Key Concept: Charges and Their Interaction

b) They repel each other

[Solution Description]

Like charges repel each other. This is a fundamental property of electric charges, as observed in activities with charged balloons and refills.

Key Concept: Transfer of Charge, Electroscope

b) They repel each other

[Solution Description]

An electroscope works on the principle that like charges repel each other. When a charged object is brought near the electroscope, the charge is transferred to the aluminum foil strips through the metal conductor (paper clip). Since both strips receive the same charge, they repel each other and become wide open.

Key Concept: Lightning

a) Collision of ice particles in clouds

[Solution Description]

Lightning is caused by the buildup and discharge of electrical energy in the atmosphere. During a thunderstorm, the movement of ice particles within clouds creates an electric charge separation. When the charges become strong enough, they discharge as lightning.

Key Concept: Natural Phenomena

c) Lightning

[Solution Description]

Earthquakes and volcanic eruptions are directly associated with the movement of tectonic plates. Lightning, however, is related to atmospheric conditions and not tectonic activity.

Key Concept: Electric Discharge

c) Electric discharge

[Solution Description]

When negative and positive charges meet, they produce streaks of bright light and sound. This process is known as electric discharge, which is what we see as lightning.

Key Concept: Lightning and Earthquakes

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] The assertion states that lightning is caused by the accumulation of charges in the clouds, which is a scientifically established fact. The reason mentions that Benjamin Franklin demonstrated that lightning is similar to the spark produced when woollen clothes are rubbed. This is also true, as Franklin's experiments with electricity confirmed this similarity. However, the reason does not explain why lightning occurs due to charge accumulation in clouds. It only draws a parallel between two phenomena. Therefore, both the assertion and reason are true, but the reason does not correctly explain the assertion.

Key Concept: Structural safety, Earthquake precautions

b) Fix them to the walls[Solution Description]In earthquake-prone areas, cupboards and shelves should be fixed to the walls to prevent them from falling easily during an earthquake. This reduces the risk of injury from falling objects.

Key Concept: Emergency response, Earthquake safety

c) Stay where you are and protect your head

[Solution Description]

If you are indoors during an earthquake and cannot find shelter under a table, the next safest action is to stay where you are and protect your head with your arms or any available cushioning. Moving around increases the risk of injury from falling debris.

Key Concept: Lightning Conductor

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion is true because lightning conductors are indeed installed to protect buildings from lightning strikes by safely directing the electric charge to the ground. The reason is also true as the primary function of a lightning conductor is to provide a low-resistance path for the electric discharge, ensuring that the building is not damaged by the lightning strike. Furthermore, the reason correctly explains the assertion since the purpose of the lightning conductor is fulfilled by providing this low-resistance path.

Key Concept: Lightning Safety

c) Using a wired phone

[Solution Description]

During a thunderstorm, it is unsafe to use wired phones as lightning can strike telephone cords. It is safer to use mobile phones and cordless phones. Additionally, open vehicles, elevated places, and carrying umbrellas are not safe. Therefore, the correct answer is using a wired phone.

Key Concept: Lightning and Static Electricity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the ancient Greeks observed that rubbing amber with fur attracted light objects. This is historically accurate as it aligns with the knowledge from 600 B.C. The reason explains that this attraction is due to the transfer of electric charge, which is also correct. Benjamin Franklin later demonstrated that this same principle underlies lightning. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Earthing, Electric Charges

c) It provides a path for excess charge to flow into the earth.

[Solution Description]

Earthing provides a safe path for the leakage of electrical current to the earth. When there is any leakage of electrical current in a building, the earthing system transfers the excess charge directly to the ground. This prevents the buildup of charge on electrical appliances or surfaces, which could otherwise lead to electrical shocks if a person comes into contact with them.

Key Concept: Historical Background of Lightning

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that the ancient Greeks believed lightning was caused by the wrath of gods due to their lack of understanding of its scientific cause. This is true as per the syllabus, which mentions that ancient people were afraid of lightning and attributed it to the wrath of gods. The reason states that Benjamin Franklin's experiment in 1752 showed that lightning is an electrical phenomenon, which is also true as per the syllabus. Franklin's experiment provided a scientific explanation for lightning, helping people understand its true nature. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Scientific Discovery

d) Benjamin Franklin

[Solution Description]

Benjamin Franklin, an American scientist, demonstrated in 1752 that lightning and the spark from clothes are essentially the same phenomena.

