction) and their efficiency in different materials, with real-world applications.
1. Friction (Triboelectricity)
Mechanism: Physical rubbing transfers electrons between insulating materials.
Efficiency: Highly effective for materials such as plastic and glass. It is generally less effective for handheld conductors because charge can escape through the human body.
Applications:
Van de Graaff generators.
Static spray painting.
2. Conduction
Mechanism: Direct physical contact allows charge to flow from a charged body to a conductor.
Efficiency: Highly effective for conductors but generally ineffective for insulators because they lack freely mobile charges.
Applications:
Charging conductors through contact.
Capacitive touchscreen operation, which involves electrical charge effects.
3. Induction
Mechanism: A charged object is brought near a neutral conductor without touching it. This causes charge separation. Grounding allows like charges to leave, leaving the conductor with a charge opposite to that of the inducing object.
Efficiency: Effective for conductors, while insulators generally experience polarization rather than permanent charge separation by this method.
Applications:
Electrostatic air purifiers.
Electrostatic precipitators.
Electrostatic charging systems.
5. Design a procedure to charge two metal spheres via electrostatic induction and discuss real-world uses of induction.
Procedure to Charge Two Spheres with Opposite Charges
1. Setup: Take two identical, uncharged metal spheres A and B, mounted on insulating stands. Bring them into physical contact with each other.
2. Bring a Charged Rod Near: Bring a negatively charged rod close to sphere A without touching it. Free electrons in the spheres are repelled towards sphere B, leaving sphere A with a net positive charge.
3. Separate the Spheres: While keeping the negative rod in position near sphere A, separate sphere B from sphere A.
4. Remove the Rod: Remove the charged rod. Sphere A retains a positive charge, while sphere B retains an equal negative charge.
Diagram Description
Step 1: [Sphere A] [Sphere B] — Both spheres are in contact and neutral.
Step 2: (Negative rod) [Positive A, Negative side] [Negative B] — Electrons move towards sphere B.
Step 3: (Negative rod) [Positive A] [Negative B] — The spheres are separated while the rod remains nearby.
Step 4: [Positive A] [Negative B] — The spheres retain opposite charges.
Real-World Uses of Induction
Electrostatic Painting: Car bodies and paint droplets can be given opposite charges so that paint is attracted towards the surface, helping to produce an even coating and reduce overspray.
Photocopiers: Photoconductive drums are selectively charged to create an electrostatic image pattern that attracts toner particles.
6. Analyse electric fields and how field lines help visualize field intensity, with applications such as capacitors and shielding.
Electric Field
An electric field is the force experienced per unit positive test charge placed at a point.
Its SI unit is newton per coulomb (N/C), which is equivalent to volt per metre (V/m).
Electric Field Lines
The following rules describe electric field lines:
Field lines originate from positive charges and terminate on negative charges.
The tangent to a field line at any point gives the direction of the electric field at that point.
The relative density of field lines represents the strength of the electric field.
Closely spaced lines indicate a strong field, while widely spaced lines indicate a weak field.
Applications
Parallel-Plate Capacitors:
Between two oppositely charged parallel plates, electric field lines are uniform, straight and parallel. This produces a uniform electric field.
Electrostatic Shielding (Faraday Cage):
Inside a closed hollow conductor in electrostatic equilibrium:
Free charges redistribute over the outer surface, shielding sensitive electronic equipment inside from external electric fields.
7. Explain how an electroscope works to detect charge and suggest improvements for detecting weaker charges.
Working Principle
A gold-leaf electroscope consists of a metal knob connected by a conducting rod to two thin, flexible gold leaves inside a glass jar.
When a charged body touches or comes near the knob, charge transfers or is induced through the conducting rod to the leaves.
Since both leaves receive charges of the same polarity, they repel each other and move apart. The angle of divergence indicates the relative amount of charge.
