Key Concept: Pressure, Force, Area

d) The pressure increases fivefold
[Solution Description]
The pressure exerted by an object is given by $Pressure = \frac{Force}{Area}$. Initially, the area in contact with the table is 2 m × 1 m = 2 $m^2$. So, the initial pressure is $\frac{100 N}{2 m^2} = 50 Pa$. When the block is rotated to stand on its shortest side (1 m), the new contact area becomes 2 m × 0.1 m = 0.2 $m^2$ (assuming thickness is 0.1 m). The new pressure is $\frac{100 N}{0.2 m^2} = 500 Pa$. Thus, the pressure increases.

Key Concept: Atmospheric Pressure

c) Millibar (\$mb)
[Solution Description]
Atmospheric pressure is measured in various units, but one common practical unit is millibar (mb), which is equal to 100 pascals (Pa). Another unit is hectopascal (hPa), also equivalent to 100 Pa. Both are widely used in meteorology.

Key Concept: Pressure Comparison

a) Box A
[Solution Description]
Pressure is inversely proportional to area when force is constant. Since $P = \frac{F}{A}$, Box A with smaller area \(2\,\mathrm{m}^2\) will exert greater pressure than Box B \(4\ \mathrm{m}^2\).
Conclusion: Box A exerts more pressure.

Key Concept: Comparative pressure analysis, multi-step reasoning

c) 5 times
[Solution Description]
Calculate pressures individually.
Flat brick:
$P_{flat} = \frac{10}{100 \times 10^{-4}} = 1000 \text{ Pa}$
Vertical brick:
$P_{vertical} = \frac{10}{20 \times 10^{-4}} = 5000 \text{ Pa}$
The ratio of pressures is:
$\frac{P_{vertical}}{P_{flat}} = \frac{5000}{1000} = 5$
Thus, the vertical brick exerts 5 times more pressure.

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 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 is pascal (Pa), which is correct as per the syllabus. The reason explains that pressure is defined as force per unit area, with SI units of newton (N) for force and metre$^2$ ($m^2$) for area. This is also correct, and it correctly justifies why the unit of pressure is Pa, since $1\,Pa = 1\,N/m^2$. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Pressure and Area Relationship

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 because it exerts more pressure. This is true since pressure ($P$) is calculated as $P = \frac{F}{A}$, where $F$ is force and $A$ is area. For the same force, reducing the area (sharp edge) increases pressure. The reason correctly explains the inverse relationship between pressure and area when force is constant. Thus, both assertion and reason are true, and the reason correctly explains the assertion.

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, Cutting Tools

b) Sharp edge exerts 10 times more pressure
[Solution Description]
Pressure is calculated as $P = \frac{F}{A}$. For the sharp edge ($A_1 = 0.01\,mm^2$), $P_1 = \frac{F}{0.01}$ = $100F$ (in $N/mm^2$). For the blunt edge ($A_2 = 0.1\,mm^2$), $P_2 = \frac{F}{0.1}$ = $10F$. Thus, $P_1 = 10P_2$, meaning the sharp edge exerts 10 times more pressure.

Key Concept: Pressure and Area Relationship

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 the force being applied over a smaller area, which is true as per the syllabus examples (cutting with sharp knives). The reason correctly explains this by stating that pressure ($P = \frac{F}{A}$) is inversely proportional to the area - smaller area increases pressure, making cutting easier. Thus, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Atmospheric Pressure and Suction Cup

d) The higher external atmospheric pressure pushes the sucker onto the surface.
[Solution Description]
When the sucker is pressed, most of the air inside is pushed out, reducing the internal pressure. The external atmospheric pressure ($P_{atm}$) becomes higher than the pressure inside ($P_{in}$). This difference in pressure ($P_{atm} - P_{in}$) creates a net inward force that holds the sucker firmly against the surface.

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: 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: Sucker Experiment Analysis

c) External air pressure exceeds the reduced pressure inside the sucker.
[Solution Description]
When the sucker is pressed, air inside it is expelled, reducing internal pressure. The external atmospheric pressure ($P_{\text{atm}}$) becomes higher than the internal pressure, creating a net inward force ($F = P_{\text{atm}} \times A$). This pressure difference makes it difficult to pull the sucker off.

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: 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: SI Unit of Pressure

c) Pascal (Pa)
[Solution Description]
The SI unit of pressure is newton per square meter ($N/m^2$), also called pascal (Pa). Other units like millibar (mb) or hectopascal (hPa) are derived from it, where 1 mb = 100 Pa and 1 hPa = 100 Pa. The question tests recall of the fundamental unit.

