Pressure, Winds, Storms, and Cyclones

Correct option: (d) equal in all three vessels

Justification: Liquids find their own level when connected to each other. Since all three vessels P, Q, and R are connected at the bottom (communicating vessels), the water level will equalise and become the same in all three vessels, regardless of their shapes or sizes. This is a direct consequence of the principle that liquid pressure depends only on the height of the liquid column, not on the shape or volume of the container.

Correct option: (c) M will stick but N will not stick.

Justification: A rubber sucker works by expelling air from between its cup and the surface, creating a low-pressure region inside. The higher atmospheric pressure outside then holds it firmly to the surface. This works only on a smooth, flat surface where a proper airtight seal can be formed. On a rough surface, air leaks in through the gaps, so the pressure inside equals the atmospheric pressure outside, and the sucker does not stick. Hence, M (on smooth surface) sticks but N (on rough surface) does not.

Correct option: (a) increase the height 'H' at which the tank is placed.

Justification: The pressure exerted by a liquid depends on the height of the liquid column above the point of measurement. The greater the height 'H' of the water tank above the ground floor, the greater the pressure of water at the taps on the ground floor. Increasing 'H' increases the height of the water column, thereby increasing the water pressure.

Correct option: (b) \mathrm{P_A} = \mathrm{P_B},\ \mathrm{F_A} < \mathrm{F_B}

Justification:
- Pressure at the bottom of a liquid column depends only on the height of the liquid column (not on the shape or base area of the vessel). Since both vessels have water up to the same level, the pressure at the bottom is equal: $P_A = P_B$.
- Force = Pressure × Area. Since vessel B has a larger base area than vessel A (as seen from Fig. 6.22 where B is wider), the force exerted at the bottom of B is greater: F_A < F_B.

Thus, $P_A = P_B$ and F_A < F_B.

(i) True [T]
Air always moves from a region of higher pressure to a region of lower pressure. This pressure difference is the primary cause of wind formation.

(ii) False [F]
Liquids exert pressure not only at the bottom of a container but also on the sides (walls) of the container. In fact, liquids exert pressure in all directions.

(iii) False [F]
The eye of a cyclone is the calm, clear centre of the storm. The weather at the eye is calm and clear, not stormy. The stormy conditions (strong winds, heavy rain) exist in the region surrounding the eye.

(iv) True [T]
During a thunderstorm, it is safer to be inside a car (with windows closed) than to be in the open. The metal body of the car acts as a Faraday cage and conducts the lightning charge safely to the ground, protecting the occupants inside.

Given:
- Fig. 6.23a: Boy lying horizontally on sand (large contact area).
- Fig. 6.23b: Boy standing vertically on sand (small contact area — only feet touching).
- The weight (force) of the boy is the same in both cases.

Concept used:
$$\text{Pressure} = \frac{\text{Force}}{\text{Area}}$$

Reasoning:
- When the boy lies horizontally, his body is spread over a large area of the sand. The same weight is distributed over a larger area, so the pressure exerted on the sand is less.
- When the boy stands vertically, only his feet are in contact with the sand, which is a much smaller area. The same weight is now concentrated over a smaller area, so the pressure exerted on the sand is much greater.

Conclusion: The boy sinks more in sand when standing vertically (Fig. 6.23b), because the pressure exerted on the sand is greater due to the smaller contact area.

Given:
- Weight of elephant (Force) $= 20000\ \text{N}$
- Area covered by one foot $= 0.25\ \text{m}^2$
- Number of feet $= 4$

Step 1: Calculate total area of contact.
$$\text{Total Area} = 4 \times 0.25 = 1.0\ \text{m}^2$$

Step 2: Apply the formula for pressure.
$$\text{Pressure} = \frac{\text{Force}}{\text{Area}}$$

$$\text{Pressure} = \frac{20000\ \text{N}}{1.0\ \text{m}^2}$$

$$\boxed{\text{Pressure} = 20000\ \text{N/m}^2 = 20000\ \text{Pa}}$$

Answer: The pressure exerted by the elephant on the ground is $\mathbf{20000\ Pa}$ (or $20000\ \text{N/m}^2$).

