Key Concept: Pressure and Area

c) The pressure increases
[Solution Description]
Pressure is defined as $Pressure = \frac{Force}{Area}$. If the force remains constant and the area decreases, the denominator in the equation becomes smaller, resulting in an increase in pressure. Therefore, pressure increases when the area decreases for a constant force.

Key Concept: Pressure and Area

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

Key Concept: Pressure, Force per Unit Area

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that a sharp needle exerts more pressure on the skin than a blunt needle for the same force. This is true because pressure ($P$) is calculated as $P = \frac{F}{A}$, where $F$ is the force and $A$ is the area. A sharp needle has a smaller contact area ($A$) compared to a blunt needle, so for the same force ($F$), the pressure increases as the area decreases.
The Reason states that pressure is inversely proportional to the area of contact when the force remains constant, which is also true. This directly explains why the sharp needle exerts more pressure—it follows from the definition of pressure.
Therefore, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Definition of pressure

c) Force per unit area
[Solution Description]
Pressure is defined as the force applied per unit area. The formula for pressure is given by:
$Pressure = \frac{Force}{Area}$
The SI unit of pressure is pascal (Pa), which is equivalent to N/m$^2$.

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: Multi-concept: Pressure calculation with unit conversion

b) 0.637 MPa
[Solution Description]
Force = 5 kN = 5000 N.
Radius of piston = $\frac{10\,cm}{2} = 5\,cm = 0.05\,m$.
Area = $\pi r^2 = \pi (0.05)^2 = 0.00785\,m^2$.
Pressure = $\frac{Force}{Area} = \frac{5000}{0.00785} ≈ 636,942\,Pa$.
Convert to MPa: $636,942\,Pa = 0.637\,MPa$.

Key Concept: Definition of Pressure

c) \(\text{pascal} = \mathrm{Pa}\)
[Solution Description]
The SI unit of pressure is pascal \(\mathrm{Pa}\), which is equivalent to \(\mathrm{N/m^2}\).

Key Concept: Application of Pressure

d) Sharp knives increase the pressure due to smaller contact area
[Solution Description]
Sharp knives have a smaller contact area with the object being cut. According to the pressure formula:
$Pressure = \frac{Force}{Area}$
A smaller area results in higher pressure for the same force, making the knife more effective at cutting.

Key Concept: Pressure and Area

c) The sharp knife has a smaller edge area, increasing pressure
[Solution Description]
A sharp knife has a smaller edge area compared to a blunt knife. Pressure is calculated as $Pressure = \frac{Force}{Area}$. For the same applied force, a smaller area results in higher pressure, making it easier to cut through objects. The higher pressure allows the sharp knife to penetrate materials with less effort.

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: 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 Concept

b) Atmospheric pressure
[Solution Description]
The pressure exerted by air around us is known as atmospheric pressure, as explained in the syllabus. It acts on all objects and balances the internal pressure of our bodies.

Key Concept: 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: Units of Pressure Conversion

c) $1013 \hspace{0.1em} \text{mb}$
[Solution Description]
1 hectopascal \(1\ \mathrm{hPa} = 1\ \mathrm{millibar} = 1\ \mathrm{mb}\). Thus, the conversion is direct:
$1013 \hspace{0.1em} \text{hPa} = 1013 \hspace{0.1em} \text{mb}$

Key Concept: Sucker Experiment Analysis

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

Key Concept: Atmospheric pressure, force calculation

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

Key Concept: 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: 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: Directional pressure, Real-world application

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

Key Concept: Pressure Exerted by Air, Surface Area Effect

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

Key Concept: Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] When air is blown into a balloon, it fills the space inside and exerts pressure on the inner walls of the balloon. This pressure acts equally in all directions, causing the balloon to expand uniformly. The reason correctly explains why the balloon expands in all directions because air indeed exerts pressure uniformly in all directions.

Key Concept: 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: Effect of air pressure, Pressure equilibrium

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

Key Concept: Pressure 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: Atmospheric Pressure

b) Air exerts pressure equally in all directions inside the balloon.
[Solution Description]
When air is blown into a balloon, it exerts pressure on the inner walls of the balloon. Since air exerts pressure equally in all directions (isotropic nature of pressure), the balloon expands uniformly in all directions. This is due to the balanced force distribution caused by the air molecules colliding with the balloon's surface.

