Key Concept: Atmospheric Pressure

c) The pressure inside our body equals the atmospheric pressure
[Solution Description]
The atmospheric pressure acts on our bodies from all directions, but the pressure inside our body balances it. This equilibrium prevents us from feeling crushed.

Key Concept: Applications of Pressure

d) The smaller area of a sharp edge increases pressure
[Solution Description]
A sharp knife has a smaller contact area, which increases the pressure for the same force applied, making cutting easier:
$Pressure = \frac{Force}{Area}$
Lower area leads to higher pressure.

Key Concept: Pressure, Force per Unit Area

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that a sharp needle exerts more pressure on the skin than a blunt needle for the same force. This is true because pressure ($P$) is calculated as $P = \frac{F}{A}$, where $F$ is the force and $A$ is the area. A sharp needle has a smaller contact area ($A$) compared to a blunt needle, so for the same force ($F$), the pressure increases as the area decreases.
The Reason states that pressure is inversely proportional to the area of contact when the force remains constant, which is also true. This directly explains why the sharp needle exerts more pressure—it follows from the definition of pressure.
Therefore, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: SI unit of pressure

c) Pascal (\$Pa\$)
[Solution Description]
The SI unit of pressure is newton per square metre (N/m$^2$), which is also called pascal (Pa). This is derived from the definition of pressure as force per unit area.

Key Concept: Pressure Definition

c) Force applied per unit area
[Solution Description]
Pressure is defined as force per unit area. The formula is: $Pressure = \frac{Force}{Area}$.

Key Concept: Pressure & Area Relationship

a) Surface with 2 m$^2$
[Solution Description]
Pressure is inversely proportional to area when the force is constant. Since $P = \frac{F}{A}$, a smaller area results in higher pressure. Here, 2 m$^2$ is smaller than 4 m$^2$, so the first surface will experience greater pressure.

Key Concept: Pressure and Area

d) 40 Pa
[Solution Description]
To find the pressure, use the formula:
$Pressure = \frac{Force}{Area}$
Substituting the given values:
$Pressure = \frac{200\,N}{5\,m^2} = 40\,Pa$
The pressure exerted is 40 Pa.

Key Concept: Practical Applications

c) It concentrates force over a smaller area increasing pressure
[Solution Description]
A sharp knife has a smaller edge area, so the same force produces higher pressure, making it easier to cut.

Key Concept: Pressure and Area Relationship

c) Broad straps distribute the weight over a larger area, reducing pressure.
[Solution Description]
Pressure is defined as force per unit area, given by $Pressure = \frac{Force}{Area}$. Broad straps distribute the weight of the bag over a larger area of the shoulders, reducing pressure and making it more comfortable to carry. Narrow straps concentrate the same weight over a smaller area, increasing pressure and causing discomfort.

Key Concept: Pressure Applications

d) The pressure decreases
[Solution Description]
If the strap width is doubled, the area of contact increases. Since pressure is inversely proportional to area ($Pressure = \frac{Force}{Area}$), increasing the area reduces the pressure for the same applied force (weight of the bag). Therefore, the pressure exerted on the shoulder decreases.

Key Concept: Sucker Experiment, Pressure Difference

c) External atmospheric pressure exceeds internal pressure of the sucker.
[Solution Description]
Pressing the sucker expels most of the air trapped between its cup and the surface. This reduces the internal air pressure, creating a pressure difference: the external atmospheric pressure is now higher than the pressure inside the sucker. The higher external pressure pushes the sucker against the surface, making it difficult to pull off. Only a strong enough force can overcome this pressure difference.

Key Concept: Atmospheric Pressure Concept

b) Atmospheric pressure
[Solution Description]
The pressure exerted by air around us is known as atmospheric pressure, as explained in the syllabus. It acts on all objects and balances the internal pressure of our bodies.

Key Concept: Atmospheric Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that atmospheric pressure is the force exerted by air on all objects, which is correct as discussed in the syllabus. The Reason explains that air exerts pressure because it has mass and occupies space, which is also true and correctly explains the Assertion since the weight of air molecules creates pressure.

Key Concept: Atmospheric Pressure and Area

c) The force doubles because force is directly proportional to area under constant pressure.
[Solution Description]
Atmospheric pressure is defined as force per unit area ($P = F/A$). If the area ($A$) doubles while pressure ($P$) remains constant (since it's atmospheric pressure), the force ($F$) must also double to maintain the equation $F = P \times A$. Thus, the force increases with increased area.

