Key Concept: Atmospheric Pressure, Rubber Sucker

d) 785 N
[Solution Description]
The suction force is due to the pressure difference between the inside and outside of the sucker. The area of the sucker is $\pi r^2$, where $r = 5$ cm = 0.05 m. So, area = $\pi (0.05)^2 ≈ 0.00785 m^2$. The force required to overcome atmospheric pressure is $Pressure × Area = 10^5 Pa × 0.00785 m^2 = 785 N$.

Key Concept: Pressure, Force and Area

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that the pressure exerted by a liquid depends only on the height of the liquid column and not on the shape or cross-sectional area of the container. This is true because pressure at any point in a liquid is given by $P = \rho g h$, where $\rho$ is the density of the liquid, $g$ is the acceleration due to gravity, and $h$ is the height of the liquid column. The Reason states that pressure in liquids is transmitted equally in all directions due to Pascal's principle, which is also true. However, Pascal's principle explains how pressure is distributed, not why it depends only on the height. Therefore, the Reason does not correctly explain the Assertion.

Key Concept: Practical Application

b) Sharp knife reduces area, increasing pressure
[Solution Description]
A sharp knife has a smaller edge area compared to a blunt knife. Since $P = \frac{F}{A}$, for the same applied force, the pressure is higher when the area is smaller. Thus, the sharp knife cuts more easily due to higher pressure.

Key Concept: Pressure calculation, real-world application

a) 1:500
[Solution Description]
First, convert areas to m$^2$ for consistency:
Hammer head area = $5 \times 10^{-4}$ m$^2$,
Nail tip area = $1 \times 10^{-6}$ m$^2$.
Pressure by hammer head:
$P_{hammer} = \frac{200}{5 \times 10^{-4}} = 400,000 \text{ Pa}$
Pressure by nail tip:
$P_{nail} = \frac{200}{1 \times 10^{-6}} = 200,000,000 \text{ Pa}$
Ratio:
$\frac{P_{hammer}}{P_{nail}} = \frac{400,000}{200,000,000} = \frac{1}{500}$

Key Concept: Practical Unit of Air Pressure

b) Millibar (mb)
[Solution Description]
While the SI unit of pressure is pascal (Pa), the practical unit of air pressure is millibar (mb), where 1 mb = 100 Pa.

Key Concept: Pressure and its SI unit

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the SI unit of pressure, pascal (Pa), is equivalent to $N/m^2$. This is correct because pressure is defined as force divided by area ($P = F/A$). Since the SI unit of force is newton (N) and that of area is metre$^2$ ($m^2$), the derived SI unit of pressure is indeed $N/m^2$, which is called pascal (Pa). The reason correctly explains the assertion by defining pressure and mentioning the units of force and area. Hence, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Pressure, Force Distribution, Applications

c) It increases the pressure due to smaller contact area
[Solution Description]
A sharp knife has a smaller edge area, so the same force is applied over a smaller area, resulting in higher pressure, which makes cutting easier.

Key Concept: Pressure, Atmospheric Pressure, Liquid Pressure

b) 117600 N
[Solution Description]
Pressure at the base due to water column = $\rho gh = 1000 \times 9.8 \times 3 = 29400 \text{ Pa}$.
Area of the base = $2 \text{ m} \times 2 \text{ m} = 4 \text{ m}^2$.
Force on the base = Pressure $\times$ Area = $29400 \times 4 = 117600 \text{ N}$.

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: Pressure, Force Distribution

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a sharp knife cuts better due to its smaller contact area, which increases pressure for the same force. The reason correctly explains this by stating the inverse relationship between pressure and area under constant force (P = F/A). Thus, both are true and the reason is the correct explanation.

Key Concept: Atmospheric Pressure, Force-Area Relationship

c) Air exerts more pressure on the larger surface area of the unfolded sheet.
[Solution Description]
When the unfolded sheet covers the paper plate, it has a larger surface area exposed to the air. Atmospheric pressure exerts force on all surfaces in contact with the air. Since pressure is force per unit area ($P = F/A$), a larger area results in a greater total force acting on the sheet, making it harder to lift. Conversely, the folded sheet has a smaller exposed area, meaning less force from atmospheric pressure acts on it, requiring less effort to lift.