Given:
- Boat A: Base area $= 7\ \text{m}^2$, Number of persons $= 5$
- Boat B: Base area $= 3.5\ \text{m}^2$, Number of persons $= 3$
- Weight of each person $= 700\ \text{N}$

Step 1: Calculate total force (weight) on each boat.
$$F_A = 5 \times 700 = 3500\ \text{N}$$
$$F_B = 3 \times 700 = 2100\ \text{N}$$

Step 2: Calculate pressure on the base of each boat.
$$P_A = \frac{F_A}{\text{Area}_A} = \frac{3500}{7} = 500\ \text{N/m}^2$$
$$P_B = \frac{F_B}{\text{Area}_B} = \frac{2100}{3.5} = 600\ \text{N/m}^2$$

Step 3: Compare the pressures.
$$P_B - P_A = 600 - 500 = 100\ \text{N/m}^2$$

Conclusion: Boat B experiences more pressure on its base. The pressure on Boat B is greater than on Boat A by $\mathbf{100\ \text{N/m}^2}$ (i.e., $100\ \text{Pa}$).

Answer: No, lightning would not occur if air and clouds were good conductors of electricity.

Reasoning:
- Lightning occurs because electric charges build up in clouds due to the rubbing of ice particles with water droplets caused by strong upward and downward winds.
- These charges accumulate in different parts of the cloud (positive charges at the top and negative charges at the bottom) because air is a poor conductor (insulator) of electricity. This prevents the charges from flowing freely and allows a large charge difference (potential difference) to build up.
- When the potential difference becomes very large, the air temporarily breaks down as an insulator and a massive discharge of electricity occurs — this is lightning.
- If air and clouds were good conductors, the charges would flow freely and continuously as soon as they were generated. No large accumulation of charge would be possible, and therefore no significant potential difference would build up.
- Without a large potential difference, there would be no sudden, massive discharge, and hence no lightning.

Conclusion: Lightning is a result of the insulating property of air allowing charge to accumulate. If air and clouds were good conductors, charges would dissipate immediately and lightning would not occur.

Given: Two identical balloons A and B are attached to the sides of a bottle. Water is filled into the bottle up to a certain height. (Assumption based on the figure: Balloon A is at a lower position and Balloon B is at a higher position on the bottle, or both are at the same height — since the figure is not visible, the standard textbook interpretation is used: both balloons are at the same height from the bottom.)

Answer: Yes, both balloons will bulge. If both balloons A and B are attached at the same height from the bottom of the bottle, they will bulge equally.

Explanation:
- Water exerts pressure not only at the bottom of a container but also on the sides (walls) of the container in all directions.
- The pressure exerted by a liquid at a given point depends only on the height of the liquid column above that point, not on the direction or the shape of the container.
- Since both balloons are at the same height from the bottom, the height of the water column above each balloon is the same.
- Therefore, the pressure exerted by water on both balloons is equal, and both balloons will bulge to the same extent.

Conclusion: Both balloons bulge equally because the pressure exerted by the water column is the same at equal heights, regardless of the direction.

Formation of a Cyclone from a Storm:

Step 1 — Warm, moist air rises:
Over warm ocean surfaces (especially in tropical regions), large amounts of water evaporate and warm, moist air rises rapidly upward. This creates a region of low pressure near the ocean surface.

Step 2 — Cooler air rushes in:
Cooler air from surrounding high-pressure regions rushes in to fill the low-pressure area. This incoming air also gets warmed by the ocean and rises, continuing the cycle.

