Class 8 Science Chapter 6 Pressure, Winds, Storms, And Cyclones (New Course)

Key Concept: Liquid Pressure

c) Because the pressure exerted by the water column is the same
[Solution Description]
The extent of the bulge in the balloons depends on the pressure exerted by the water column, not the volume or weight of the water. Since the water columns in both pipes are of the same height, they exert the same pressure on the balloons, causing them to bulge equally regardless of the pipe diameter.

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 bag with broad straps exerts less pressure on the shoulders than a bag with narrow straps when the weights are equal. This is correct because pressure is calculated as $Pressure = \frac{Force}{Area}$. For the same force (weight of the bag), increasing the area (broad straps) reduces the pressure. Therefore, the assertion is true.
The reason states that pressure is inversely proportional to the area over which the force is applied. This is also correct because the formula for pressure shows that as area increases, pressure decreases for a constant force. Moreover, this correctly explains why broad straps reduce pressure. Hence, the reason is true and correctly explains the assertion.

Key Concept: Definition of Pressure

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion (A) claims that a sharp knife exerts less pressure on vegetables compared to a blunt knife, making cutting easier. However, this is incorrect because according to the formula $Pressure = \frac{Force}{Area}$, reducing the area (as with a sharp knife) increases pressure, not decreases it. Therefore, the assertion is false.
The reason (R) states correctly that the area of contact is smaller for a sharp knife, leading to higher pressure. This explanation aligns with the definition of pressure but contradicts the assertion. Hence, the reason is true.
Combining these observations:
1. Assertion (A) is false.
2. Reason (R) is true.

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: 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: Practical Unit of Air Pressure

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

Key Concept: Formula for Pressure

b) \(50\ \mathrm{N/m^2}\)
[Solution Description]
Pressure tells us how much force is acting on each unit area. Here, a force of 200 N is spread over an area of 4 square metres. To find the pressure, divide the force by the area. So, each square metre receives 50 N of force. Therefore, the pressure exerted is 50 pascals (Pa).

Key Concept: Pressure, Force, Area

c) It increases by a factor of 5
[Solution Description]
Initial area of contact = $10 \text{ cm} \times 5 \text{ cm} = 50 \text{ cm}^2$.
Pressure initially = $\frac{\text{Force}}{\text{Area}} = \frac{50 \text{ N}}{50 \text{ cm}^2} = 1 \text{ N/cm}^2$.
When turned to smallest face, new area = $5 \text{ cm} \times 2 \text{ cm} = 10 \text{ cm}^2$ (assuming thickness is 2 cm).
New pressure = $\frac{50 \text{ N}}{10 \text{ cm}^2} = 5 \text{ N/cm}^2$.
Therefore, the pressure increases by a factor of 5.

Key Concept: Pressure, Area Relationship

b) Pressure doubles
[Solution Description]
The pressure is given by $P = \frac{F}{A}$. When the strap width is halved (from 5 cm to 2.5 cm), the area of contact decreases since $A$ is proportional to width for a given length. Thus, pressure doubles because $P$ is inversely proportional to $A$.

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: Atmospheric Pressure, Force-Area Relationship

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

Key Concept: 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: 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: 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: Magnitude of Atmospheric Pressure

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

Key Concept: Atmospheric pressure, 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: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lifting the paper plate covered with folded chart paper requires less effort compared to unfolded chart paper. This is true because in the experiment, it was observed that the unfolded chart paper has a larger surface area exposed to air pressure, making it harder to lift. The reason explains that air exerts more pressure on a larger surface area, which is correct according to the observation and concept of atmospheric pressure. Therefore, both the assertion and reason 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: Force calculation, Pressure-area relationship

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

Key Concept: Air pressure, Surface area comparison

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

Key Concept: Effect of air pressure, Pressure equilibrium

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

Key Concept: 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 behavior, Balloon inflation dynamics

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

Key Concept: Pressure Exerted by Air

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

Key Concept: Air Pressure

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: Air Pressure’s Role in Daily Life

