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: Liquid Pressure, Pipe Diameter

c) The pressures are equal
[Solution Description]
Liquid pressure depends only on the height of the liquid column and its density, not on the cross-sectional area or shape of the container. Since both pipes are filled to the same height with water, the pressure at the bottom of both pipes is the same: $P = \rho g h$, where $\rho$ is density, $g$ is gravity, and $h$ is height.

Key Concept: Wind pressure conceptual complexity

a) 100 Pa
[Solution Description]
Original pressure:
$P_{original} = \frac{800}{8} = 100 \text{ Pa}$
New billboard area = 4 m$^2$. If force scales with area, new force:
$F_{new} = 100 \times 4 = 400 \text{ N}$
Pressure on new billboard remains the same because:
$P_{new} = \frac{400}{4} = 100 \text{ Pa}$
Thus, the pressure tolerance required is unchanged.

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 conversion, real-world application

b) 102,000 Pa, higher than 1 atm
[Solution Description]
1 hectopascal (hPa) = 100 Pa.
Given pressure = 1020 hPa = 1020 × 100 Pa = 102,000 Pa.
Standard atmospheric pressure = 101,325 Pa.
Comparison: 102,000 Pa is slightly higher than standard atmospheric pressure.

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 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, 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 and Area Relationship

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

Key Concept: Sucker Experiment, Pressure Difference

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

Key Concept: Units of Pressure

c) 101300 Pa
[Solution Description]
Since $1 \text{ mb} = 100 \text{ Pa}$, we convert millibars to pascals by multiplying by $100$. Therefore:
$1013 \text{ mb} = 1013 \times 100 \text{ Pa} = 101300 \text{ Pa}$

Key Concept: Atmospheric Pressure and Area

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

Key Concept: Atmospheric Pressure

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: Atmospheric Pressure, Pressure-Force Relationship

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that atmospheric pressure exerts a force of 2250 N over an area of 225 cm$^2$. From the given information, we know that the force exerted by the atmospheric air column over an area of 15 cm $\times$ 15 cm (225 cm$^2$) is nearly equal to 2250 N, which matches the Assertion. The Reason explains that the internal pressure in our bodies balances the external atmospheric pressure, preventing us from being crushed, which is also correct. However, the Reason does not explain why the Assertion is true (i.e., it does not justify the force calculation). Thus, both are true, but the Reason is not the correct explanation of the Assertion.

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: Air Pressure Experiment

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

Key Concept: 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: Pressure and Area Relationship

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

Key Concept: 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 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: 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: 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: 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: 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-area relationship, Atmospheric force

d) Pressure exerts more force over larger areas.
[Solution Description]
When unfolded, the chart paper has a larger surface area exposed to atmospheric pressure. Since pressure is force per unit area ($P = F/A$), a larger area means greater total force exerted by atmospheric pressure downward, requiring more effort to lift the plate. This demonstrates that atmospheric pressure acts over surfaces, and its effect scales with area.

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: 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: 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: 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: 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: 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, Pressure gradient

b) 0.083 m/s$^2$
[Solution Description]
The pressure gradient force per unit mass is calculated as $\frac{1}{\rho} \frac{\Delta P}{\Delta x}$. First, convert the pressure difference to pascals: $\Delta P = 1010 \text{ mb} - 990 \text{ mb} = 20 \text{ mb} = 2000 \text{ Pa}$. The distance $\Delta x = 20 \text{ km} = 20000 \text{ m}$. The force per unit mass is $\frac{1}{1.2} \times \frac{2000}{20000} = \frac{1}{12} \approx 0.0833 \text{ m/s}^2$.

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: 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: 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: Sea breeze, Land breeze

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

Key Concept: Pressure difference and wind speed

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

Key Concept: 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: 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

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: Pressure Differences, Sea Breeze

b) 1007 hPa
[Solution Description]
During the day, land heats up faster than the sea, creating a low-pressure area over the land. The cooler air over the sea has higher pressure, causing a sea breeze. Since the land pressure is given as 1005 hPa (low pressure), the sea pressure must be higher. Typically, pressure differences for sea breezes range between 2-4 hPa. Thus, the sea pressure is approximately 1007 hPa to 1009 hPa.

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, 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: Bernoulli's principle, Airflow dynamics

c) Low pressure forms between them due to fast airflow
[Solution Description]
When air is blown between the balloons, the speed of airflow increases, creating a low-pressure region between the balloons (as per Bernoulli's principle). The surrounding higher-pressure air exerts a force on the outer surfaces of the balloons, pushing them towards each other. Hence, the movement occurs due to the pressure difference caused by high-speed airflow.