Step 3 — Rotation begins:
Due to the rotation of the Earth (Coriolis effect), the incoming winds do not blow straight into the low-pressure centre but begin to spiral inward, rotating around the centre. In the Northern Hemisphere, this rotation is anti-clockwise; in the Southern Hemisphere, it is clockwise.

Step 4 — Intensification:
As more and more warm, moist air rises, it cools at higher altitudes and condenses, releasing latent heat. This heat warms the surrounding air further, causing it to rise faster, which further lowers the pressure at the centre. This draws in more air, creating a self-reinforcing cycle.

Step 5 — Cyclone forms:
The system of rapidly rotating winds around a central low-pressure area (the eye) intensifies into a cyclone. The eye is calm and clear, while the surrounding region (the eye wall) has the strongest winds and heaviest rainfall.

Conditions necessary: Warm ocean water (above 26–27°C), moisture in the atmosphere, and the Coriolis effect due to Earth's rotation are essential for cyclone formation.

Conclusion: A storm becomes a cyclone when a system of low pressure develops over warm ocean water, and the surrounding winds spiral inward with increasing speed due to Earth's rotation, forming a large, organised rotating storm system.

Given: The figure shows trees bending in a particular direction along the sea coast on a summer afternoon. (Assumption: The trees are bending towards side A, indicating wind is blowing from B towards A.)

Concept — Sea Breeze:
During a summer afternoon, the land heats up much faster than the sea. The air above the land becomes warm and light, rises upward, and creates a low-pressure area over the land. The air above the sea remains cooler and denser, creating a high-pressure area over the sea. As a result, wind blows from the sea towards the land — this is called sea breeze.

Identification:
- The trees are bending in the direction the wind is blowing.
- Since sea breeze blows from sea to land during a summer afternoon, the trees will bend towards the land side.
- The trees are bending towards side A, which means the wind is coming from side B.
- Therefore, side B is the sea and side A is the land.

Conclusion: Side A is the land. The trees bend towards the land (side A) because during a summer afternoon, sea breeze blows from the sea (side B, high pressure) towards the land (side A, low pressure).

Activity to demonstrate that air flows from high pressure to low pressure:

Materials required: Two identical rubber balloons, one drinking straw, rubber bands or thread.

Procedure:
1. Take two identical thin rubber balloons and a drinking straw.
2. Insert one end of the straw into the neck of the first (uninflated) balloon and secure it tightly with a rubber band so that no air leaks.
3. Inflate the second balloon fully and hold its mouth tightly with your fingers to prevent air from escaping.
4. Carefully insert the free end of the straw into the neck of the inflated balloon and secure it tightly with a rubber band. Ensure no air leaks from either balloon.
5. Now you have a setup where one end of the straw is inside the inflated balloon (high pressure) and the other end is inside the uninflated balloon (low pressure).
6. Release your fingers and observe what happens to both balloons.

Observation:
- The inflated balloon gradually deflates (decreases in size).
- The uninflated balloon gradually inflates (increases in size).
- After some time, both balloons reach almost the same size and the air flow stops.

Inference:
- The air pressure inside the inflated balloon is higher than in the uninflated balloon.
- Air flows through the straw from the inflated balloon (high pressure) to the uninflated balloon (low pressure).
- The flow stops when the pressure in both balloons becomes equal.

Conclusion: This activity clearly demonstrates that air flows from a region of higher pressure to a region of lower pressure.

What is a Thunderstorm?
A thunderstorm is a violent weather condition characterised by strong winds, heavy rainfall, thunder, and lightning. It develops in a very short time and is accompanied by dark clouds, thunder, and lightning.

Process of Formation of a Thunderstorm:

Step 1 — Conditions required:
Thunderstorms develop in hot and humid (moist) tropical areas. The two most important requirements are:
- Abundant moisture in the atmosphere.
- Strong upward and downward winds (updrafts and downdrafts).

Step 2 — Rising of warm, moist air:
During a hot day, the ground heats up rapidly. The air near the ground becomes warm and moist due to evaporation of water. This warm, light air rises rapidly upward in the form of strong updrafts.

