Key Concept: Pressure and Area

d) 40 $N/m^2$
[Solution Description]
Pressure is calculated using the formula:
$Pressure = \frac{Force}{Area} = \frac{200\,N}{5\,m^2} = 40\,N/m^2$

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: Application of pressure

d) The pointed end decreases the area, increasing pressure
[Solution Description]
The pointed end of a nail has a smaller surface area compared to the blunt end. Since pressure is inversely proportional to area ($Pressure = \frac{Force}{Area}$), the smaller area results in higher pressure, making it easier to penetrate surfaces.

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: Pressure Calculation

c) 40 Pa
[Solution Description]
Pressure is calculated using the formula $P = \frac{F}{A}$. Here, $F = 200\,N$ and $A = 5\,m^2$. Substituting the values, we get:
$P = \frac{200\,N}{5\,m^2} = 40\,N/m^2 = 40\,Pa$
Thus, the pressure is 40 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, Force and Area

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 higher pressure from a smaller contact area. This is true because pressure ($P$) is given by $P = \frac{F}{A}$, where $F$ is force and $A$ is area. For a sharp knife, $A$ is smaller, resulting in higher $P$. The reason correctly explains this relationship as pressure indeed varies inversely with area for a constant force. Thus, both are true, and the reason explains the assertion correctly.

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: Pressure in Tools

c) The pointed end has a smaller area, increasing pressure.
[Solution Description]
The pointed end of a nail has a smaller contact area compared to the flat end. According to $Pressure = \frac{Force}{Area}$, a smaller area results in higher pressure for the same applied force, making it easier to penetrate the wood.

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, Atmospheric Pressure, Force and Area Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the activity, when the paper plate is covered with an unfolded chart paper, the area exposed to atmospheric pressure increases compared to when it is folded. Since pressure ($P$) is defined as force ($F$) per unit area ($A$), i.e., $P = \frac{F}{A}$, an increase in area for the same atmospheric pressure results in a greater total force exerted on the plate. The Assertion correctly states that more effort is needed to lift the unfolded sheet due to increased area. The Reason correctly explains this using the relationship between pressure, force, and area. Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Pressure units, conversion

c) 1,020 mb
[Solution Description]
Both hectopascal (hPa) and millibar (mb) are equal in magnitude: $1 \text{ hPa} = 1 \text{ mb}$.
Therefore, $1,020 \text{ hPa} = 1,020 \text{ mb}$.

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: 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: 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: Directional pressure, Real-world application

c) Air moves from high-pressure to low-pressure regions
[Solution Description]
1) Air inside the balloon is at higher pressure than outside (atmospheric pressure).
2) According to the key concept, air moves from high-pressure to low-pressure regions.
3) The open mouth creates a path for air to escape until pressures equalize.
4) This demonstrates that air exerts pressure in all directions, including outward on the balloon walls.

Key Concept: Force calculation, Pressure-area relationship

c) 6 N
[Solution Description]
1) Given: Force $F_1 = 12$ N for area $A_1 = 0.392$ m$^2$.
2) New area $A_2 = A_1/2 = 0.196$ m$^2$.
3) Since pressure $P = F/A$ is constant (atmospheric pressure), $P = \frac{F_1}{A_1} = \frac{F_2}{A_2}$.
4) Thus, $F_2 = \frac{F_1 × A_2}{A_1} = \frac{12 × 0.196}{0.392} = 6$ N.

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: Atmospheric Pressure Directionality

b) Air pressure inside the balloon acts uniformly in all directions
[Solution Description]
Air inside the balloon exerts pressure uniformly in all directions. This property of gases ensures that the pressure is not limited to one direction, causing the balloon walls to expand outward equally in all directions until internal pressure balances external forces.

Key Concept: Pressure gradient application

d) Air would flow until pressures equalize, with the balloon shrinking noticeably.
[Solution Description]
The inflated balloon's high-pressure air would flow into the rigid container (which initially has lower pressure) until pressures equalize. Since the container cannot expand, the balloon would shrink significantly while the container's pressure increases minimally due to its large volume. The final sizes wouldn't equalize unlike with two balloons.

Key Concept: Air Pressure Equilibrium

c) The inflated balloon decreases in size while the uninflated one increases until pressures equalize.
[Solution Description]
Initially, the inflated balloon has higher air pressure than the uninflated one. Air flows from the high-pressure region (inflated balloon) to the low-pressure region (uninflated balloon) through the straw. This transfer continues until the air pressure in both balloons becomes equal, resulting in both balloons attaining similar sizes.