Key Concept: Atmospheric pressure, directionality

c) An inflated balloon expands uniformly when more air is blown into it.
[Solution Description]
Air exerts pressure equally in all directions. An inflated balloon expands uniformly because the air inside pushes outward in all directions. This is a direct observation of omnidirectional pressure.

Key Concept: Activity-Based Question

d) Air exerts greater pressure on the larger area of the unfolded sheet
[Solution Description]
Air exerts force on the surface area in contact. Since the unfolded sheet has a larger area than the folded one, the total force (and thus pressure) acting on it increases, requiring more effort to lift.

Key Concept: Air Pressure - Basic Concept

d) More air pressure acts on the larger surface area of the unfolded sheet
[Solution Description]
Unfolding the chart paper increases the surface area exposed to air pressure. Air exerts pressure on all surfaces, and a larger area means more force acts downward due to atmospheric pressure. Hence, lifting requires more effort to overcome this increased force. The weight of the sheet remains the same, but the pressure force increases with area ($P = F/A$ implies $F = PA$). Thus, the correct reasoning relates to the effect of increased area.

Key Concept: Air Pressure Experiment

c) The unfolded chart paper has a larger surface area exposed to atmospheric pressure
[Solution Description]
When the unfolded chart paper covers the paper plate, it increases the surface area exposed to atmospheric pressure. Air exerts force per unit area (pressure) on all objects. The larger area of the unfolded sheet means more air pressure acts downward, making it harder to lift. The folded sheet has less area, so less air pressure acts on it, requiring less effort.

Key Concept: Pressure and Area Relationship

b) Atmospheric pressure exerts a force proportional to the area exposed
[Solution Description]
Since the weight of the covering sheets remains constant but the effort varies with area, the increase in effort must be due to air pressure. Pressure ($P$) is force ($F$) per unit area ($A$), given by $P = F/A$. Larger area means more total force from atmospheric pressure acting downward, hence greater lifting effort.

Key Concept: Air pressure, Surface area comparison

c) 1:4
[Solution Description]
1) When the chart paper is folded twice, its area reduces to 1/4th of the original (since folding halves the length and width: (70/2) × (56/2) = 35 × 28 cm).
2) The unfolded sheet maintains full area (70 × 56 cm).
3) Air pressure $P = F/A$, where F remains constant (same weight of chart paper), but A changes.
4) Pressure ratio = $\frac{P_{\text{unfolded}}}{P_{\text{folded}}} = \frac{F/A_{\text{unfolded}}}{F/A_{\text{folded}}} = \frac{A_{\text{folded}}}{A_{\text{unfolded}}}$
5) Calculation: $\frac{35×28}{70×56} = \frac{980}{3920} = \frac{1}{4}$
Thus, the pressure ratio is 1:4.

Key Concept: Pressure Exerted by Air

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that when air is blown between two inflated balloons, they move towards each other. This is true because, as demonstrated in Activity 6.6, blowing air between the balloons creates a low-pressure area between them. The higher air pressure outside the balloons then pushes them towards the low-pressure region, causing them to move closer.
The Reason correctly explains the Assertion, as it describes the phenomenon of high air pressure moving towards low air pressure, which causes the balloons to move towards each other. Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Air movement due to pressure difference

c) Air moves from high-pressure regions to low-pressure regions.
[Solution Description]
In Activity 6.5, air moves from the inflated balloon to the uninflated one because the air pressure inside the inflated balloon is higher than that in the uninflated balloon. Air flows from a region of high pressure to a region of low pressure until the pressures equalize.

Key Concept: Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] When air is blown into a balloon, it fills the space inside and exerts pressure on the inner walls of the balloon. This pressure acts equally in all directions, causing the balloon to expand uniformly. The reason correctly explains why the balloon expands in all directions because air indeed exerts pressure uniformly in all directions.