Key Concept: Benjamin Franklin's discovery

c) Benjamin Franklin

[Solution Description]

Benjamin Franklin, an American scientist, showed in 1752 that lightning and the spark from clothes are the same phenomena.

Key Concept: Ancient beliefs about lightning

b) The wrath of gods

[Solution Description]

Ancient people did not understand the scientific cause of lightning. They believed it was caused by the wrath of gods.

Key Concept: Benjamin Franklin’s experiment

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Benjamin Franklin conducted an experiment in 1752 using a kite to demonstrate that lightning is a form of static electricity. This experiment showed that the sparks observed when rubbing certain materials, like amber or clothes, are similar to lightning. The Reason correctly explains the Assertion because Franklin’s experiment directly linked the two phenomena.

Key Concept: Earthing and Electric Discharge

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The Assertion states that during a thunderstorm, the accumulation of positive charges near the ground and negative charges at the lower edges of clouds leads to lightning. This is true because the separation of charges in a thunderstorm creates a large electric potential difference. The Reason states that lightning occurs when the air, which is normally a poor conductor of electricity, can no longer resist the flow of accumulated charges, resulting in an electric discharge. This is also true because when the charge buildup becomes too great, the air breaks down and allows the charges to flow, causing lightning. The Reason correctly explains the Assertion because it describes the mechanism by which the accumulated charges lead to lightning.

Key Concept: Charge Separation, Electric Discharge

c) 0 C

[Solution Description]

The total charge causing the electric discharge is the sum of the positive and negative charges accumulated in the cloud. Given the positive charge as $+5 \text{ C}$ and the negative charge as $-5 \text{ C}$, the total charge $Q$ is calculated as:

$Q = (+5 \text{ C}) + (-5 \text{ C}) = 0 \text{ C}$

Therefore, the total charge that causes the electric discharge is $0 \text{ C}$.

Key Concept: Electric Discharge, Lightning

d) $3 \times 10^9 \text{ V}$

[Solution Description]

The minimum potential difference $V$ required for lightning to occur can be calculated using the formula:

$V = E \times d$

where $E$ is the electric field strength and $d$ is the distance. Given $E = 3 \times 10^6 \text{ V/m}$ and $d = 1000 \text{ m}$:

$V = 3 \times 10^6 \text{ V/m} \times 1000 \text{ m} = 3 \times 10^9 \text{ V}$

Therefore, the minimum potential difference needed for lightning to occur is $3 \times 10^9 \text{ V}$.

Key Concept: Transfer of Charge

c) They repel each other and move apart

[Solution Description]

When an electroscope is charged by touching it with a charged refill, the charge is transferred to the metal paper clip and then to the aluminum foil strips. Since both strips receive the same type of charge, they repel each other and move apart.

Key Concept: Charge Transfer, Electric Discharge

c) An electric discharge occurs, producing lightning

[Solution Description]

When the magnitude of the accumulated charges becomes very large, the air, which is normally a poor conductor of electricity, can no longer resist the flow of charges. This leads to an electric discharge where the negative and positive charges meet, producing streaks of bright light and sound, which we observe as lightning.

Key Concept: Charges in Clouds

c) Near the upper edges of the clouds

[Solution Description]

According to the syllabus, during a thunderstorm, the positive charges collect near the upper edges of the clouds. This is due to the vigorous movement of air currents and water droplets, which causes the separation of charges.

Key Concept: Lightning, Charges in clouds

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

During a thunderstorm, air currents move upward while water droplets move downward, leading to the separation of charges. Positive charges accumulate near the upper edges of the clouds, and negative charges accumulate near the lower edges. Additionally, positive charges also accumulate near the ground. When the magnitude of these charges becomes very large, the air, which is normally a poor conductor of electricity, can no longer resist their flow, resulting in an electric discharge seen as lightning. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Charging by Rubbing, Materials

b) The balloon attracts the steel spoon

[Solution Description]

When a balloon is rubbed with polythene, it acquires a negative charge as electrons are transferred from the polythene to the balloon. Steel is a conductor, so when the negatively charged balloon is brought close to the steel spoon, the free electrons in the spoon are repelled to the far side of the spoon, leaving the near side positively charged. This causes an attractive force between the negatively charged balloon and the positively charged side of the spoon. Therefore, the balloon will be attracted to the steel spoon.