Key Concept: Air pressure behavior, Balloon inflation dynamics

c) Air escapes until internal pressure equals external atmospheric pressure.
[Solution Description]
When the balloon's mouth is open, the high-pressure air inside escapes to the lower-pressure environment outside. The air continues to flow out until the pressure inside equals the atmospheric pressure outside, causing complete deflation. Without a closed system, equilibrium is achieved only when internal pressure matches external pressure.

Key Concept: Pressure Exerted by Air

b) The balloon inflates because the air inside exerts pressure on its walls.
[Solution Description]
When air is blown into an uninflated balloon, the air exerts pressure on the inner walls of the balloon in all directions. This causes the balloon to expand uniformly as the rubber stretches to accommodate the increased pressure inside. The expansion continues until the internal air pressure balances with the elastic forces of the rubber.

Key Concept: Units of pressure, Real-world application

d) 100,000 Pa
[Solution Description]
First, convert mass to force: $F = mg = 225 \, \text{kg} \times 10 \, \text{m/s}^2 = 2250 \, \text{N}$. Then, convert area to square meters: $225 \, \text{cm}^2 = 0.0225 \, \text{m}^2$. Pressure $P = F/A = 2250 / 0.0225 = 100,\!000 \, \text{Pa}$ (or $1000 \, \text{hPa}$).

Key Concept: Force and Area

d) The increased area experiences greater atmospheric pressure force.
[Solution Description]
The unfolded chart paper has a larger surface area exposed to atmospheric pressure compared to the folded one. Since pressure is force per unit area ($P = \frac{F}{A}$), a larger area results in a greater total force exerted by atmospheric pressure on the paper plate. Hence, more effort is needed to lift it.

Key Concept: Pressure and Area

b) 10 $N/m^2$
[Solution Description]
The pressure exerted can be calculated using the formula: $P = \frac{F}{A}$. Here, $F = 50\;N$ and $A = 5\;m^2$. Substituting these values into the formula gives:
$P = \frac{50}{5} = 10\;N/m^2$
Therefore, the pressure exerted is 10 $N/m^2$.

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: 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: Formation of Wind, Pressure Difference, Land and Sea Breeze

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
During the day, land gets heated faster than water, causing the air above it to warm up and rise, creating a low-pressure area. The sea remains cooler, maintaining higher pressure. According to the principle of pressure difference, air flows from the high-pressure region (sea) to the low-pressure region (land), resulting in a sea breeze. The reason correctly explains the assertion because it highlights the fundamental principle that governs the movement of air, which is the pressure gradient.

Key Concept: High-speed winds and air pressure

c) High-speed winds reduce air pressure above the roof
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure below the roof remains relatively higher, causing an upward force. If the pressure difference is large enough and the roof is weak, it may be blown away.

Key Concept: Sea breeze and land breeze

c) Because the air above the land rises, creating a low-pressure region
[Solution Description]
During the day, land heats up faster than water, causing the air above the land to become warmer and rise, creating a low-pressure region. Cooler air from the sea (high-pressure region) moves towards the land to fill this gap, resulting in a sea breeze.

Key Concept: Basic Concept

b) Difference in air pressure
[Solution Description]
Wind is caused by the movement of air from a high pressure region to a low pressure region. This happens because air naturally moves to balance the pressure differences between two regions.

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: 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: 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: Sea breeze and land breeze

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

Key Concept: Formation of Wind - High and Low Pressure Regions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion correctly states that air moves from a high-pressure region to a low-pressure region, which is the fundamental principle behind wind formation. The Reason explains why this happens: warm air rising creates low pressure, and cool air sinking creates high pressure, leading to the movement of air from high to low pressure. Thus, the Reason correctly explains the Assertion.

Key Concept: Pressure difference and wind speed

c) Wind speed increases
[Solution Description]
The speed of the wind is directly proportional to the pressure difference between two regions. A higher pressure difference results in a higher wind speed as air moves more rapidly from the high-pressure region to the low-pressure region.

Key Concept: Warm air rising and cool air sinking

d) Warm air rises over the land, creating a low-pressure zone that draws in cooler air from the sea.
[Solution Description]
On a typical sunny day, the land heats up faster than the sea. This causes the air above the land to become warmer and rise, creating a low-pressure area. To balance this, cooler air from the higher pressure region over the sea moves towards the land, forming what is known as a sea breeze.