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 a rubber sucker sticks to a smooth surface when pressed because the air pressure inside becomes lower than the atmospheric pressure outside. This is correct because pressing the sucker removes most of the air inside, creating a lower pressure zone compared to the higher external atmospheric pressure. The reason given is also correct: atmospheric pressure acts in all directions and exerts a force on the sucker, making it hard to pull off. Furthermore, the reason correctly explains the assertion, as the external atmospheric pressure's force is what keeps the sucker stuck due to the pressure difference. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Atmospheric Pressure Basics

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

Key Concept: Magnitude of Atmospheric Pressure

c) The pressure inside our body balances the atmospheric pressure
[Solution Description]
Atmospheric pressure exerts a large force, but our body’s internal pressure balances it, preventing us from being crushed. This equilibrium ensures no net force acts on our bodies.

Key Concept: Atmospheric pressure, force calculation

d) 2,250 N
[Solution Description]
First, convert the area from \(\mathrm{cm}^2 \rightarrow \mathrm{m}^2\) : $15 \text{ cm} \times 15 \text{ cm} = 225 \text{ cm}^2 = 0.0225 \text{ m}^2$.
Atmospheric pressure is given as $100 \text{ kPa} = 100,000 \text{ Pa} = 100,000 \text{ N/m}^2$.
Force required to overcome atmospheric pressure is calculated using $F = P \times A$:
$F = 100,000 \text{ N/m}^2 \times 0.0225 \text{ m}^2 = 2,250 \text{ N}$.

Key Concept: Atmospheric Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The rubber sucker sticks to a smooth surface because when it is pressed against the surface, air inside it is expelled, reducing the internal pressure. The higher external atmospheric pressure then pushes the sucker firmly onto the surface. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Pressure Exerted by Air, Surface Area Effect

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the given experiment, the assertion states that more effort is needed to lift the paper plate when covered with an unfolded sheet than a folded one, which is true as observed in Activity 6.3. The reason claims this is due to higher atmospheric pressure on the unfolded sheet because of its larger surface area, which is also true. Moreover, the reason correctly explains the assertion since atmospheric pressure acts on all surfaces, and its effect increases with the surface area exposed to air. Hence, both the assertion and reason are true, and the reason is the correct explanation for the assertion.

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: Direction of Air Pressure

c) Air pressure acts uniformly in all directions
[Solution Description]
Air inside the balloon exerts pressure uniformly in all directions (a property of fluids/gases). This isotropic pressure causes the balloon's walls to stretch outward equally, leading to expansion in all directions. The explanation aligns with the concept that air pressure is not directional but acts omnidirectionally.

Key Concept: Air Pressure Experiment

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that more effort is needed to lift the paper plate with an unfolded chart paper than a folded one, which matches observations from Activity 6.3. The reason explains this phenomenon by stating that air exerts pressure on the sheet's surface area, and since $\text{Pressure} = \frac{\text{Force}}{\text{Area}}$, a larger area increases the force exerted by air pressure. Therefore, both statements are true, and the reason correctly explains the assertion.

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: 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 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: Direction of Air Pressure

c) Air exerts pressure equally in all directions, causing uniform expansion.
[Solution Description]
Air molecules inside the balloon move randomly and collide with the inner walls, exerting pressure uniformly in all directions. Since the rubber material of the balloon is flexible, it expands outward equally to balance the internal air pressure.

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: Air Pressure

c) The balloon deflates as air escapes
[Solution Description]
When an inflated balloon is left open, the air inside escapes because the pressure inside the balloon is higher than the atmospheric pressure outside. The air moves from a region of high pressure (inside the balloon) to a region of low pressure (outside the balloon). This is why the balloon deflates.

Key Concept: Atmospheric pressure, Suction cup

d) The air inside is at higher pressure than the surrounding atmosphere.
[Solution Description]
The air inside the balloon is at higher pressure than the atmospheric pressure outside. Air always moves from high-pressure regions to low-pressure regions. Hence, when the balloon's mouth is left open, the high-pressure air inside escapes to the lower-pressure surroundings until equilibrium is achieved.