Step 3 — Formation of cumulonimbus clouds:
As the warm, moist air rises, it cools and the water vapour condenses to form large, towering cumulonimbus clouds (thunderclouds). These clouds can extend to great heights in the atmosphere.

Step 4 — Development of strong winds:
Within these clouds, there are both strong upward currents (updrafts) and downward currents (downdrafts) of air. These strong winds carry water droplets upward where they freeze into ice particles.

Step 5 — Charge development:
The strong winds cause ice particles and water droplets to rub against each other, generating static electric charges. Positive charges accumulate near the top of the cloud and negative charges accumulate near the bottom.

Step 6 — Lightning and thunder:
When the charge difference becomes very large, a massive electrical discharge occurs — either within the cloud, between clouds, or between the cloud and the ground. This discharge is seen as lightning. The rapid heating and expansion of air around the lightning bolt produces a loud sound called thunder.

Step 7 — Heavy rainfall:
The strong winds and electrical activity are accompanied by heavy rainfall, completing the thunderstorm.

Conclusion: A thunderstorm is formed due to the rapid rising of warm, moist air, formation of large clouds, development of electric charges through friction between ice and water particles, and their subsequent discharge as lightning and thunder.

Process that Causes Lightning:

Step 1 — Formation of thunderclouds:
During a thunderstorm, strong updrafts (upward winds) carry warm, moist air to great heights. Water vapour condenses to form large cumulonimbus clouds containing both water droplets and ice particles.

Step 2 — Charge generation by friction:
Within the thundercloud, there are powerful upward and downward winds. These winds cause ice particles (at higher, colder levels) and water droplets (at lower, warmer levels) to collide and rub against each other continuously.
- This friction causes separation of electric charges (similar to rubbing a glass rod with silk).
- Positive charges accumulate near the top of the cloud.
- Negative charges accumulate near the bottom of the cloud.

Step 3 — Induction on the ground:
The large negative charge at the bottom of the cloud repels the negative charges on the ground below and attracts positive charges. This creates a region of positive charge on the ground directly below the cloud.

Step 4 — Electrical discharge (Lightning):
As the charge difference (potential difference) between the bottom of the cloud and the ground (or between different parts of the cloud) becomes very large, the insulating property of air breaks down.
- A massive and sudden flow of electric current occurs between the cloud and the ground (or within/between clouds).
- This discharge is seen as a bright flash of light — called lightning.
- The enormous heat generated (about 30,000 K) causes the surrounding air to expand explosively, producing the loud sound of thunder.

Conclusion: Lightning is caused by the build-up of large static electric charges in thunderclouds due to friction between ice particles and water droplets, followed by a sudden and massive electrical discharge between the cloud and the ground or between clouds.

Explanation:

Concept used: High-speed winds are accompanied by reduced (low) air pressure (as established in Activity 6.6). When wind blows over a surface, it creates a pressure difference between the two sides of the surface.

Reason for holes in banners and hoardings:

1. When strong winds blow, they strike the front surface of a banner or hoarding. The wind speed on the front side is reduced (as wind is blocked), so the air pressure on the front is relatively higher.
2. On the back side of the banner, the wind moves faster (as it goes around the edges), creating a region of lower pressure.
3. This pressure difference between the front (high pressure) and back (low pressure) creates a large net force pushing the banner backward. If this force is too large, it can tear the banner or topple the hoarding, causing damage.
4. When holes are made in banners and hoardings, the wind can pass through the holes from the front to the back. This equalises the pressure on both sides of the banner.
5. With equalised pressure, the net force on the banner is greatly reduced, preventing it from being torn or the hoarding from being toppled.

Conclusion: Holes are made in banners and hoardings so that wind can pass through them, equalising the air pressure on both sides and reducing the net force on the structure, thereby preventing damage during strong winds.