Key Concept: Charging by Rubbing

c) The dry leaves will be attracted to the refill.

[Solution Description]

When a plastic refill is rubbed with polythene, it becomes charged. When this charged refill is brought close to dry leaves, the leaves will be attracted to it due to the opposite charge induced on them. This phenomenon is similar to the attraction observed with paper pieces in Activity 12.1.

Key Concept: Charging by Rubbing

c) It does not become charged

[Solution Description]

As per Table 12.1, when a steel spoon is rubbed with polythene, it does not attract pieces of paper and remains uncharged. Therefore, the steel spoon does not acquire any charge.

Key Concept: Charging by Rubbing

b) The balloon sticks to the wall due to electrostatic attraction

[Solution Description]

When the balloon is rubbed with woollen cloth, it acquires a static electric charge. When the charged balloon is brought close to a neutral wall, the charge on the balloon induces an opposite charge on the surface of the wall. This results in an electrostatic attraction between the balloon and the wall, causing the balloon to stick to the wall temporarily.

Key Concept: Charging by Rubbing

a) Plastic comb and dry hair

[Solution Description]

When two different materials are rubbed together, one material loses electrons and becomes positively charged, while the other gains electrons and becomes negatively charged. This process is called charging by rubbing. In this case, a plastic refill rubbed with polythene acquires a charge and can attract small pieces of paper. Similarly, a plastic comb rubbed with dry hair becomes charged and attracts paper pieces. Based on this, the correct combination is a plastic comb and dry hair.

Key Concept: Charging by Rubbing, Materials and Charging

c) The dry leaf pieces would still be attracted because the eraser becomes charged.

[Solution Description]

When an eraser is rubbed with wool, it acquires a negative charge, attracting the neutral dry leaf pieces. If the eraser is rubbed with polythene, it would also acquire a charge, though the type of charge may vary depending on the materials involved. Polythene tends to gain electrons when rubbed, making the eraser negatively charged. Therefore, the dry leaf pieces would still be attracted to the eraser regardless of whether it is rubbed with wool or polythene.

Key Concept: Types of Charges, Charge Convention

b) Positive charge

[Solution Description]

According to the convention, when a glass rod is rubbed with silk, it acquires a positive charge. This means that the charge on the glass rod is opposite to that acquired by the plastic refill rubbed with polythene (which is negative). Therefore, the charge on the glass rod is positive.

Key Concept: Types of Charges

b) Negative charge

[Solution Description]

When a plastic straw is rubbed with polythene, it acquires a negative charge. This is a convention based on the type of materials used.

Key Concept: Like charges repel, unlike charges attract

b) They attract each other

[Solution Description]

According to the principle of unlike charges attracting each other, a negatively charged plastic straw and a positively charged glass rod will attract each other.

Key Concept: Like charges repel, unlike charges attract

a) They repel each other

[Solution Description]

According to the principle of like charges repelling each other, two positively charged glass rods will repel each other.

Key Concept: Interaction of Like Charges

b) They will repel each other

[Solution Description]

When two balloons are rubbed with a woollen cloth, they acquire the same type of charge (both become negatively charged). According to the principle of like charges repelling each other, the two balloons will repel each other when brought close.

Key Concept: Types of Charges and Their Interaction, Balloon and Refill Experiment

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that a charged balloon and a charged refill attract each other. This is consistent with the observation from Activity 12.3, where it was noted that a charged balloon attracts a charged refill. The reason explains that the balloon and the refill acquire opposite types of charges when rubbed with different materials. This is also correct, as rubbing different materials can result in one object acquiring a positive charge and the other acquiring a negative charge, leading to attraction between them. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Earthing, Transfer of Charge

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that a charged object loses its charge when connected to the earth through a conductor. This is true because the process of transferring charge from a charged object to the earth is known as earthing. The reason claims that earthing happens because the earth acts as an infinite reservoir of charge, which balances the charge of the object. This is also true, as the earth can absorb or provide electrons to neutralize the charge on the object. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Earthing

b) Earthing

[Solution Description]

The process of transferring charge from a charged object to the earth is called earthing. This helps in discharging the object and making it neutral.

