NCERT Solutions Curiosity Chapter 6 Chapter-end exercise, Questions 1–13 — Keep the curiosity alive

(i) (d) equal in all three vessels  •  (ii) (c) M will stick but N will not stick  •  (iii) (a) increase the height ‘H’ at which the tank is placed  •  (iv) (b) P_A = P_B, F_A < F_B

(i) In Fig. 6.21 the three vessels P, Q and R are of different shapes and widths, but they are joined to one another by tubes at the bottom. Water poured into R therefore spreads into all three. The pressure a liquid exerts depends only on the height of its column, so the water can be at rest only when the column height is the same everywhere — if one vessel had a taller column, its greater pressure would push water along the connecting tube into the others. Hence the level settles equal in all three vessels, whatever their shape or width.

(ii) A sucker sticks because pressing it out drives the air from under the cup, so the pressure inside becomes lower than the atmospheric pressure outside, and the outside air holds it down. On a flat smooth surface the rim seals properly, the air cannot creep back, and sucker M sticks. On a rough surface air leaks in continuously through the tiny gaps under the rim, the pressure inside never stays low, no pressure difference is maintained, and sucker N falls off.

(iii) Pressure at a tap depends on the height of the water column above it. Raising the tank raises that column, so the pressure on the ground floor increases. Options (c) and (d) change only how much water the tank holds, at the same height — and the quantity of water makes no difference to the pressure, as Activity 6.1 showed with a broad and a narrow pipe.

(iv) The two vessels hold water to the same level, so the height of the water column is the same and the pressure at the bottom is the same: P_A = P_B. But vessel B is wider, so the area of its base is larger. Since force = pressure × area, the same pressure acting on a larger base gives a larger force.

(i) True  •  (ii) False  •  (iii) False  •  (iv) True

The boy sinks more when he is standing vertically (Fig. 6.23 b).

His weight is exactly the same in both pictures — lying down does not make him lighter. What changes is the area of sand supporting that weight. Standing, he touches the sand only with the soles of his two feet, a very small area. Lying down, his back, legs, arms and head all rest on the sand, an area many times larger. Since pressure is force divided by area, the pressure on the sand is far greater when he stands, and loose sand grains give way under high pressure — so he sinks in deeper.

The elephant exerts a pressure of 20000 N/m², that is 20000 Pa, on the ground.

The elephant stands on all four feet, so its whole weight is shared over the total area of the four feet. First find that total area, then divide the weight by it.

Boat B experiences the greater pressure on its base — greater by 100 N/m² (100 Pa).

For each boat, first add up the weight of the persons sitting in it, then divide by the base area.

Notice that boat A carries the greater load — 3500 N against 2100 N — and yet feels the smaller pressure. Its base is twice as wide, so the load is spread over twice the area.

No, lightning would not occur. Lightning depends on air being an insulator, and on the cloud being able to hold separated charges.

Lightning happens in three stages. First, strong up-and-down winds rub ice particles against water droplets so that the cloud becomes charged. Second, the charges separate and build up — light positive ice particles collect in the upper part of the cloud, heavier negative water droplets in the lower part — and the negative base makes the ground below positively charged. Third, when the build-up becomes very large, the insulating property of air breaks down and the charges flow suddenly across, producing a bright flash.

Now change the materials. If the clouds were good conductors, the positive and negative charges could not stay apart within the cloud; they would move through it and neutralise each other as fast as they were produced, so no large build-up could ever form. If the air were a good conductor, the charge would leak away steadily and continuously into the surrounding air and the ground, instead of being stored until air suddenly gives way. Either way there would be no huge accumulation and no sudden discharge — and lightning is precisely that sudden discharge.

Yes, both balloons will bulge — and they will bulge equally.

In Fig. 6.24 the two balloons are fixed to short side tubes near the bottom of the bottle, and both tubes are at the same height from the base. When water is poured in through the funnel, it fills the bottle and pushes into both side tubes.

Water presses on the walls of its container in all directions, so it presses outwards into each side tube and stretches the rubber — that is why both balloons bulge. How much each bulges is decided by the pressure at the tube's mouth, and that pressure depends only on the height of the water column standing above it. Since the two tubes are at the same height, the column above each is the same, the pressure at each is the same, and the two identical balloons stretch by the same amount.

A storm becomes a cyclone when it forms over warm ocean water, where the heat released by condensation keeps feeding it, so the central pressure falls lower and lower and the Earth's rotation sets the inrushing air spinning.