Key Concept: Effect of air pressure, Pressure equilibrium

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the given scenario, the inflated balloon has higher internal pressure compared to the uninflated one. According to the concept of pressure equilibrium, air will flow from the high-pressure region (inflated balloon) to the low-pressure region (uninflated balloon) through the straw. The flow continues until the pressures in both balloons become equal, resulting in both balloons attaining similar sizes. The reason correctly explains why the air moves, as it aligns with the principle that air moves from high to low pressure regions. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Atmospheric Pressure

b) Air exerts pressure equally in all directions inside the balloon.
[Solution Description]
When air is blown into a balloon, it exerts pressure on the inner walls of the balloon. Since air exerts pressure equally in all directions (isotropic nature of pressure), the balloon expands uniformly in all directions. This is due to the balanced force distribution caused by the air molecules colliding with the balloon's surface.

Key Concept: Air Pressure and Area Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The activity demonstrates that when the area of the chart paper covering the inverted plate increases (unfolded), more effort is needed to lift it. This happens because air exerts pressure on all surfaces, and the total force increases with the area, following $P = F/A$. Thus, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When a sucker is pressed against a smooth surface, most of the air inside is expelled, creating a low-pressure region inside the sucker. The atmospheric pressure outside is much higher than the pressure inside the sucker, resulting in a net force pressing the sucker onto the surface. Thus, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Pressure Difference

c) The difference in air pressure inside and outside the sucker creates a strong sticking force.
[Solution Description]
When the rubber sucker is pressed against a smooth surface, most of the air trapped inside its cup is pushed out. This reduces the internal air pressure compared to the atmospheric pressure outside. The higher external air pressure presses the sucker firmly onto the surface, making it difficult to pull off.

Key Concept: Thunderstorms, Charge separation

c) Collision of water droplets and ice particles causes charge separation.
[Solution Description]
During thunderstorms, strong updrafts and downdrafts cause water droplets and ice particles to collide, leading to charge separation. Positive charges accumulate at the top and negative charges at the bottom of clouds. When the charge difference becomes large enough, it discharges as lightning.

Key Concept: Effect of high-speed winds on pressure

d) A low-pressure area forms above the roof
[Solution Description]
High-speed winds create a low-pressure area above the roof. The higher air pressure inside the house pushes the roof upwards due to the pressure difference, which can cause the roof to be blown off if it's weak.

Key Concept: Pressure difference, Wind formation

a) Wind is formed with speed proportional to the pressure difference.
[Solution Description]
When air moves from a high-pressure region to a low-pressure region, it results in the formation of wind. The greater the pressure difference, the faster the wind speed. This is because nature tends to balance pressure differences by moving air from areas of higher pressure to areas of lower pressure.

Key Concept: Pressure difference and wind direction

c) Difference in air pressure between regions
[Solution Description]
Air moves from a region of high pressure to a region of low pressure due to the pressure difference between the two regions. This movement of air is what we perceive as wind.

Key Concept: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that air moves from a region of high pressure to a region of low pressure, which is true as demonstrated in Activity 6.5 where air flows from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The reason explains that this pressure difference causes the movement of air, which is also true and correctly explains the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Land and Sea Breeze

d) Low pressure develops over the land
[Solution Description]
During the day, land heats up faster than water, creating a low pressure area over the land. The cooler air from the high pressure region over the sea moves towards the land, resulting in a sea breeze.

Key Concept: Units of Pressure

c) Newton per cubic meter ($N/m^3$)
[Solution Description]
Air pressure can be measured in pascals ($Pa$), millibars ($mb$), or hectopascals ($hPa$). Newtons per cubic meter ($N/m^3$) measures density, not pressure.

Key Concept: Pressure Difference and Wind Formation

d) The flow of air increases
[Solution Description]
The speed of wind is directly proportional to the pressure difference between two regions. A higher pressure difference results in faster wind movement as air flows from high-pressure to low-pressure regions until equilibrium is reached.

Key Concept: Formation of Wind, Pressure Differences

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during the day, sea breeze occurs due to warm air rising over the land, creating a low-pressure area, which causes cooler air from the sea (high-pressure area) to move toward the land. This is correct as per the concept of pressure differences in wind formation. The reason explains the general principle that air moves from high-pressure regions to low-pressure regions until equilibrium is reached, which directly explains why the sea breeze occurs. Both are true, and the reason correctly explains the assertion.

Key Concept: Sea breeze, Land breeze

c) Due to the differential heating and cooling rates of land and sea.
[Solution Description]
During the day, the land heats up faster than the sea, causing the air above the land to rise (creating low pressure). Cooler air from the sea moves towards the land (sea breeze). At night, the land cools faster than the sea, so the warmer air over the sea rises (low pressure), and cooler air from the land moves towards the sea (land breeze).