Key Concept: Electroscope

c) They repel each other

[Solution Description]

When a charged object is brought near the metal clip of an electroscope, the charge is transferred to the aluminum foil strips through the metal clip. Since both strips receive the same type of charge, they repel each other and move apart.

Key Concept: Earthing

b) They collapse to their original state

[Solution Description]

When the metal clip of an electroscope is touched by a hand, the charge on the foil strips is transferred to the earth through the body. This process is called earthing, and as a result, the foil strips lose their charge and collapse back to their original position.

Key Concept: Charge Transfer Mechanism

c) It conducts the charge to the foil strips

[Solution Description]

The paper clip acts as a conductor that transfers charge from the charged object to the aluminium foil strips. Being made of metal, it efficiently conducts electricity, allowing the charge to move from the charged object to the foil strips. Without the paper clip, the charge would not be able to reach the foil strips.

Key Concept: Electroscope, Charge Detection

c) They will repel each other and move apart

[Solution Description]

When a positively charged body is touched with the paper clip of an uncharged electroscope, the positive charge is transferred to the electroscope through the metal conductor (paper clip). The aluminium foil strips receive the same positive charge and repel each other due to like charges. Thus, the foil strips will diverge or open up.

Key Concept: Earthing

c) To prevent electrical shocks

[Solution Description]

Earthing in buildings is provided to protect us from electrical shocks due to any leakage of electrical current. By connecting electrical systems to the earth, any excess current is safely transferred to the ground, preventing accidents.

Key Concept: Earthing

c) They collapse and lose charge

[Solution Description]

When foil strips are touched after being charged, they collapse because the charge on the strips is discharged through the person's body to the earth. This is an example of earthing, where the charge is transferred from the charged object (foil strips) to the earth.

Key Concept: Transfer of Charge

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

When a charged object is brought near an uncharged electroscope, the charge is transferred through the metal conductor (paper clip) to the aluminium foil strips. Since both strips receive the same type of charge, they repel each other. This repulsion indicates that the object is charged. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Charge Transfer, Electroscope

b) The foil strips repel each other due to similar charges.

[Solution Description]

When a positively charged rod is brought near the paper clip, the electrons in the paper clip are attracted towards the rod, leaving the aluminium foil strips with a positive charge. Since both strips carry the same positive charge, they repel each other and move apart.

Key Concept: Transfer of Charge, Earthing, Protection from Electric Shocks

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

Earthing is a safety measure used in buildings to prevent electrical shocks. When there is a leakage of current in an electrical appliance, the excess charge flows through the earthing wire into the ground, preventing it from causing harm to humans. This is because the earth acts as a reservoir for electric charge, and it can neutralize any excess charge. The assertion states that earthing protects us from electrical shocks due to current leakage, which is true. The reason explains that earthing is the process of transferring charge from a charged object to the earth, which is also true. Furthermore, the reason correctly explains why earthing provides protection from electrical shocks, as it allows the excess charge to flow safely into the ground. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Discharging

c) The object loses its charge

[Solution Description]

When a charged object is touched by a human hand, the charge is transferred to the earth through the body. This process is known as discharging, and it causes the object to lose its charge.

Key Concept: Charge Separation in Thunderstorms

c) Due to the upward movement of air currents carrying positive charges

[Solution Description]

During a thunderstorm, air currents move upward while water droplets move downward. This vigorous movement causes the separation of charges. The positive charges are lighter and are carried upwards by the air currents, accumulating near the upper edges of the clouds. Meanwhile, negative charges, being heavier, accumulate near the lower edges of the clouds due to the downward movement of water droplets.

Key Concept: Electric Discharge

b) Electric discharge

[Solution Description]

When negative and positive charges meet, they produce streaks of bright light and sound. This process is known as electric discharge.