Key Concept: Atmospheric pressure, Suction cup

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

Key Concept: Force and Area

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

Key Concept: 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: Sea breeze and land breeze

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

Key Concept: Formation of Wind

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: 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 wind moves from high pressure to low pressure, which is correct as per Activity 6.5 where air moved from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The reason is also true because air flows to equalize the pressure difference, as observed when both balloons attained the same size and air flow stopped. Moreover, the reason correctly explains the assertion since the movement of wind due to pressure differences is a fundamental concept in wind formation.

Key Concept: Pressure difference and wind direction

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

Key Concept: Formation of Wind

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that air moves from a region of high pressure to a region of low pressure, which is true as demonstrated in Activity 6.5 where air flows from the inflated balloon (high pressure) to the uninflated balloon (low pressure). The reason explains that this pressure difference causes the movement of air, which is also true and correctly explains the assertion. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: 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: 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, Wind speed

b) 0.5 N
[Solution Description]
The force exerted by the air can be calculated using the pressure difference and the area of the straw. First, convert the pressures to pascals: $P_1 = 1100 \text{ mb} = 1100 \times 100 \text{ Pa} = 110000 \text{ Pa}$, $P_2 = 1050 \text{ mb} = 105000 \text{ Pa}$. The pressure difference is $\Delta P = P_1 - P_2 = 5000 \text{ Pa}$. Using the formula for pressure, $Force = \Delta P \times Area = 5000 \times 10^{-4} = 0.5 \text{ N}$.

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, 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: Thunderstorms, Pressure difference

d) The pressure difference between outside and inside the house lifts the roof.
[Solution Description]
High-speed winds create a low-pressure area above the roof. The pressure inside the house remains higher, creating a pressure difference. If this difference is large enough and the roof is weak, it can be blown away. Keeping windows open equalizes the pressure.

Key Concept: Land breeze formation

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

Key Concept: 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) Sea breeze occurs because the sea is cooler and has higher pressure compared to the land.
[Solution Description]
During the day, land heats up faster than the sea. The air above the land becomes warm and rises, creating a low-pressure area. As a result, cooler air from the high-pressure region over the sea moves towards the land, forming a sea breeze.

Key Concept: Sea Breeze and Pressure Differences

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that during the day, the wind blows from the sea to the land because the land is warmer than the sea. This is true as per the syllabus, which explains that during the day, the land becomes warmer and lighter, creating a low-pressure area. The reason given is that the land heats up faster than the sea during the day, creating a low-pressure region over the land. This is also true and correctly explains why the wind blows from the high-pressure sea to the low-pressure land, resulting in a sea breeze. Therefore, both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Pressure Differences

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

Key Concept: 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: High-speed winds and air pressure

c) The balloons move towards each other due to reduced pressure between them.
[Solution Description]
When high-speed wind is blown between the two balloons, it creates a low-pressure region between them. The higher air pressure on the outer sides of the balloons pushes them towards each other. This demonstrates that high-speed winds lower air pressure.

Key Concept: High-Speed Winds Result in Lowering of Air Pressure

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

Key Concept: 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: 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: 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: High-speed winds, pressure difference

c) It reduces the pressure difference between the inside and outside of the house
[Solution Description]
High-speed winds create a low-pressure area above the roof due to Bernoulli's principle. If windows and doors are kept closed, the higher air pressure inside the house pushes upwards on the roof while the lower pressure outside pulls the roof upwards. This large pressure difference can cause the roof to blow off. Keeping windows and doors open allows wind to pass through the house, equalizing the pressure inside and outside, thus reducing the risk of roof damage.

Key Concept: Pressure Difference, Roof Safety

b) The roof might blow away due to the pressure difference
[Solution Description]
When high-speed winds blow over the roof, they create a low-pressure area above it due to Bernoulli's principle. If windows and doors are closed, the air pressure inside remains higher than outside. This significant pressure difference exerts an upward force on the roof, which may cause it to get blown away if the structure is weak.

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: Air Pressure, Speed-Pressure Relationship

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

Key Concept: Inference from Activity

c) High-speed winds reduce air pressure.
[Solution Description]
The activity shows that when air moves at high speed between the balloons, the air pressure decreases. This indicates that high-speed winds are accompanied by reduced air pressure.