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a rubber sucker sticks to a smooth surface because of the difference in air pressure, which is correct. When the sucker is pressed against a smooth surface, most of the air inside is pushed out, reducing the internal pressure. The external atmospheric pressure, being higher, exerts a force that keeps the sucker firmly attached to the surface. The reason correctly explains why this happens: it describes the process of expelling air from inside the sucker, creating a low-pressure region, and how the higher external pressure maintains the attachment. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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: 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: High-speed winds and air pressure

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

Key Concept: Sea breeze and land breeze

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

Key Concept: Pressure difference, Wind formation

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

Key Concept: 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 Formation

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

Key Concept: Assertion-Reason

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

Key Concept: Units of Pressure

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

Key Concept: 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: 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: Role of warm air rising and cool air sinking

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

Key Concept: Sea breeze and land breeze

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

Key Concept: 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: Sea Breeze, Land Breeze, Pressure Differences

d) Assertion is false, but Reason is true.
[Solution Description]
Let’s analyze both Assertion (A) and Reason (R):
1. **Assertion (A):** “During the day, wind blows from the sea to the land because the land is cooler than the sea.”
– This is false. During the day, the land gets heated faster than the sea, making the air above the land warmer and lighter. This creates a low-pressure area over the land, causing wind (sea breeze) to blow from the high-pressure sea region to the low-pressure land region.
2. **Reason (R):** “At night, the land cools faster than the sea, creating a high-pressure area over the land.”
– This is true. At night, the land loses heat faster than the sea, making the air above it cooler and denser, which leads to high pressure over the land. As a result, wind (land breeze) blows from the land to the sea where the pressure is lower.
Since Assertion (A) is false but Reason (R) is true, the correct option is the one stating this relationship.

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: Pressure Differences

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

Key Concept: Sea and land breezes

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

Key Concept: High-Speed Winds Result in Lowering of Air Pressure, Bernoulli’s Principle

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

Key Concept: Bernoulli’s principle, 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 and Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that when high-speed winds blow over a house, the roof may get blown away due to the pressure difference. This is true because high-speed winds reduce the air pressure above the roof (as per Bernoulli’s principle), while the air pressure inside the house remains relatively higher. If this pressure difference is significant and the roof is not strong enough, it can be blown away. The reason correctly explains why the roof may get blown away: high-speed winds indeed create a low-pressure area above the roof, while the air pressure inside the house remains higher. Thus, the reason is the correct explanation for the assertion.

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: 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: 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: 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: 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: 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: Observation of Balloons

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

Key Concept: Bernoulli’s Principle, Air Pressure

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

Key Concept: Pressure Gradient, Force Calculation

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

Key Concept: 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: Pressure difference, Roof stability

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

Key Concept: Impact of Pressure Differences on Structures

b) It reduces the pressure difference between inside and outside.
[Solution Description]
Keeping doors and windows open allows the high-speed wind to move through the house, reducing the pressure difference between the inside and outside. This prevents the roofs from being blown off due to excessive pressure difference.

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: Formation of Thunderstorms

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

Key Concept: Lightning Formation, Thunderstorms

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lightning occurs when the charge buildup is sufficient to overcome air’s insulating property, which is accurate as per the syllabus. The reason explains the mechanism where the negatively charged lower part of the cloud induces a positive charge on the ground, creating a potential difference that results in lightning. Since the reason correctly explains the cause of the phenomenon described in the assertion, both are true and the reason is the correct explanation.

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: Formation of thunderstorms and lightning

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

Key Concept: Safety Measures During Cyclones

d) To equalize air pressure inside and outside the house
[Solution Description]
Keeping doors and windows open allows wind to flow through the house, reducing the pressure difference between the inside and outside. This minimizes the risk of the roof being blown away due to high pressure inside.