Key Concept: Lightning and Its Effects

b) Accumulation of charges in the clouds

[Solution Description]

Lightning is caused by the accumulation of charges in the clouds. When the magnitude of these charges becomes very large, the air, which is normally a poor conductor of electricity, can no longer resist their flow. This results in an electric discharge between clouds or between clouds and the earth, which we see as lightning.

Key Concept: Lightning Safety

b) Under a tall tree in an open field

[Solution Description]

During a thunderstorm, open vehicles like motorbikes, tractors, construction machinery, and open cars are not safe. Open fields, tall trees, shelters in parks, and elevated places also do not protect us from lightning strokes. A house or a building, or being inside a car or bus with windows and doors shut, are safe places.

Key Concept: Formation of Lightning

a) Movement of air currents and water droplets

[Solution Description]

The separation of charges in a thunderstorm occurs due to the vigorous movements of air currents and water droplets. Air moves upward while water droplets move downward, leading to the accumulation of positive charges near the upper edges of clouds and negative charges near the lower edges.

Key Concept: Charge Separation, Electric Discharge

c) Movement of air currents and water droplets

[Solution Description]

During the development of a thunderstorm, air currents move upward while water droplets move downward. These vigorous movements cause the separation of charges. Positive charges collect near the upper edges of the clouds, and negative charges accumulate near the lower edges. This separation of charges leads to the buildup of electric potential, which eventually results in lightning.

Key Concept: Electric Discharge

b) Electric discharge occurs

[Solution Description]

When the magnitude of the accumulated charges in a thundercloud becomes very large, the air, which is normally a poor conductor of electricity, is no longer able to resist their flow. This results in negative and positive charges meeting, which produces streaks of bright light and sound. This process is known as electric discharge. Therefore, the correct answer is that electric discharge occurs.

Key Concept: Movement of Charges, Charge Accumulation

c) Air currents push positive charges upwards and water droplets pull negative charges downwards

[Solution Description]

In a thundercloud, positive charges accumulate near the upper edges while negative charges accumulate near the lower edges due to the vigorous upward movement of air currents and downward movement of water droplets, which cause the separation of charges.

Key Concept: Electric Discharge

c) As a result of induction from the negatively charged cloud base

[Solution Description]

During a thunderstorm, air currents move upward while water droplets move downward. This vigorous movement causes the separation of charges. Positive charges accumulate near the upper edges of the clouds, and negative charges accumulate near the lower edges. The presence of positive charges near the ground is due to the induction effect caused by the negatively charged cloud base. When the magnitude of these accumulated charges becomes very large, it leads to an electric discharge, which we observe as lightning.

Key Concept: Electric discharge, Lightning safety

c) To minimize the risk of electric shock and damage

[Solution Description]

During a thunderstorm, lightning can cause a sudden surge of electrical current through power lines. This surge can travel into homes and damage electrical appliances connected to the mains. Even if the appliances are turned off, they can still be affected by the surge. Hence, it is safer to unplug them to prevent any potential damage or risk of electric shock.

Key Concept: Lightning Safety

a) Inside a house with windows closed

[Solution Description]

During a thunderstorm, a house or a building is considered a safe place. Additionally, being inside a car or bus with windows and doors shut is also safe. Open vehicles like motorbikes, tractors, and open cars are not safe. Therefore, the safest place is indoors.

Key Concept: Do’s and Don’ts during a Thunderstorm, Inside the House

d) Taking a bath

[Solution Description]

During a thunderstorm, contact with telephone cords, electrical wires, and metal pipes should be avoided because lightning can strike these objects. It is safer to use mobile phones and cordless phones. Bathing should also be avoided to prevent contact with running water, which can conduct electricity.

Key Concept: Safe Practices During Thunderstorm

d) Stay away from metal pipes and use a mobile phone

[Solution Description]

To stay safe during a thunderstorm, one should avoid contact with electrical wires, metal pipes, and running water. Using mobile phones is safer than using wired phones. Electrical appliances should be unplugged except for electrical lights which can remain on.

Key Concept: Finding a Safe Place, Inside the House

a) Unplugging all electrical appliances

[Solution Description]

During a thunderstorm, lightning can strike telephone cords, electrical wires, and metal pipes. It is safer to avoid contact with these. Mobile phones and cordless phones can be used, but it is not wise to call someone using a wired phone. Electrical appliances should be unplugged, but electrical lights can remain on as they do not cause harm. Bathing should also be avoided to prevent contact with running water.