Key Concept: High-Speed Winds and Air Pressure

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that when air is blown between two inflated balloons, they move towards each other, which is observed in Activity 6.6. This happens because blowing air between the balloons increases wind speed in the gap, reducing air pressure (Bernoulli’s principle). The higher pressure outside then pushes the balloons together. The reason correctly explains why this occurs by linking high-speed wind to low pressure and the resulting pressure difference.

Key Concept: Pressure equalization, Structural safety

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

Key Concept: Pressure Difference Due to High-Speed Winds

b) The wind reduces air pressure above the roof, causing an upward force
[Solution Description]
High-speed winds create a low-pressure area above the roof, while the pressure inside the house remains higher. The pressure difference causes an upward force on the roof. If the roof is weak or not properly secured, this force can blow it away.

Key Concept: Bernoulli's Principle Application

c) The balloons move closer together due to reduced pressure between them
[Solution Description]
Blowing air between the balloons creates a low-pressure region in between. The higher pressure on the outer sides of the balloons pushes them toward each other. This demonstrates Bernoulli's principle, where increased air speed results in decreased pressure.

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: Charge Separation in Clouds

b) Positive and negative charges separate within the cloud
[Solution Description]
Strong winds blowing upwards and downwards cause rubbing between water droplets and ice particles, leading to the development of static electric charges. The positively charged lighter ice particles move to the upper part of the cloud, while the negatively charged heavier water droplets occupy the lower part, creating charge separation.

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

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: 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: 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: Charge separation, Lightning formation, Cloud dynamics

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

Key Concept: Thunderstorms and Lightning

d) Positive charges in the upper part and negative charges in the lower part
[Solution Description]
Before lightning, positive charges accumulate in the upper part of the cloud (ice particles), while negative charges (water droplets) collect in the lower part of the cloud.

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: 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: Assertion and Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that thunderstorms are more frequent in hot, humid, and tropical regions like India, which is true because such conditions provide the necessary moisture and temperature contrasts for thunderstorm development. The Reason explains that warm air rises to great heights, forming ice particles and causing charge separation, which is also correct and directly explains why thunderstorms occur more frequently in these regions. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: 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: 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 separation, Lightning mechanism

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

Key Concept: Lightning formation

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: Static charge development within clouds leading to lightning

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

Key Concept: 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: Charge Separation in Clouds

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

Key Concept: Charge Separation, Lightning 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: Lightning Formation

c) A sudden flow of charges occurs, producing lightning
[Solution Description]
When the charge separation in the cloud becomes significant, the insulating property of air breaks down, allowing a sudden flow of charges. This discharge produces a bright flash of light called lightning. Lightning can occur within a cloud, between clouds, or between a cloud and the ground.

Key Concept: Eye of the Cyclone

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

Key Concept: Cyclone 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 and Characteristics

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that the eye of a cyclone has calm winds and clear skies, which is true because it is a region of descending air that suppresses cloud formation. However, the reason claims that the eye forms due to subsidence of warm air, which is incorrect. The eye forms due to the centrifugal force balancing the pressure gradient force, resulting in minimal wind movement. Thus, the Assertion is true, but the Reason is false.

Key Concept: Eye of the Cyclone

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

Key Concept: 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: Energy release, Cyclone formation

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

Key Concept: Eye of Cyclone

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

Key Concept: 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: 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: Eye of the cyclone, High-speed winds

c) Wind speed increases to 220 km/h
[Solution Description]
The wind speed in a cyclone is driven by the pressure gradient force. A lower pressure in the eye means a steeper pressure gradient between the eye and the surrounding regions. This steeper gradient results in stronger winds. Since the pressure drops from 950 hPa to 930 hPa, the gradient increases, leading to higher wind speeds.

Key Concept: 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

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: Cyclone Formation

c) The absence of warm, moist air over land causes the cyclone to weaken.
[Solution Description]
A cyclone derives its energy from warm, moist air rising from the ocean surface. When it moves over land, this source of moist air is cut off, causing the cyclone to gradually weaken and lose its strength.

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: Impact of Cyclones on Land, Flooding

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that storm surges from cyclones can flood coastal and inland areas due to high water walls. This is true because cyclones generate powerful winds that push seawater, creating storm surges as mentioned in the syllabus. The reason explains the mechanism behind storm surges, which aligns with the assertion. Hence, both are true, and the reason correctly explains the assertion.

Key Concept: Cyclone 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: 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: Emergency preparedness, IMD alerts

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

Key Concept: Cyclone 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.