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: 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: 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: 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

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: Pressure equilibrium

d) The air pressure in both balloons becomes equal.
[Solution Description]
The air flow stops when the pressure inside both balloons becomes equal. Initially, air flows from the inflated (high-pressure) balloon to the uninflated (low-pressure) balloon. Once the pressures balance, there is no further net movement of air, and both balloons reach nearly the same size.

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: 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

b) Narrow straps concentrate pressure on a smaller area
[Solution Description]
Broad straps increase the area over which the weight of the bag is distributed. Since pressure is defined as force per unit area ($P = \frac{F}{A}$), increasing the area reduces the pressure exerted on the shoulders. Therefore, carrying a bag with broad straps feels more comfortable because the pressure is spread out over a larger area.

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: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that during the day, wind blows from the sea to the land because of the formation of a low-pressure region over the land. This is true as the land heats up faster than water, causing the air above it to warm up, rise, and create a low-pressure area. The Reason explains the cause of this phenomenon by stating that land gets heated faster, leading to the rising of air and creation of a low-pressure area. Since the Reason correctly explains the Assertion, both are true and logically connected.

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: 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: High-speed winds, Air pressure

d) Low pressure above the roof creates an upward force.
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure below the roof (inside the house) remains higher. This pressure difference exerts an upward force on the roof. If the roof is not strong enough to withstand this force, it gets blown away.

Key Concept: Bernoulli's principle, Wind speed

c) 57.7 m/s
[Solution Description]
Using Bernoulli's equation, the pressure difference $\Delta P = \frac{1}{2} \rho v^2$. Convert $\Delta P$ to pascals: $1000 \text{ mb} - 980 \text{ mb} = 20 \text{ mb} = 2000 \text{ Pa}$. Rearranging for $v$: $v = \sqrt{\frac{2 \Delta P}{\rho}} = \sqrt{\frac{4000}{1.2}} \approx 57.7 \text{ m/s}$.

Key Concept: Pressure difference and wind formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that winds form due to the movement of air from high-pressure to low-pressure regions, which is correct as seen in activities like balloon experiments and sea/land breezes. The Reason states that wind speed increases with greater pressure differences, which is also true and explains why stronger winds occur in cases of larger pressure differentials, such as during storms or cyclones. Therefore, both the 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]
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: Land breeze formation

d) Cooler air sinks over the land, creating a high-pressure zone that pushes air towards the sea.
[Solution Description]
At night, the land cools down faster than the sea. As a result, the air above the land becomes cooler and denser, leading to a high-pressure area over the land. Meanwhile, the sea retains heat longer, keeping its surface warmer. The cooler air from the high-pressure land area moves towards the lower pressure over the sea, creating a land breeze.

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 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: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
During the day, the land heats up faster than the sea due to differences in specific heat capacity. This causes the air above the land to warm up and rise, creating a low-pressure region. Cooler air from the higher-pressure region over the sea moves toward the low-pressure region over the land, resulting in a sea breeze. Thus, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

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, Land Breeze, Wind Speed

b) Sea breeze winds are stronger than land breeze winds
[Solution Description]
Wind speed is proportional to the pressure difference. Since the sea breeze has a higher pressure difference (3 hPa) compared to the land breeze (2 hPa), the wind speed during the sea breeze will be stronger. This aligns with the relationship $\text{Wind speed} \propto \text{Pressure difference}$.

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 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: 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]
According to the syllabus, high-speed winds blowing over a surface create a low-pressure area because the fast-moving air reduces the number of air molecules applying force per unit area. This explains why roofs may be blown off during storms when high-speed winds lower the pressure outside while higher pressure inside pushes upward. Therefore, the Assertion is true because high-speed winds indeed reduce air pressure, and the Reason correctly explains this phenomenon by describing how fast-moving air leads to fewer air molecules exerting 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: 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 and storm damage

b) It balances indoor and outdoor air pressure
[Solution Description]
During a storm, high-speed winds over the roof create low pressure outside. If windows are closed, the higher pressure inside the house can push the roof upward, causing damage. Opening windows equalizes the pressure inside and outside, reducing the risk of roof damage.

Key Concept: Practical Example of Low Pressure

c) The balloons move towards each other.
[Solution Description]
Blowing air between two balloons increases the wind speed in that region, creating a low-pressure area between them. The surrounding higher-pressure air pushes the balloons inward, causing them to come closer together. This illustrates how high-speed winds reduce 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: 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: 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: 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 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 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 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: 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: 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: 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: 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 Names

c) Kalboishakhi
[Solution Description]
In West Bengal, Bihar, and Jharkhand, local thunderstorms occurring before the monsoon are called Kalboishakhi. These storms help kharif crops grow by providing rain.