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: 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 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: 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 Result in Lowering of Air Pressure, Bernoulli's Principle

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that keeping doors and windows closed during a storm increases the likelihood of roofs blowing off. This is true because when high-speed winds pass over the roof, they create a low-pressure area above it (Bernoulli's principle). If the house remains airtight (doors/windows closed), the higher air pressure inside the house exerts an upward force on the roof, potentially causing it to blow off. The reason correctly explains this phenomenon using Bernoulli's principle, which relates high-speed winds ($v$) and reduced pressure ($P$): $P + \frac{1}{2} \rho v^2 = \text{constant}$. Thus, both assertion and reason are true, and the reason is the correct explanation for the assertion.

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

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: 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: High-speed winds and air pressure

b) Air pressure decreases
[Solution Description]
When high-speed winds blow over a surface, they create a region of lower air pressure because the fast-moving air reduces the pressure in that area. This is due to Bernoulli's principle, which states that increasing the speed of a fluid decreases its pressure.

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, pressure difference

c) 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 due to Bernoulli's principle. If windows and doors are kept closed, the higher air pressure inside the house pushes upwards on the roof while the lower pressure outside pulls the roof upwards. This large pressure difference can cause the roof to blow off. Keeping windows and doors open allows wind to pass through the house, equalizing the pressure inside and outside, thus reducing the risk of roof damage.

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: Air Pressure, Speed-Pressure Relationship

c) The distance will decrease less in the second case.
[Solution Description]
According to Bernoulli's principle, lower airspeed results in higher pressure between the balloons. Reduced blowing speed means less pressure difference is created, so the balloons will move towards each other slower.
For 20 m/s: Distance decreases significantly as balloons accelerate faster due to high pressure difference.
For 10 m/s: Distance decreases at half the rate since pressure difference is smaller.
After 2 seconds, the distance covered in the second case will be less than in the first case, meaning the balloons will be farther apart compared to the first scenario.

Key Concept: Observation of Balloons

c) The balloons move towards each other.
[Solution Description]
When air is blown between the balloons, a low-pressure area is created between them. The higher air pressure from the surrounding areas pushes the balloons towards each other. This demonstrates that high-speed winds result in lower air pressure.

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: High-Speed Winds Result in Lowering of 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 balloons, they move towards each other, which aligns with the activity observations. The reason explains that this movement occurs due to the creation of a low-pressure zone between the balloons and higher surrounding pressure pushing them together. Both statements are true, and the reason correctly explains the assertion based on the principle that high-speed winds (blowing air) lower air pressure between the balloons, leading to their movement.

Key Concept: Pressure difference, Roof stability

b) 250,000 N
[Solution Description]
Given:
- Pressure below roof ($P_{\text{below}}$) = $1.01 \times 10^5$ Pa
- Pressure difference ($\Delta P$) = 5000 Pa
- Roof area (A) = 50 $m^2$
The net upward force (F) can be calculated using the formula: $F = \Delta P \times A$
Step 1: Plug in the values
$F = 5000 \times 50 = 250000 N$
Therefore, the upward force is 250,000 N.

Key Concept: Pressure equalization, Structural safety

b) It equalizes air pressure inside and outside the house
[Solution Description]
According to the Bernoulli's principle and the syllabus content:
1) High-speed winds create low pressure above the roof
2) With windows closed, higher pressure inside pushes up on roof
3) When windows are open, wind flows through house reducing inside pressure
4) This minimizes pressure difference across roof structure
5) Reduced pressure difference means less upward force on roof
Therefore, it helps equalize pressures and prevent structural damage.

Key Concept: Activity Observation

c) The balloons move towards each other faster.
[Solution Description]
Blowing harder increases the speed of air between the balloons, creating an even lower pressure area between them. The higher surrounding air pressure pushes the balloons closer together at a faster rate.

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: 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 formation, Charge separation

d) Strong winds rubbing against each other
[Solution Description]
The charge separation in thunderclouds is caused by strong winds rubbing against each other, creating static electric charges. Positively charged ice particles move upwards while negatively charged water droplets remain in the lower part of the cloud. This separation builds up until a discharge (lightning) occurs.