Improvements for Detecting Weaker Charges
1. Lightweight and Thinner Foil: Use extremely thin foil, such as ultra-thin aluminium, to reduce the mass of the leaves and make them more sensitive.
2. Vacuum Enclosure: A vacuum enclosure can reduce air resistance and limit charge leakage through the surrounding air.
3. Electronic Signal Amplification: Replace visual observation of the leaves with a high-input-impedance MOSFET-based electrometer circuit to detect very small charge levels, such as pico-coulombs.
8. Analyse electrostatic precipitators and photocopiers, and propose ways to improve their energy efficiency and performance.
Electrostatic Precipitator (ESP)
Operation:
A high negative voltage is applied to wires, ionizing the surrounding air through corona discharge.
Dust particles in the dirty gas stream gain negative ions.
These negatively charged particles are attracted towards positively charged collector plates.
Improvements:
Use pulsed high-voltage power supplies instead of continuous DC to reduce energy consumption.
Use optimized automated mechanical rapping schedules to remove accumulated dust from collector plates.
Photocopier (Xerography)
Operation:
A photoconductive drum made of selenium or an organic photoconductor is charged electrostatically.
Light reflected from the original image discharges the illuminated areas.
A latent electrostatic charge pattern remains in the darker areas.
Toner powder sticks to the charged areas and is transferred onto paper.
Improvements:
Use low-energy LED exposure arrays instead of high-power lamps.
Develop low-melting-point toner particles to reduce the energy required by the fuser heater.
9. Investigate electric breakdown, how it occurs, and suggest safety measures for high-voltage environments.
Mechanism of Electric Breakdown
Electric breakdown occurs when an applied electric field exceeds the dielectric strength of an insulating material.
For air, the approximate dielectric strength is:
An intense electric field accelerates free electrons. These electrons gain enough kinetic energy to ionize neutral gas atoms when they collide with them. This process, called a Townsend avalanche, rapidly transforms the insulating medium into a conductive plasma channel, producing a spark or arc.
Safety Measures in High-Voltage Environments
1. Increase Radii of Curvature: Use rounded conductors and smooth corona rings to prevent electric field concentration at sharp corners.
2. High-Dielectric-Strength Insulators: Use suitable insulating materials and systems, such as transformer oil or insulating gases, around high-voltage equipment.
3. Safety Clearance and Interlocks: Maintain safe physical distances and install automatic grounding switches with safety interlocks.
10. Explain lightning production and how lightning rods work, proposing additional safety measures for lightning-prone areas.
Lightning Production
Collisions between falling hail or ice particles and rising water droplets inside storm clouds cause charge separation.
Positive charges gather near the cloud top, while negative charges gather near the cloud base.
The negative charge at the cloud base induces a large positive charge on the Earth’s surface beneath it.
When the electric field exceeds the breakdown strength of air, a downward stepped leader meets an upward streamer.
This creates a high-current lightning channel called the return stroke.
Function of Lightning Rods
A lightning rod provides a conducting path for lightning current to travel safely to the ground. It is connected to the Earth through low-impedance conductors, helping protect the building structure.
Additional Safety Measures
1. Surge Protection Devices (SPDs): Install transient voltage surge suppressors on power and communication lines.
2. Equipotential Bonding Grid: Interconnect structural metalwork, pipes and electrical grounding systems to reduce dangerous side-flash hazards.
3. Early Warning Detection Systems: Use atmospheric electric-field monitoring systems to detect lightning threats and allow sensitive facilities to take protective action.
Numerical Response Questions
Q1. A positive test charge of \(30\,\mu C\) is placed in an electric field. The force on it is \(0.600\,N\). What is the magnitude of the electric field at the location of the test charge?
Given Data:
Test charge:
Electrostatic force:
To Find:
Magnitude of electric field:
Formula:
Calculation:
Substitute the given values:
Answer:
The magnitude of the electric field at the location of the test charge is:
Notes by Sir Dilawar
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