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: 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: 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: Cyclone formation, Pressure systems, Eye wall dynamics

c) Descending air in the eye inhibits cloud formation and wind activity.
[Solution Description]
The eye forms due to descending air, which suppresses cloud formation and reduces wind speed, creating a low-pressure center. Surrounding the eye is the eyewall, where rising warm moist air leads to intense thunderstorms and high-speed winds.

Key Concept: Formation of Storms

d) Low pressure above the roof and high pressure below it
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the air pressure inside the house remains higher. The large pressure difference can cause weak roofs to be blown away.

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 keeping windows and doors open during a storm reduces the pressure difference above and below the roof. This is true because allowing air to flow through the house balances the pressure inside and outside, preventing structural damage. The Reason explains that high-speed winds create low pressure above the roof, which is also correct. Furthermore, the Reason provides the correct explanation for why opening windows and doors helps, as it equalizes the pressure difference caused by high-speed winds. 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: 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 Clouds

c) Friction between water droplets and ice particles creates opposite charges
[Solution Description]
Strong upward and downward winds cause friction between water droplets and ice particles, leading to static electric charges. Positively charged lighter ice particles move to the upper part of the cloud, while negatively charged heavier water droplets occupy the lower part. This results in charge separation.

Key Concept: Role of Winds, Moisture, and Temperature

a) Warm air has lower pressure than cold air.
[Solution Description]
Warm air rises because it is less dense than cooler air. When land gets heated, the warm and moist air becomes lighter and creates a low-pressure area, causing it to rise. This rising motion sets up the wind circulation necessary for thunderstorm development.

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 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 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 precautions during lightning

d) Crouching in a low-lying open area
[Solution Description]
During a thunderstorm, crouching in a low-lying open area minimizes the risk of being struck by lightning as it reduces exposure and avoids tall objects that attract lightning.

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: 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: 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: Thunder Formation

b) Rapid heating and expansion of air caused by lightning
[Solution Description]
Thunder is produced when lightning rapidly heats the surrounding air, causing it to expand explosively. This expansion creates a shockwave that manifests as a loud sound known as thunder.

Key Concept: Cyclone Formation

c) Heating of ocean water causes warm air to rise and form low-pressure areas.
[Solution Description]
The primary cause of a cyclone's formation is the heating of ocean water, which leads to the rise of warm and moist air. As this air rises, water vapour condenses into raindrops, releasing heat that further warms the air and lowers atmospheric pressure. This cycle continues, drawing in more air from surrounding areas, which also rises due to Earth's rotation, creating a spinning system of winds and clouds.

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 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: Cyclone Formation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, cyclones indeed form over warm ocean waters where warm, moist air rises, creating a low-pressure area. This is correctly explained by the fact that Earth's rotation (Coriolis effect) causes the moving air to spin, forming a cyclone. Both statements are true, and the reason accurately explains the assertion.

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: 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: 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: 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: 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: 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: 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: Effects of Cyclones

c) Flooding caused by storm surges pushing seawater inland
[Solution Description]
Cyclones cause destructive effects such as storm surges, which push seawater inland, leading to flooding. Other impacts include heavy rainfall, landslides, blocked roads due to debris, power outages, and contamination of drinking water sources.

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: 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: 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: Agricultural Impact

c) Makes soil less fertile
[Solution Description]
Seawater contains high levels of salt, which makes the soil less fertile when it intrudes into farmland. This reduces crop yields and negatively impacts agriculture.

Key Concept: Collaborative efforts, technology

d) Combined resources and shared data improve tracking accuracy and warning systems.
[Solution Description]
International collaborations pool resources, expertise, and global satellite data to improve cyclone tracking accuracy. For example, sharing meteorological data from multiple countries helps in better modeling cyclone paths and issuing early warnings, reducing response time and increasing preparedness.

Key Concept: Emergency preparedness, IMD alerts

c) Stay updated with IMD alerts, prepare emergency kits, and move to shelters when advised.
[Solution Description]
The first step is to stay updated with IMD warnings. Next, the community should ensure emergency kits are ready with essentials like food, water, and medicines. Finally, during the cyclone, they must move to designated shelters for safety. These steps align with the syllabus guidelines for cyclone preparedness.

Key Concept: Cyclone preparedness

a) Emergency kit with essential items
[Solution Description]
People in cyclone-prone areas must keep an emergency kit with essential items ready to quickly move to a designated cyclone shelter when needed.

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.