Key Concept: Pressure difference and wind speed

d) Higher pressure difference increases wind speed.
[Solution Description]
Wind speed depends on the pressure gradient (difference in pressure) between two regions. A larger pressure difference ($\Delta P$) results in a stronger force pushing the air from the high-pressure region to the low-pressure region, which increases the wind speed. Conversely, a smaller pressure difference leads to slower wind speeds.

Key Concept: Thunderstorms, Pressure difference

d) The pressure difference between outside and inside the house lifts the roof.
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure inside the house remains higher, creating a pressure difference. If this difference is large enough and the roof is weak, it can be blown away. Keeping windows open equalizes the pressure.

Key Concept: Pressure Differences

d) The pressure gradient force drives air from high to low pressure.
[Solution Description]
Air moves from high-pressure regions to low-pressure regions due to the pressure gradient force. This natural movement occurs because the atmosphere seeks to balance pressure differences.

Key Concept: Sea and Land Breezes

b) The land heats up faster than the sea, causing air to move from the sea to the land.
[Solution Description]
The land heats up faster than the sea during the day, causing the air above it to warm and rise, creating a low-pressure area. The cooler, high-pressure air over the sea moves towards the land, resulting in a sea breeze.

Key Concept: Sea and land breezes

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sea breeze occurs during the day because the land gets heated faster than the sea. This is true as land heats up faster than water, causing the air above it to rise and create a low-pressure area, drawing in cooler air from the sea (high-pressure area). The reason states that air moves from a region of high pressure to a region of low pressure, which is a fundamental principle behind wind formation. The reason correctly explains why the sea breeze occurs, as it describes the movement of air due to pressure differences caused by uneven heating of land and sea.

Key Concept: Sea and Land Breezes

a) Sea breeze occurs because the sea is cooler and has higher pressure compared to the land.
[Solution Description]
During the day, land heats up faster than the sea. The air above the land becomes warm and rises, creating a low-pressure area. As a result, cooler air from the high-pressure region over the sea moves towards the land, forming a sea breeze.

Key Concept: Effect of high-speed winds

c) The pressure difference created by high-speed winds lifts the leaves.
[Solution Description]
High-speed winds create a low-pressure area above the leaves. The higher air pressure below the leaves pushes them upwards, lifting them into the air.

Key Concept: Bernoulli's Principle

c) Bernoulli's Principle
[Solution Description]
The relationship between wind speed and air pressure is described by Bernoulli's principle, which states that an increase in wind speed leads to a decrease in pressure, as shown by the equation $P + \frac{1}{2} \rho v^2 = \text{constant}$.

Key Concept: High-speed winds and air pressure

c) The balloons move towards each other due to reduced pressure between them.
[Solution Description]
When high-speed wind is blown between the two balloons, it creates a low-pressure region between them. The higher air pressure on the outer sides of the balloons pushes them towards each other. This demonstrates that high-speed winds lower air pressure.

Key Concept: Pressure difference in storms

c) Low pressure outside the roof causes it to be pushed upwards by the higher internal pressure.
[Solution Description]
During a storm, high-speed winds reduce the air pressure above the roof. The higher air pressure inside the house exerts an upward force on the roof, causing it to be lifted or blown away. Balancing internal and external pressure by keeping windows and doors open can prevent this.

Key Concept: Wind Speed and Pressure Relationship

b) Air pressure decreases due to fewer molecular collisions.
[Solution Description]
According to Bernoulli's principle, an increase in wind speed leads to a decrease in air pressure. When wind blows faster over a surface, the kinetic energy of the air molecules increases, causing fewer collisions per unit area, resulting in lower pressure in that region.

Key Concept: High-speed winds, Bernoulli's principle, Low-pressure areas

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during a storm, keeping windows and doors closed increases the likelihood of the roof blowing off due to pressure difference. This is correct because high-speed winds reduce pressure above the roof as per Bernoulli's principle ($P + \frac{1}{2} \rho v^2 = \text{constant}$). The reason explains this phenomenon correctly by mentioning the low-pressure region created by high-speed winds and the resulting pressure imbalance. Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is option a).