Key Concept: Lightning Safety

c) Squat low on the ground

[Solution Description]

During a thunderstorm, if no shelter is available in an open field, the safest action is to squat low on the ground. This position minimizes the risk of being struck by lightning by making you the smallest target. Place your hands on your knees with your head between your hands to further reduce exposure.

Key Concept: Lightning Safety

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that during a thunderstorm, using mobile phones is safer than using wired phones. This is true because mobile phones do not have any physical connection like wires that can conduct electricity from a lightning strike. The reason given is that wired phones are connected to telephone cords, which can conduct lightning strikes. This is also true because telephone cords are made of metal and can act as conductors for electrical discharge during a thunderstorm. Additionally, the reason correctly explains why using mobile phones is safer, as they eliminate the risk associated with conducting materials like telephone cords. Therefore, both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Lightning Conductors

b) They conduct electricity and can cause electric shock

[Solution Description]

Metal columns can conduct electricity if struck by lightning, posing a risk of electric shock. Therefore, it is advised not to touch them during a thunderstorm.

Key Concept: Lightning Conductors, Electric Charge Transfer

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

A lightning conductor is specifically designed to protect buildings from lightning strikes by providing a safe path for the electric charge to travel to the ground. This prevents the charge from passing through the building, which could cause damage or injury. Metal columns and water pipes can conduct electricity, but they are not intended to handle the immense energy of a lightning strike. Therefore, while they may offer some protection, they are not as reliable as a dedicated lightning conductor.

Key Concept: Lightning Conductor

b) To provide an easy route for electric charge to the ground

[Solution Description]

A lightning conductor is a device used to protect buildings from the effect of lightning. It provides an easy route for the transfer of electric charge to the ground, thereby preventing damage to the building.

Key Concept: Earthing, Lightning Conductor

c) To safely discharge the electric charge into the earth

[Solution Description]

A lightning conductor provides an easy path for the transfer of electric charge to the ground. The metallic rod installed in the building is taller than the building itself. One end of the rod is kept out in the air to attract the lightning strike, while the other end is buried deep into the ground to safely discharge the accumulated electric charge into the earth. This prevents damage to the building and reduces the risk of fire or electrical shock.

Key Concept: Lightning Safety

c) Taking shelter in a car with windows shut

[Solution Description]

During a thunderstorm, it is safe to stay inside a car or bus with windows and doors shut. Open vehicles, tall trees, and elevated places are not safe. Bathing and using wired phones should be avoided, but mobile phones can be used safely.

Key Concept: Lightning Conductors, Safety Measures

b) It diverts the lightning strike away from the building.

[Solution Description]

The lightning conductor provides an easy and direct path for the electric charge from the lightning strike to travel safely to the ground. This prevents the charge from passing through the building, thereby protecting it from damage. The metallic rod of the conductor is taller than the building, ensuring it intercepts the lightning strike before it can reach the structure. The other end of the rod is buried deep in the ground to ensure efficient dissipation of the electric charge.

Key Concept: Lightning Safety

a) Open field

[Solution Description]

During a thunderstorm, open fields, tall trees, and elevated places are not safe because they can attract lightning. A house or a building is considered a safe place. Therefore, the option that mentions an unsafe place is the correct answer.

Key Concept: Finding a Safe Place

a) Inside a house

[Solution Description]

According to the syllabus, during a thunderstorm, open places are not safe. A house or a building is considered a safe place. Additionally, if traveling by car or bus, you are safe inside with windows and doors shut. Open vehicles like motorbikes, tractors, and open cars are not safe.

Key Concept: Do’s and Don’ts during a Thunderstorm

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that staying inside a car with windows and doors shut is safer during a thunderstorm. This is true because a car provides a metal enclosure that can act as a Faraday cage, which shields the inside from electrical discharges like lightning. The reason given is that a car acts as a Faraday cage and protects occupants from lightning. This is also true because a Faraday cage distributes electric charge around its exterior, preventing it from penetrating the interior. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Do’s and Don’ts during a Thunderstorm

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that using mobile phones is safer than wired phones during a thunderstorm. This is true because mobile phones do not have physical connections like wires, which can conduct electricity if struck by lightning. The reason provided explains that lightning can strike telephone cords and electrical wires, making wired phones unsafe. This reason correctly explains why mobile phones are safer during thunderstorms. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Finding a Safe Place

b) In a car with windows and doors shut

[Solution Description]

When traveling during a thunderstorm, a car or bus with windows and doors shut is a safe place. Open vehicles like motorbikes or tractors are not safe.