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 during a storm, warm air rises and creates a low-pressure area, which is correct as per the syllabus. The reason explains that cooler air rushes in to replace the rising warm air, leading to continuous wind circulation, which is also correct and directly explains the assertion. Therefore, both are true, and the reason correctly explains the assertion.

Key Concept: Thunderstorm conditions, Safety measures

c) Standing near a tall tree in an open field
[Solution Description]
Standing near a tall tree is unsafe because lightning tends to strike the tallest object in an area. Even if crouching in an open area is advised, proximity to tall objects increases the risk of being struck.

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: 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: 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: 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: Charge Separation in Thunderstorms

b) Due to friction between water droplets and ice particles caused by strong winds.
[Solution Description]
Strong winds rub water droplets and ice particles against each other, causing static electric charges to develop. Positively charged lighter ice particles move upwards while negatively charged heavier water droplets settle in the lower part of the cloud, leading to charge separation.

Key Concept: Lightning Formation

d) Breakdown of air insulation due to excessive charge buildup
[Solution Description]
When the buildup of opposite charges (positive on the ground/objects and negative in the cloud's lower part) becomes too large, the insulating property of air breaks down. A sudden flow of charges occurs, producing lightning.

Key Concept: Localized Thunderstorms

c) They supply necessary rain before monsoon for kharif crops.
[Solution Description]
Localized thunderstorms such as Kalboishakhi occur before the monsoon and provide essential rainfall that supports the growth of kharif crops and aids in ripening fruits like mangoes, benefiting agriculture.

Key Concept: Static charge separation, Lightning mechanism

c) 10$^5$ V/m
[Solution Description]
The electric field strength ($E$) is calculated using the formula $E = \frac{V}{d}$, where $V$ is the potential difference and $d$ is the distance. Given $V = 10^8$ V and $d = 1$ km = $1000$ m, substitute the values:
$E = \frac{10^8}{1000} = 10^5 \text{ V/m}$

Key Concept: Lightning formation

b) When static charges accumulate sufficiently to break down the insulating property of air
[Solution Description]
Lightning occurs when the build-up of opposite charges becomes large enough to overcome the insulating property of air, causing a sudden discharge of electricity.

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: 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 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: 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: 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: 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: Eye of the Cyclone

b) Descending air in the eye suppresses wind activity.
[Solution Description]
The eye of the cyclone is the region of lowest pressure at the center where the ascending air stops rising and begins descending. This descending motion suppresses cloud formation and wind activity, resulting in calm conditions. Meanwhile, the surrounding regions experience strong winds and heavy rainfall due to the cyclonic circulation.

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 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: 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: Energy release, Cyclone formation

c) It provides energy to sustain and intensify the cyclone.
[Solution Description]
When water vapor condenses into raindrops, latent heat is released into the atmosphere. This additional heat warms the surrounding air, causing it to rise further, reducing surface pressure, and intensifying the cyclone by drawing more air inward, thus sustaining the storm's energy.

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: Impact of Cyclones

d) Gradual weakening due to loss of moisture source
[Solution Description]
When a cyclone reaches land, its source of moist air from the ocean is cut off, causing it to weaken gradually. However, before weakening, cyclones can cause severe destruction, including storm surges flooding coastal areas, heavy rainfall leading to landslides, and contamination of freshwater sources due to saltwater intrusion.

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, 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: Destruction Caused by Cyclones

c) Storm surges flooding coastal regions
[Solution Description]
Cyclones generate storm surges—walls of water pushed toward the shore by high-speed winds—which can flood coastal areas, damage infrastructure, and contaminate freshwater sources with saltwater.

Key Concept: Formation of Cyclones

b) Condensation of water vapor releasing heat
[Solution Description]
Cyclones form when warm, moist air rises from the ocean surface, causing water vapor to condense into raindrops and release latent heat. This further warms the surrounding air, creating a low-pressure system and initiating cyclonic circulation due to Earth's rotation.

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: 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: 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: 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

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: 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: Eye of the cyclone

b) Low pressure and calm winds
[Solution Description]
The eye of a cyclone is the region of lowest pressure at the center, where winds are calm, while the surrounding regions experience strong winds and heavy rainfall.

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

b) Weather monitoring satellites
[Solution Description]
Weather monitoring satellites provide real-time data on cloud patterns, wind speeds, and pressure systems, enabling accurate tracking and prediction of cyclone paths. Doppler radar and weather stations also contribute, but satellites offer the most comprehensive coverage.