Key Concept: High-Speed Winds and Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that high-speed winds blowing over a roof can cause it to be blown off, which is true based on the syllabus. The reason explains that this occurs because high-speed winds lower the air pressure above the roof, while the higher pressure inside the house pushes upward. Since both the assertion and reason are correct, and the reason directly explains the assertion, the correct answer is (a).

Key Concept: Pressure Difference Due to High-Speed Winds

b) The high pressure inside the house pushes the roof up as low pressure forms outside.
[Solution Description]
During storms, high-speed winds pass over the roof, reducing the air pressure above it. The higher air pressure inside the house pushes upwards on the roof due to this pressure difference. If the pressure difference is large enough, it can lift and blow the roof off. This demonstrates how high-speed winds create low-pressure areas.

Key Concept: Effect of Blowing Harder

c) The balloons move towards each other more quickly.
[Solution Description]
Blowing harder increases the speed of the air moving between the balloons, reducing the air pressure further. As a result, the difference in pressure causes the balloons to move towards each other more quickly.

Key Concept: High-Speed Winds and Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that when air is blown between two inflated balloons, they move towards each other, which is observed in Activity 6.6. This happens because blowing air between the balloons increases wind speed in the gap, reducing air pressure (Bernoulli’s principle). The higher pressure outside then pushes the balloons together. The reason correctly explains why this occurs by linking high-speed wind to low pressure and the resulting pressure difference.

Key Concept: Pressure Gradient, Force Calculation

c) 0.4 N
[Solution Description]
The pressure difference ($\Delta P$) is proportional to the square of the airspeed ($v^2$). If $v$ doubles from 30 m/s to 60 m/s, $\Delta P$ becomes $(60/30)^2 = 4$ times larger.
Since force ($F$) is directly proportional to $\Delta P$, the new force will also be 4 times the original:
$F_{\text{new}} = 4 \times 0.1\,\text{N} = 0.4\,\text{N}$.

Key Concept: Bernoulli's Principle, Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Step 1: Analyzing Assertion (A).
- The assertion states that when air is blown between two balloons, they move towards each other because of decreased pressure between them.
- From Activity 6.6, blowing air between two inflated balloons creates a region of lower pressure between them compared to the surrounding air pressure.
- The higher external pressure pushes the balloons together, confirming the assertion is true.
Step 2: Analyzing Reason (R).
- The reason refers to Bernoulli's principle, which states that an increase in fluid velocity decreases its pressure.
- In this experiment, blowing air increases its speed between the balloons, leading to lower pressure as per Bernoulli's principle.
- This directly explains why the balloons move towards each other, making the reason both true and the correct explanation of the assertion.
Conclusion: Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Pressure Difference Due to High-Speed Winds

b) The wind reduces air pressure above the roof, causing an upward force
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the pressure inside the house remains higher. The pressure difference causes an upward force on the roof. If the roof is weak or not properly secured, this force can blow it away.

Key Concept: Impact of pressure differences on structures

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, high-speed winds create low pressure over the roof while the air pressure inside the house remains higher when windows/doors are closed. This pressure difference exerts an upward force on the roof (Pressure = Force/Area). If the roof is weak, it may blow away. The Reason correctly explains why keeping windows/doors closed increases this pressure difference. Hence, both Assertion and Reason are true, and Reason is the correct explanation.

Key Concept: High-speed winds, pressure difference, structural impact

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
High-speed winds create low pressure over the roof while the air pressure inside the house remains higher if the roof is sealed. This pressure difference can cause the roof to lift or blow off. When doors and windows are open, air flows through the house, reducing the pressure difference between the inside and outside. Thus, both Assertion and Reason are true, and the Reason correctly explains why open doors and windows prevent roofs from being blown off.

Key Concept: Bernoulli's Principle Application

c) The balloons move closer together due to reduced pressure between them
[Solution Description]
Blowing air between the balloons creates a low-pressure region in between. The higher pressure on the outer sides of the balloons pushes them toward each other. This demonstrates Bernoulli's principle, where increased air speed results in decreased pressure.

Key Concept: Lightning

c) Lower part of the cloud (heavier water droplets)
[Solution Description]
In a thundercloud, heavier water droplets collect in the lower part of the cloud, accumulating negative charges. Lighter ice particles with positive charges remain suspended in the upper part of the cloud.

Key Concept: Formation of Thunderstorms

c) Upward movement of warm, moist air leading to condensation and cloud formation
[Solution Description]
The primary cause for the formation of thunderstorms is the upward movement of warm, moist air leading to condensation and the development of clouds. This process creates charge separation in clouds due to friction between rising and descending air currents.