Key Concept: Lightning Safety

d) Squat low on the ground with hands on knees and head between hands

[Solution Description]

If you are caught in an open field during a thunderstorm, the safest action is to squat low on the ground, placing your hands on your knees with your head between your hands. This position minimizes your height and reduces the risk of being struck by lightning. Do not lie flat on the ground as this increases your exposure to lightning.

Key Concept: Lightning Safety, Electrical Appliances

c) Taking a shower

[Solution Description]

To solve this question, we need to understand the risk factors associated with lightning strikes during thunderstorms. Bathing involves contact with running water, which can conduct electricity if lightning strikes plumbing. Using wired phones can also be risky because telephone cords can carry electrical discharges from lightning. However, unplugging electrical appliances reduces the risk of damage but does not directly pose a personal injury risk. Therefore, bathing during a thunderstorm is the most dangerous action among the options.

Key Concept: Electrical appliances during thunderstorms

b) Unplug all appliances except lights[Solution Description]During a thunderstorm, it is advisable to unplug electrical appliances like computers and TVs to prevent damage from lightning-induced power surges. However, electrical lights can remain on as they do not cause harm.

Key Concept: Earthquake Causes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The outermost layer of the earth is fragmented into tectonic plates, which are in continual motion. When these plates brush past one another or collide, they cause disturbances in the earth's crust. These disturbances manifest as earthquakes on the surface of the earth. Therefore, earthquakes are more likely to occur at the boundaries of tectonic plates, which are also known as seismic or fault zones. Both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Seismic Zones

c) South India

[Solution Description]

Seismic zones in India include Kashmir, Western and Central Himalayas, North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic Plain. Southern regions like Kerala are not typically considered high-risk seismic zones.

Key Concept: Earthquake Prediction, Plate Movement

c) Complexity of plate movements and stress accumulation[Solution Description]Although scientists understand that earthquakes are caused by the movement of tectonic plates, predicting the exact time and location of an earthquake is still not possible. This is because plate movements are complex and influenced by various factors that are difficult to monitor precisely. Additionally, the buildup of stress along fault lines occurs over long periods and does not always follow predictable patterns, making accurate predictions challenging.

Key Concept: Plate Tectonics

a) Movement of tectonic plates[Solution Description]Earthquakes are primarily caused by the movement of tectonic plates. These plates, which make up the earth's crust, can either brush past each other or collide, leading to disturbances that manifest as earthquakes on the surface.

Key Concept: Plate Movement, Fault Zones

d) Two plates colliding, causing one to subduct

[Solution Description]

Earthquakes are primarily caused by the movement of tectonic plates. When two plates collide, one plate may slide under the other, causing a disturbance in the earth’s crust. This disturbance leads to an earthquake. Among the given options, a collision between two tectonic plates is the most likely cause of a significant earthquake. Volcanic eruptions and meteor impacts can also cause tremors but are less common compared to plate movements.

Key Concept: Earthquake Definition

a) Movement of tectonic plates[Solution Description]Most earthquakes are caused by the movement of tectonic plates. These plates are fragments of the earth's crust that are in continual motion. When they collide or brush past each other, they create disturbances in the crust, which manifest as earthquakes on the surface.

Key Concept: Measurement of Earthquake

c) Richter scale

[Solution Description]

The power of an earthquake is measured on the Richter scale. This scale quantifies the magnitude of an earthquake, with higher values indicating more destructive energy.

Key Concept: Earthquake Causes

a) Movement of tectonic plates

[Solution Description]

An earthquake is primarily caused by the movement of tectonic plates deep inside the earth's crust. When these plates brush past one another or collide, they cause disturbances that manifest as earthquakes on the surface.