Key Concept: Sea breeze and land breeze

d) Low pressure develops over the sea
[Solution Description]
During the day, the land heats up faster than the sea, causing the air above the land to become warmer and rise, creating a low-pressure area. The cooler air from the high-pressure area over the sea moves towards the land, resulting in a sea breeze.

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) Wind is generated
[Solution Description]
Air moves from a high-pressure region to a low-pressure region due to the pressure gradient force. This movement of air is what we perceive as wind. The greater the pressure difference, the stronger the wind speed.

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: 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: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that wind moves from a high-pressure region to a low-pressure region, which is true as explained in the activity where air flows from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The Reason correctly explains this phenomenon by stating that the difference in air pressure causes the movement of air. Therefore, both Assertion and Reason are true, and Reason provides the correct explanation for the Assertion.

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: Unit of Pressure

b) Pascal ($Pa$)
[Solution Description]
The SI unit of pressure is $N/m^2$, also known as pascal ($Pa$). Other units like millibar ($mb$) and hectopascal ($hPa$) are derived from it but are not the standard SI unit.

Key Concept: Pressure difference, Wind speed

c) The wind speed doubles.
[Solution Description]
The wind speed is directly proportional to the pressure difference:
$\text{Wind speed} \propto \text{Pressure difference}$.
Initially, let the wind speed be $v_1$ when pressure difference is 12 hPa.
When pressure difference doubles to 24 hPa, the new wind speed $v_2$ can be calculated as:
$\frac{v_2}{v_1} = \frac{24 \text{ hPa}}{12 \text{ hPa}} = 2$
Therefore, $v_2 = 2v_1$. The wind speed doubles.

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: 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: Role of warm air rising and cool air sinking

c) Movement of air from high pressure to low pressure
[Solution Description]
Winds are formed due to the movement of air from high-pressure areas to low-pressure areas. When land gets heated, warm air rises, creating a low-pressure area. Cooler air from surrounding high-pressure regions flows in to replace the rising warm air, resulting in wind.

Key Concept: Land Breeze, Pressure Differences

c) Low pressure over sea, high pressure over land
[Solution Description]
At night, land cools faster than the sea, creating high pressure over the land and low pressure over the sea. The pressure difference drives the land breeze. Since wind speed $\propto$ pressure difference, the larger the temperature difference, the greater the pressure gradient. Given the land is colder ($15^\circ C$) and the sea is warmer ($20^\circ C$), the pressure gradient will cause air to flow from land (high pressure) to sea (low pressure).

Key Concept: Sea Breeze and 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, the wind blows from the sea to the land because the land is warmer than the sea. This is true as per the syllabus, which explains that during the day, the land becomes warmer and lighter, creating a low-pressure area. The reason given is that the land heats up faster than the sea during the day, creating a low-pressure region over the land. This is also true and correctly explains why the wind blows from the high-pressure sea to the low-pressure land, resulting in a sea breeze. Therefore, both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Pressure Differences

c) Wind moves from high-pressure areas to low-pressure areas.
[Solution Description]
Wind is caused by the movement of air from a region of high pressure to a region of low pressure. This principle explains phenomena like sea and land breezes.

Key Concept: Land Breeze

c) Because the sea retains heat longer and develops lower pressure.
[Solution Description]
At night, the land cools down faster than the sea. The air above the sea remains warmer, creating a low-pressure area over the sea. Consequently, the cooler air from the high-pressure region over the land moves towards the sea, forming a land breeze.

Key Concept: High-Speed Winds and Air Pressure

b) Air pressure decreases
[Solution Description]
According to Bernoulli's principle, as the speed of wind increases, the pressure decreases. This is given by the equation: $P + \frac{1}{2} \rho v^2 = \text{constant}$. High-speed winds reduce the air pressure in the region they flow over.