Key Concept: Local thunderstorms, Climate phenomena

c) Bordoisila
[Solution Description]
Bordoisila is a local thunderstorm specific to Assam, characterized by strong winds and heavy rainfall. It occurs during pre-monsoon months.

Key Concept: Lightning Conductor

b) To provide a safe path for electric charges to reach the ground
[Solution Description]
A lightning conductor provides an easy path for the transfer of electric charges from a lightning strike into the ground, preventing damage to the building. It consists of a metallic rod with one end pointed higher than the building's highest point and the other end buried deep in the ground.

Key Concept: Formation of Storms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that high-speed winds create a low-pressure area over houses, which is true as per the syllabus. The reason explains that this happens because high-speed winds are accompanied by reduced air pressure, which is also true and correctly explains why low pressure is created. Hence, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Thunderstorms and Lightning

d) Positive charges in the upper part and negative charges in the lower part
[Solution Description]
Before lightning, positive charges accumulate in the upper part of the cloud (ice particles), while negative charges (water droplets) collect in the lower part of the cloud.

Key Concept: Pressure difference, Wind speed, Roof stability

b) It reduces the pressure difference between the inside and outside of the house.
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the air pressure below remains higher. This difference can cause roofs to blow off. Keeping doors and windows open allows wind to flow through the house, balancing the pressure inside and outside, thus reducing the net force on the roof.

Key Concept: Formation of Storms, Cyclones

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that opening doors and windows during a storm reduces the risk of roofs being blown away by equalizing pressure inside and outside the house. This is true because high-speed winds lower the air pressure above the roof, creating a pressure difference between the inside and outside of the house. If the pressure difference is large enough, roofs may be blown away. Opening doors and windows allows air to flow through the house, balancing this pressure difference.
The reason explains that high-speed winds create a low-pressure area above the roof, which can cause the roof to lift if the pressure difference is significant. This is also correct and directly supports the assertion. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Assertion and Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that thunderstorms are more frequent in hot, humid, and tropical regions like India, which is true because such conditions provide the necessary moisture and temperature contrasts for thunderstorm development. The Reason explains that warm air rises to great heights, forming ice particles and causing charge separation, which is also correct and directly explains why thunderstorms occur more frequently in these regions. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Concept: Formation of Thunderstorms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When land is heated, warm and moist air rises, creating a low-pressure area. As this air rises, it expands and cools, causing the moisture in it to condense and form clouds. This process can lead to the development of thunderstorms when strong winds and charge separation occur within the clouds. Thus, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Thunderstorm formation, Charge separation in clouds

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion claims that lightning occurs exclusively between a cloud and the ground due to maximum charge separation there, which is false. Lightning can occur within a cloud, between clouds, or between a cloud and the ground. The Reason correctly describes the charge distribution in a thundercloud but does not justify the Assertion, as it explains general charge separation, not ground-specific phenomena.

Key Concept: Dangers and Precautions, Thunderstorms and Lightning

c) It reduces contact area with the ground and avoids being a tall target
[Solution Description]
Lying flat increases the body's surface area in contact with the ground, making it more susceptible to a ground current if lightning strikes nearby. Crouching reduces this contact area, minimizing risk. Additionally, staying low avoids being the tallest object, which is more likely to be struck by lightning.

Key Concept: Lightning safety measures, Static charge induction

d) They attract lightning due to charge induction
[Solution Description]
Tall objects and metallic conductors are more likely to be struck by lightning because they provide a path of least resistance for the discharge. The negatively charged base of the cloud induces a positive charge on such objects, increasing the risk of a lightning strike.

Key Concept: Safety during thunderstorms

c) Staying inside a car with windows closed
[Solution Description]
During a thunderstorm, staying inside a car or bus is safer because their metal bodies act as a shield against lightning, provided you do not touch any metallic parts.

Key Concept: Lightning formation

c) Discharge between the lower part of a cloud and the positively charged ground
[Solution Description]
When the negatively charged lower part of the cloud induces a positive charge on the ground, the buildup of opposite charges overcomes air's insulating property, leading to a sudden discharge called cloud-to-ground lightning.