Key Concept: Seismic Waves

c) Seismograph

[Solution Description]

Seismic waves are recorded by an instrument called the seismograph. The seismograph consists of a vibrating rod or pendulum that starts vibrating when tremors occur. A pen attached to the vibrating system records the seismic waves on a moving paper.

Key Concept: Tectonic plate boundaries, Seismic waves

d) Primary (P) waves

[Solution Description]

When an earthquake occurs at a subduction zone, the first waves to be detected by a seismograph are Primary (P) waves. P-waves are compressional waves that travel faster than other seismic waves and are the first to arrive at any given location after an earthquake.

Key Concept: Seismic Waves and Seismograph

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The seismograph is an instrument designed to record seismic waves, which are generated by the tremors caused by earthquakes. The assertion that a seismograph is used to record seismic waves is true. The reason that seismic waves are produced by the tremors of an earthquake is also true. Furthermore, the reason correctly explains the assertion because seismic waves are indeed the result of earthquake tremors, which the seismograph records.

Key Concept: Earthquake Safety Measures

b) Take shelter under a table

[Solution Description]

During an earthquake, if you are at home, it is recommended to take shelter under a table and stay there until the shaking stops. This helps protect you from falling objects.

Key Concept: Earthquake Effects

b) Landslides

[Solution Description]

Earthquakes can lead to various secondary effects such as landslides, tsunamis, and floods. These occur due to the shaking of the ground and displacement of large volumes of water or soil.

Key Concept: Earthquake Measurement

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The Richter scale is a logarithmic scale used to measure the magnitude of earthquakes. According to the syllabus, an increase of 2 in magnitude on the Richter scale means that the earthquake has 1000 times more destructive energy. Therefore, an earthquake with a magnitude of 6 has 1000 times more destructive energy than an earthquake with a magnitude of 4. Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Richter scale, Destructive energy

c) $E / 1000$

[Solution Description]

The difference in magnitude between 7.5 and 5.5 is 2. For the Richter scale, a difference of 2 in magnitude corresponds to 1000 times more energy. Therefore, the energy released by the magnitude 5.5 earthquake is $E$ divided by 1000. Mathematically, this is expressed as $E / 1000$.

Key Concept: Richter Scale

c) 1000 times

[Solution Description]

The Richter scale is not linear; an increase of 2 in magnitude means 1000 times more destructive energy. Therefore, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4.

Key Concept: Earthquake-Prone Areas in India

b) Indian Plate and Eurasian Plate

[Solution Description]

The Himalayan region experiences frequent earthquakes due to the collision between the Indian Plate and the Eurasian Plate. This tectonic activity makes the region highly seismic.

Key Concept: Seismic or fault zones

c) Deccan Plateau

[Solution Description]

Seismic or fault zones are areas where earthquakes are more likely to occur due to the movement of tectonic plates. In India, regions such as Kashmir, Western and Central Himalayas, North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic Plain are known to be seismic or fault zones. South India, specifically the Deccan Plateau, is generally not considered a seismic or fault zone.

Key Concept: Seismic Zones in India

c) Tamil Nadu

[Solution Description] The areas most threatened by earthquakes in India include Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic Plane. Some areas of South India also fall in the danger zone. Among the given options, Tamil Nadu is not considered a high-risk seismic zone as per the syllabus.

Key Concept: Earthquake Protection

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that in highly seismic areas, it is better to use mud or timber for construction, which is true as per the syllabus. The reason provided is that mud and timber are lighter materials and cause less damage if the structure collapses, which is also true. Furthermore, the reason correctly explains why using mud or timber is advisable in seismic zones. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Earthquake-resistant buildings

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that in highly seismic areas, using mud or timber for construction is advisable over heavy materials. This is true because lighter materials like mud and timber are less likely to cause severe damage if the building collapses during an earthquake. The reason given is that mud and timber are lighter materials, which reduces the risk of heavy damage if the structure falls. This is also true and directly explains why using these materials is advisable in seismic zones. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Precautionary Measures Indoors

b) Take shelter under a table and stay there till the shaking stops

[Solution Description]

According to the syllabus, if you are at home during an earthquake, the safest action is to take shelter under a table and stay there until the shaking stops. This protects you from falling objects and reduces the risk of injury.