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: 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: 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: Charge Separation in Thunderstorms

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

Key Concept: 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: 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: Thunderstorm formation, Charge separation in clouds

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

Key Concept: Static charge development in clouds

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

Key Concept: Static charge development within clouds leading to lightning

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, strong winds cause rubbing between water droplets and ice particles in clouds, generating static electric charges. Lighter positively charged ice particles move upwards while heavier negatively charged water droplets occupy the lower part of the cloud, creating charge separation. When the charge buildup becomes large enough, it overcomes air’s insulating property, causing a sudden discharge of electricity (lightning). Thus, both Assertion (A) and Reason (R) are true, and Reason correctly explains the occurrence of lightning as described in the syllabus.

Key Concept: Static charge development, Lightning conductor

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The lightning conductor is designed to protect buildings by providing a low-resistance path for lightning to follow, safely directing the charge into the ground. The pointed end of the conductor helps ionize the air, reducing the insulating property of air and allowing easier discharge. Both the assertion and reason are true, and the reason correctly explains why the lightning conductor works as described in the assertion.

Key Concept: Lightning types, Charge distribution

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

Key Concept: 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 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: Lightning Conductor, Charge Transfer

c) It provides a path for charges to flow into the ground
[Solution Description]
A lightning conductor is a metallic rod with one end pointed and placed higher than the building’s highest point. The other end is buried deep in the ground. When lightning strikes, the conductor provides an easy path for the electric charges to flow directly into the ground, preventing damage to the building.

Key Concept: Charge separation, Lightning and Thunder

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

Key Concept: 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: Eye of the Cyclone

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

Key Concept: Cyclone impact, Storm surge

c) 1200 meters
[Solution Description]
First, calculate the effective rise in water level due to the storm surge: $8 \, \text{m} – 2 \, \text{m} = 6 \, \text{m}$.
The land slope is 0.5%, meaning for every 100 meters horizontally, the elevation changes by 0.5 meters. To find the horizontal distance $x$ where the elevation rises by 6 meters:
$0.005 = \frac{6}{x} \implies x = \frac{6}{0.005} = 1200 \, \text{meters}$

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: Impact of Cyclones

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

Key Concept: 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: Formation Process

c) Over warm ocean waters
[Solution Description]
Cyclones form over warm ocean waters due to differences in temperature and pressure. The text states: “Cyclones are large storms that form over warm ocean waters.”

Key Concept: Cyclone formation, Eye of the cyclone

b) The eye contains descending air that suppresses wind activity.
[Solution Description]
The eye of the cyclone is a region characterized by descending air. As warm, moist air rises in the surrounding eyewall, it cools and condenses, releasing latent heat, which further drives the upward motion. At the center (eye), the descending air creates a low-pressure area but suppresses cloud formation and wind activity, leading to calm conditions.

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: Heavy rainfall, Destructive Effects

c) Soil becomes less fertile due to increased salinity
[Solution Description]
Cyclone-induced floods often bring saline seawater inland. A salinity level of 10 ppt is harmful to crops as it makes the soil less fertile by increasing salt concentration. This reduces the soil’s ability to retain water and nutrients, negatively affecting crop yield.

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

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

Key Concept: Formation of Cyclones

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

Key Concept: Agricultural impact, soil salinity

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

Key Concept: Impact of cyclones on land

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that cyclones cause extensive flooding in coastal areas due to storm surges, which is true because cyclones generate strong winds that push seawater inland, leading to flooding. The reason explains that storm surges occur when strong winds push seawater towards the shore, forming a wall of water that floods coastal areas. Both the assertion and reason are correct, and the reason accurately explains why cyclones lead to flooding through storm surges.

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: 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: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the IMD issues alerts and warnings for cyclones, which is true as per the syllabus. The Reason states that weather monitoring satellites help track cyclones and predict their path, which is also true as mentioned in the syllabus. The Reason correctly explains why the IMD can issue alerts and warnings because satellite data is crucial for cyclone prediction and tracking. Hence, both Assertion and Reason are true, and the Reason is the correct explanation of the 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.

Key Concept: Cyclone Safety Measures

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

Key Concept: Measures to monitor 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.