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: 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: Low-pressure areas

c) Low pressure created by wind pulls them together
[Solution Description]
When air is blown between two balloons, it creates a low-pressure zone between them. The higher pressure outside the balloons pushes them inward, causing them to move closer. This demonstrates how high-speed winds reduce pressure in an area.

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: 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: 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: 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: 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: 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, pressure difference, structural impact

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

Key Concept: 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: 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: 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: 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: 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: 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 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

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: 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: 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 thunderstorms

c) Rubbing between water droplets and ice particles due to upward and downward winds
[Solution Description]
Strong winds blowing upwards and downwards cause rubbing between water droplets and ice particles, leading to static electric charge separation. The positively charged lighter ice particles move to the top of the cloud, while negatively charged heavier water droplets settle at the bottom.

Key Concept: Role of Warm Air in Thunderstorms

c) It leads to condensation and cloud formation
[Solution Description]
Warm and moist air rises because it is lighter, creating a low-pressure area. Cooler air from surrounding high-pressure areas moves in to replace it, forming wind circulation. As the rising air expands and cools, moisture condenses into water droplets, leading to cloud formation. Eventually, these droplets merge and fall as rain, hail, or snow.

Key Concept: Formation of Storms, Thunderstorms and Lightning

a) Warm air rises → Moisture condenses → Wind circulation → Charge separation → Lightning
[Solution Description]
First, warm moist air rises, creating a low-pressure area. Cooler air moves in to replace it, causing wind circulation. As the air rises, it cools, condenses into clouds, and forms water droplets. In thunderstorms, strong updrafts lift droplets to freezing heights, converting them to ice particles. Friction between these particles creates static charges, separating positive (top) and negative (bottom) charges in the cloud. When the charge difference becomes large enough, air's insulating property breaks down, causing a sudden discharge (lightning).

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: 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: 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: 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, Lightning Formation

c) Friction between water droplets and ice particles
[Solution Description]
The charge separation in clouds occurs due to the rubbing of ice particles and water droplets by strong upward and downward winds. The lighter ice particles gain a positive charge and move to the upper part of the cloud, while the heavier water droplets gain a negative charge and occupy the lower part. This creates a charge separation within the cloud.

Key Concept: Formation of Thunder

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

Key Concept: Thunderstorm Formation, Charge Build-Up

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

Key Concept: Formation of Lightning

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Lightning is caused by the movement of electric charges, as mentioned in the Assertion (A). The Reason (R) explains the process by which these charges are generated—through the rubbing of water droplets and ice particles in the clouds. Since the Reason correctly explains the Assertion, both are true and logically connected.

Key Concept: Cyclone dynamics, Coriolis effect

c) It would start rotating clockwise
[Solution Description]
Cyclones rotate counterclockwise in the Northern Hemisphere due to the Coriolis effect, which deflects moving air to the right. In the Southern Hemisphere, the Coriolis effect deflects moving air to the left, causing cyclones to rotate clockwise. Therefore, the cyclone's rotation would reverse direction upon crossing the equator.

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

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 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, Eye of 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 has calm winds and clear skies due to descending warm air. This is true because the sinking air in the eye inhibits cloud formation, leading to clear conditions. The Reason explains that descending air increases pressure and reduces moisture, suppressing clouds. Both statements are true, and the Reason correctly explains the Assertion as the descending air's warming effect prevents condensation and cloud formation.

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: Storm surge, Cyclone impact

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

Key Concept: Cyclone Formation

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

Key Concept: 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: Eye of the Cyclone, High-Speed Winds

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that the eye of a cyclone has high-speed winds and heavy rainfall. However, this is false because the eye is actually a calm region with clear skies and no rain. The surrounding region, called the eyewall, experiences the high-speed winds and heavy rainfall.
The Reason claims that the eye is formed due to rapid rotation of air around a low-pressure center, generating centrifugal force pushing air outward. This is true because the centrifugal force balances the pressure gradient force, creating the calm eye at the center.
Since the Assertion is false but the Reason is true, the correct choice is:

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: 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: Cyclone formation, storm surge impact

c) 3.0 meters
[Solution Description]
To solve this:
1. Storm surge height = 8 m
2. Coastal elevation = 5 m
3. Effective flood height = surge - elevation = 8 - 5 = 3 m
4. Water will spread inland with decreasing depth but for approximation in low-lying areas close to shore, we can consider similar depth.
5. Therefore, approximate floodwater depth would be 3 m.

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: 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: 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: 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 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

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.