Key Concept: High-speed winds, Pressure difference, Structural safety

b) 100365 Pa
[Solution Description]
According to Bernoulli's principle, the total energy per unit volume remains constant. The pressure difference can be calculated using the formula $\Delta P = \frac{1}{2}\rho v^2$. Given $\rho = 1.2$ kg/m$^3$ and $v = 40$ m/s, we have:
Step 1: Compute $\frac{1}{2}\rho v^2$
$\frac{1}{2} \times 1.2 \times (40)^2 = 0.6 \times 1600 = 960$ Pa.
Step 2: Subtract this from the internal pressure
Pressure above roof $= P_{\text{inside}} - \Delta P = 101325 - 960 = 100365$ Pa.
Thus, the pressure above the roof is approximately $100365$ Pa.

Key Concept: Application of Bernoulli's Principle

c) Because of the low pressure created by high-speed winds above the roof
[Solution Description]
During storms, high-speed winds create low pressure above the roof, while the pressure inside the house remains higher. This difference in pressure causes the roof to lift off. This effect is explained by Bernoulli's principle.

Key Concept: Pressure equalization, Wind effects on structures

b) To reduce the pressure difference between inside and outside
[Solution Description]
When high-speed winds blow over a house, they create a low-pressure region above the roof. If the windows are tightly shut, the pressure inside the house remains higher compared to the outside, creating a significant pressure difference. This can cause the roof to lift or blow away. By keeping windows and doors slightly open, the wind flows through the house, reducing the pressure difference between the inside and outside, thereby minimizing structural damage.

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: 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: Bernoulli's Principle, Air Pressure

c) They move towards each other due to the low pressure created between them
[Solution Description]
According to Bernoulli's principle, when air is blown between the two balloons, the speed of air increases in that space, creating a low-pressure region between them. The surrounding higher air pressure pushes the balloons toward each other. This demonstrates that high-speed winds result in reduced air pressure.

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 create low-pressure areas, which is correct according to the given syllabus. The reason explains why this happens: fast-moving air reduces the air pressure in the region it passes over. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Effect of Blowing Harder on Balloons

c) They move towards each other faster.
[Solution Description]
Blowing harder increases the speed of the air between the balloons, further reducing the air pressure in that region. The greater difference in pressure compared to the surrounding air causes the balloons to move towards each other at a faster rate.

Key Concept: Effect of High-Speed Wind on Air Pressure

c) The air pressure decreases.
[Solution Description]
When air is blown between the two balloons, it creates a region of high-speed wind, which reduces the air pressure in that area. The surrounding higher air pressure pushes the balloons towards each other. Thus, the air pressure between the balloons decreases.

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: Balloon Movement Due to Pressure Difference

c) The high-pressure air outside pushes them inward.
[Solution Description]
Blowing air between the balloons creates a low-pressure zone due to the high-speed wind. The higher atmospheric pressure outside the balloons exerts a force, pushing them toward the low-pressure region between them, causing them to move closer.

Key Concept: High-Speed Winds and Air Pressure

b) The air pressure decreases above the roof.
[Solution Description]
According to Bernoulli's principle, high-speed winds create a region of low pressure above the roof compared to the higher pressure inside the house. This pressure difference can cause the roof to be blown off if it is weak.

Key Concept: Pressure reduction, Wind speed

b) 98,900 Pa
[Solution Description]
Given:
- Initial pressure ($P_{\text{initial}}$) = $1.02 \times 10^5$ Pa
- Pressure reduction = 3% of initial pressure
Step 1: Calculate pressure reduction amount
$0.03 \times 1.02 \times 10^5 = 3060$ Pa
Step 2: Calculate new pressure
$1.02 \times 10^5 - 3060 = 0.9894 \times 10^5$ Pa = $98,940$ Pa
Therefore, the new pressure is approximately 98,900 Pa

Key Concept: Unit of Pressure

b) Pascal ($Pa$)
[Solution Description]
The SI unit of pressure is $N/m^2$, also known as pascal ($Pa$). Other units like millibar ($mb$) and hectopascal ($hPa$) are also used, but $Pa$ is the standard SI unit.