Key Concept: Static charge development in clouds

d) Strong winds causing rubbing between water droplets and ice particles
[Solution Description]
Inside a thundercloud, strong winds cause rubbing between water droplets and ice particles. The positively charged lighter ice particles move upward, while the negatively charged heavier water droplets settle at the lower part of the cloud, creating charge separation.

Key Concept: Lightning Formation

c) A sudden flow of charges occurs, producing lightning
[Solution Description]
When the charge separation in the cloud becomes significant, the insulating property of air breaks down, allowing a sudden flow of charges. This discharge produces a bright flash of light called lightning. Lightning can occur within a cloud, between clouds, or between a cloud and the ground.

Key Concept: Formation of Thunder

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that thunder is produced due to rapid expansion of air caused by the intense heat from lightning. According to the syllabus, thunder indeed results from the explosive expansion of air heated by lightning. The reason explains that lightning involves a sudden flow of electric charges, which heats the air. This directly supports the assertion because the mechanism described in the reason leads to the effect mentioned in the assertion. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Thunderstorm Formation, Charge Build-Up

c) Excessive build-up of electric charges
[Solution Description]
Normally, air acts as an insulator and prevents opposite charges from meeting. However, when the build-up of charges becomes very large, the insulating property of air breaks down, allowing a sudden flow of charges. This produces a bright flash of light known as lightning.

Key Concept: Lightning Mechanism

d) When the buildup of opposite charges exceeds air's insulation capacity
[Solution Description]
Lightning occurs when the buildup of opposite charges becomes very large, causing the insulating property of air to break down. A sudden flow of charges produces a bright flash of light, known as lightning. This can happen within a cloud, between clouds, or between clouds and the ground.

Key Concept: Cyclone Formation and Characteristics

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that the eye of a cyclone has calm winds and clear skies, which is true because it is a region of descending air that suppresses cloud formation. However, the reason claims that the eye forms due to subsidence of warm air, which is incorrect. The eye forms due to the centrifugal force balancing the pressure gradient force, resulting in minimal wind movement. Thus, the Assertion is true, but the Reason is false.

Key Concept: Cyclone formation, Pressure difference

b) $0.167 \, \text{hPa/km}$
[Solution Description]
The pressure gradient is calculated as the change in pressure divided by the distance over which this change occurs. Here, the pressure difference is $1000 \, \text{hPa} - 950 \, \text{hPa} = 50 \, \text{hPa}$. The distance is 300 km.
Therefore, the pressure gradient is:
$\frac{50 \, \text{hPa}}{300 \, \text{km}} = \frac{1}{6} \, \text{hPa/km} \approx 0.167 \, \text{hPa/km}$

Key Concept: Cyclone Impact

c) Saltwater making soil more fertile.
[Solution Description]
When a cyclone reaches land, it loses its source of moist air and gradually weakens. However, it still causes destruction through strong winds, storm surges, heavy rainfall, flooding, landslides, and contamination of freshwater sources. Improved soil fertility is not an impact; instead, seawater intrusion makes soil less fertile due to salt deposits.

Key Concept: Eye of the Cyclone

c) Calm winds and low pressure
[Solution Description]
The eye of the cyclone is a region of calm winds and low pressure at the center of the storm, surrounded by intense winds and heavy rainfall.

Key Concept: Impact

c) Strong winds, heavy rain, and storm surges
[Solution Description]
When a cyclone reaches land, it loses its source of moist air and weakens, but before that, it causes destruction through strong winds, heavy rainfall, storm surges, and flooding. The text explains: "Once the cyclone reaches land, the source of moist air is cut off and it gradually loses its strength... Cyclones can be extremely destructive..."

Key Concept: Basic Concept

c) Eye of the cyclone
[Solution Description]
The eye of the cyclone is the central region where the pressure is the lowest and winds are calm. This is mentioned in the text as: "In a cyclone, the region of lowest pressure is at the centre, known as the eye of the cyclone."

Key Concept: Storm surge, Cyclone impact

c) Widespread flooding and saltwater intrusion.
[Solution Description]
A storm surge is a rapid rise in sea level caused by a cyclone's strong winds pushing water toward the shore. A 10-meter surge can flood coastal regions extensively, damaging infrastructure, contaminating freshwater sources, and causing loss of life due to drowning or structural collapse.