Key Concept: High-speed winds and pressure difference

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When high-speed winds blow over a house, they create a low-pressure area above the roof. Inside the house, the air pressure remains higher compared to the outside. This pressure difference exerts an upward force on the roof. If the roof is weak or not properly secured, it can be blown off due to this force. Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Formation of thunderstorms and lightning

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that lightning occurs due to the sudden flow of charges when the insulating property of air breaks down, which is correct as per the syllabus. The reason mentions that rapid heating of air around lightning causes expansion, resulting in thunder, which is also true as per the syllabus. However, the reason does not explain why lightning occurs but instead explains the phenomenon of thunder, which is a consequence of lightning. Therefore, both are true, but the reason is not the correct explanation of the assertion.

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 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: 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: Charge separation, Lightning formation, Cloud dynamics

c) The negatively charged lower part of the cloud induces a positive charge on the ground.
[Solution Description]
The negative charges accumulate in the lower part of the cloud due to heavier water droplets, while positive charges rise to the upper part. When the negative charge in the cloud induces a positive charge on the ground, the potential difference increases until it discharges as lightning.

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: Precautions during thunderstorms

c) Crouch down in a low-lying open area
[Solution Description]
During thunderstorms, it is safer to crouch down in a low-lying open area to minimize contact with the ground and avoid being the tallest object, reducing the risk of being struck by lightning.

Key Concept: Lightning conductors

c) To provide a safe path for electric discharge into the ground
[Solution Description]
A lightning conductor provides an easy path for electric charges to travel safely into the ground, preventing damage to the building by discharging the accumulated static electricity from lightning strikes.

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: Thunderstorms and Lightning, Regional Variations

c) Assam; helps kharif crops and mango ripening
[Solution Description]
Bordoisila is a local name for thunderstorms in Assam. These thunderstorms occur before the monsoon and are crucial for agriculture. They provide necessary rainfall for kharif crops and aid in ripening mangoes, supporting regional farming activities.

Key Concept: Lightning types, Charge distribution

c) Collision of opposite charges inside a single cloud
[Solution Description]
Intra-cloud lightning occurs due to the collision of opposite charges within the same cloud. The positively charged ice particles accumulate at the top, while negatively charged water droplets gather at the bottom. When the charge buildup is sufficient, discharge occurs within the cloud.

Key Concept: Static charge development within clouds leading to lightning

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that lightning occurs only when a cloud is negatively charged at the bottom and the ground is positively charged. This is false because lightning can also occur within a single cloud (intra-cloud lightning) or between two different clouds (inter-cloud lightning). The Reason, however, is true as it correctly describes the process of charge separation in clouds due to the rubbing of water droplets and ice particles from strong winds.

Key Concept: Static charge development in clouds

b) Friction between ice particles and water droplets due to wind movement
[Solution Description]
The strong upward and downward winds within a cloud cause rubbing between water droplets and ice particles, leading to charge separation. Lighter positively charged ice particles move upwards while heavier negatively charged water droplets remain at the lower part of the cloud.

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: Charge Separation in Clouds

b) Friction between rising and falling ice and water particles
[Solution Description]
According to the syllabus, strong winds blowing upwards and downwards facilitate rubbing between water droplets and ice particles. This friction causes static electric charges to develop, with positively charged lighter ice particles moving to the upper part of the cloud and negatively charged heavier water droplets occupying the lower part. Thus, charge separation occurs due to the movement and collision of these particles.

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: Charge separation, Lightning and Thunder

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that thunder is formed due to the rapid heating and expansion of air from lightning, which is correct because the intense heat of lightning causes the surrounding air to expand explosively, creating the sound wave we hear as thunder. The Reason explains why lightning occurs—due to the breakdown of air's insulating property when charge buildup is excessive, which is also correct. However, the Reason does not explain why thunder is formed; it only explains lightning. Thus, both statements are true, but the Reason does not explain the Assertion.

Key Concept: Precautions During Thunderstorms

c) Using an umbrella with a metallic rod
[Solution Description]
The syllabus advises avoiding tall objects, staying away from open water, and not lying flat on the ground. Using an umbrella with a metallic rod is unsafe because metal conducts electricity, increasing the risk of being struck by lightning. Seeking shelter inside a car or low-lying area is safer.

Key Concept: Cyclone Impact

c) Earthquakes due to wind pressure
[Solution Description]
Cyclones cause destruction through strong winds, storm surges, and heavy rains, but they do not typically trigger earthquakes.

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 Formation

b) Warm ocean waters
[Solution Description]
Cyclones derive their energy from warm ocean waters. The heat and moisture from the ocean surface fuel the storm, causing air to rise and form low-pressure systems.

Key Concept: Cyclone formation and Earth's rotation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Coriolis effect caused by Earth's rotation deflects moving air, creating the spinning motion characteristic of cyclones. If Earth stopped rotating, this deflection would not occur, preventing the development of the cyclonic spin. Therefore, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Cyclone Formation

c) Heating of ocean water leading to rising warm, moist air
[Solution Description]
Warm ocean water heats the air above it, making the air warm and moist. As this warm and moist air rises, the water vapor condenses into raindrops, releasing heat back into the atmosphere. This further warms the ascending air, causing it to rise even more. The process creates a low-pressure area, drawing in surrounding air.

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 form over warm ocean waters, which is true as per the syllabus. The reason explains that warm ocean waters cause the air above them to rise, creating low pressure and leading to cyclone formation, 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: Eye of Cyclone

c) It is characterized by calm winds but surrounded by strong winds
[Solution Description]
The eye of a cyclone is the central region where the pressure is the lowest. Here, winds are calm, but the surrounding areas experience high-speed winds and heavy rainfall. This happens because the air descends in the eye, creating a relatively clear and calm zone.

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: Eye of the Cyclone

b) Low pressure with calm winds
[Solution Description]
The eye of a cyclone is the central region with the lowest pressure, characterized by calm winds and clear skies. This is because the air in the eye descends slightly, inhibiting cloud formation.

Key Concept: Structure of a cyclone: Eye of the cyclone

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the eye of a cyclone is a region of low atmospheric pressure, which is true as per the syllabus. The reason explains that the calm wind in the eye is due to descending air that warms up and suppresses cloud formation, which is also correct and provides the explanation for why the eye has calm winds. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

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: 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 Agriculture

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that cyclones cause long-term agricultural damage due to seawater contamination, which is true as cyclones often bring storm surges that flood coastal regions. The reason states that salt from seawater reduces soil fertility, which is also true and directly explains why the assertion is correct since saline water adversely affects crops and soil quality.

Key Concept: Cyclone Characteristics

b) Eye
[Solution Description]
The eye of the cyclone is the region of lowest pressure. It is characterized by calm winds, while the surrounding areas experience high-speed winds and heavy rainfall.

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: Agricultural impact, soil salinity

c) 80 cm
[Solution Description]
To solve:
1. Current salinity = 12 dS/m
2. Target salinity = 6 dS/m
3. Required reduction = 50%
4. Leaching requirement = $\frac{EC_i}{EC_d \times LF}$ where LF = leaching fraction
5. With 25\% efficiency, need to apply 4 times the theoretical amount
6. Root zone depth = 40 cm
7. Theoretical requirement = 40 cm × (50% reduction) = 20 cm
8. Actual requirement = 20 cm × 4 = 80 cm

Key Concept: Role of satellites, IMD warnings

d) Satellites provide continuous, wide-area coverage essential for tracking cyclones over oceans.
[Solution Description]
Satellites provide real-time data on atmospheric conditions over vast oceanic areas where ground-based weather stations cannot be installed. They monitor cloud patterns, wind speeds, and sea surface temperatures, enabling accurate prediction of cyclone formation and movement. The India Meteorological Department (IMD) uses this satellite data to issue timely warnings.

Key Concept: Measures to monitor and predict cyclones

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that weather satellites are essential for accurately predicting the path of a cyclone, which is true because they help in tracking the movement and intensity of cyclones. The reason given is that satellites provide real-time data on atmospheric conditions, which is also true since this data is crucial for monitoring cyclones. Moreover, the reason correctly explains why satellites are essential for prediction, as their data allows meteorologists to make accurate forecasts. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

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.