Key Concept: Cyclone Formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that cyclones lose strength over land due to the absence of warm, moist air, which is correct as per the syllabus. The Reason explains that warm ocean water provides heat and moisture to sustain cyclones, which is also accurate. Since the Reason correctly explains why cyclones weaken over land (due to the loss of this energy source), both statements are true and logically connected.

Key Concept: Formation of Cyclones

c) By heating the air, causing it to rise and form a low-pressure zone
[Solution Description]
Cyclones form over warm ocean waters when the moist air above the heated water rises. As it rises, the water vapour condenses into raindrops, releasing heat, which warms the air further and causes it to rise more. This creates a low-pressure area that draws in surrounding air, forming a cyclonic system.

Key Concept: Eye of the cyclone, High-speed winds

c) Wind speed increases to 220 km/h
[Solution Description]
The wind speed in a cyclone is driven by the pressure gradient force. A lower pressure in the eye means a steeper pressure gradient between the eye and the surrounding regions. This steeper gradient results in stronger winds. Since the pressure drops from 950 hPa to 930 hPa, the gradient increases, leading to higher wind speeds.

Key Concept: Eye of the Cyclone

c) The area of low pressure at the center with calm winds
[Solution Description]
The eye of a cyclone is the central region where the atmospheric pressure is the lowest. It is characterized by calm winds and clear skies, while the surrounding areas experience high-speed winds and heavy rain.

Key Concept: Safety Measures, High-speed winds

b) Constructing cyclone-resistant buildings
[Solution Description]
For cyclones with high wind speeds like 250 km/h, the most effective safety measures include early warning systems for evacuation and sturdy shelters to withstand the winds. Public awareness campaigns and timely IMD alerts are also crucial. Among the options, constructing cyclone-resistant buildings provides long-term protection against such high-speed winds.

Key Concept: Impact of Cyclones on Land, Flooding

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that storm surges from cyclones can flood coastal and inland areas due to high water walls. This is true because cyclones generate powerful winds that push seawater, creating storm surges as mentioned in the syllabus. The reason explains the mechanism behind storm surges, which aligns with the assertion. Hence, both are true, and the reason correctly explains the assertion.

Key Concept: Storm Surge Damage

c) It causes devastating flooding, damaging homes and infrastructure.
[Solution Description]
A storm surge is a wall of water pushed towards the shore by strong cyclone winds. It can flood coastal areas, contaminate freshwater sources, destroy infrastructure, and cause significant loss of life and property.

Key Concept: Impact of Cyclones

a) Flooding caused by storm surges
[Solution Description]
Cyclones generate strong winds that push ocean water towards the shore, creating a storm surge. This can flood coastal areas and even regions far inland. The flooding contaminates drinking water sources and damages farmland due to saltwater intrusion.

Key Concept: Impact of Cyclones on Agriculture

c) Salt from seawater makes the soil less fertile, reducing crop yields.
[Solution Description]
When seawater floods agricultural land due to a cyclone, the high salt content in the water can make the soil less fertile. This reduces crop productivity for months or even years until the salt is washed away naturally or through human intervention such as soil treatment.

Key Concept: Cyclone Safety Measures

b) To ensure immediate access to essential supplies if services are disrupted
[Solution Description]
Cyclones can disrupt power, water supply, and communication networks. An emergency kit with essentials like food, water, medicines, and flashlights ensures survival during and immediately after the cyclone until help arrives or normalcy is restored.

Key Concept: Measures to monitor cyclones

d) India Meteorological Department
[Solution Description]
The India Meteorological Department (IMD) monitors weather conditions, including cyclones, and issues alerts and warnings to the public.

Key Concept: Cyclone monitoring, Prediction measures

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that IMD uses weather satellites to predict cyclone paths and intensity accurately. This is true as satellites provide critical data for monitoring cyclones. The Reason explains that satellites supply real-time data on atmospheric conditions necessary for tracking cyclones, which is also true. Moreover, the Reason correctly explains why satellites are used for cyclone prediction, as they provide essential parameters like wind speed and pressure. Hence, both statements are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Cyclone Formation

c) Warm ocean waters with temperatures above $26.5^{\circ}$C
[Solution Description]
A cyclone forms when warm ocean water heats the air above it, causing it to rise and create a low-pressure zone. As the moist air rises, condensation releases heat, further lowering pressure and drawing in surrounding air. The Earth's rotation causes the air to spin, forming a cyclone. Therefore, warm ocean waters are crucial for cyclone formation.