Follow the steps:

1. Warm moist air rises. The ocean water is heated, the warm and moist air above it becomes lighter and rises, and a low-pressure region forms below.
2. Water vapour condenses and releases heat. As the moist air rises it cools and the vapour condenses into raindrops. During evaporation water had taken up heat; on condensing, that heat is released back into the atmosphere.
3. The rising becomes stronger. This released heat warms the ascending air further, so it rises even higher, and the pressure at the centre becomes even lower than before.
4. Air rushes in from all around. Air from the surrounding higher-pressure regions rushes towards the centre, and it too begins to rise — bringing more moisture with it, so the cycle repeats and strengthens.
5. The Earth's rotation makes it spin. The moving air does not travel straight in; the rotation of the Earth makes it turn, so the whole system begins to revolve around the low-pressure centre.
6. A cyclone is born. The result is a very low-pressure area with high-speed winds revolving around it — a spinning system of clouds, winds and rain, with a calm eye at the centre.

Side A is the land (and side B is the sea).

Read the wind direction from the trees first. Their trunks lean towards A and their fronds are swept towards A, so the wind must be blowing from B towards A.

Now use the time of day. It is a summer afternoon, when land heats up much faster than water. The air over the land becomes warm and lighter, rises, and leaves a low-pressure area over the land. The air over the sea stays cooler and is at higher pressure. Air therefore blows from the sea to the land — this is the sea breeze. Since the wind here blows from B to A, B must be the sea and A must be the land.

Activity 6.5 — the two-balloon and straw experiment — shows it directly.

What you need: two similar balloons of thin rubber, a drinking straw, rubber bands or thread.

How to do it:

1. Push one end of the straw into the neck of the first balloon and tie it tightly with a rubber band or thread, so no air can leak past the joint. Leave this balloon uninflated.
2. Inflate the second balloon and hold its mouth closed with your fingers so the air does not escape.
3. Push the free end of the straw into the neck of the inflated balloon and tie that joint too, making sure no air leaks out while you do it. One end of the straw is now inside the inflated balloon and the other inside the limp one.
4. Write down your prediction, then release your fingers and watch both balloons.

What you observe: the inflated balloon gets smaller and the limp one fills up. After a while both are of almost the same size and the movement stops.

What it shows: the air pressure in the inflated balloon was higher than in the uninflated one, so air travelled through the straw from the high-pressure balloon to the low-pressure balloon. The flow continued only while the pressures differed, and stopped as soon as they became equal. This is exactly how wind blows outdoors.

A thunderstorm is a storm accompanied by lightning and thunder. (A storm itself is strong winds accompanied by rain.)

How it forms:

1. Heating and rising. The land gets heated, and the warm, moist air above it — being lighter — rises, creating a low-pressure area.
2. Circulation sets in. Cooler air from the surrounding high-pressure areas flows in to take its place, gets heated in turn and rises. This makes a continuous circulation of wind.
3. Clouds form. The rising air expands and cools, the moisture in it condenses into water droplets, and clouds are formed. The droplets merge into heavier drops which fall as rain, hail or snow. Strong winds together with this rain make a storm. Such storms are frequent in hot, humid and tropical regions like India.
4. Ice particles appear. Under certain conditions the warm air rises to such great heights that the low temperature there converts the water droplets into ice particles.
5. The cloud gets charged. Strong winds blowing upwards and downwards make the ice particles and water droplets rub against each other. As you learnt in Exploring Forces, rubbing charges objects — so static electric charges develop within the cloud.
6. Lightning and thunder. Positively charged lighter ice particles gather in the upper part of the cloud and negatively charged heavier water droplets in the lower part. When the build-up is large enough, air stops insulating and a sudden flow of charge gives a flash of lightning; the air it heats expands violently and produces the loud sound we call thunder. A storm with lightning and thunder is a thunderstorm.

Lightning is the sudden flow of charge that takes place when the charge built up in a cloud becomes so large that air can no longer insulate it.

1. Charging by rubbing. Inside a tall storm cloud, strong winds blow upwards and downwards. They make ice particles and water droplets rub against each other, and — as with any two objects rubbed together — the rubbing leaves them electrically charged.
2. Charge separation. The positively charged, lighter ice particles are carried upwards and occupy the upper part of the cloud. The negatively charged, heavier water droplets stay in the lower part. The cloud now has its charges separated, top from bottom.
3. Induced charge on the ground. When the negatively charged lower part of the cloud comes closer to the ground, it makes the ground and nearby objects such as trees and buildings become positively charged.
4. Air breaks down. Normally air acts as an electrical insulator and keeps the opposite charges apart. But when the build-up of charge becomes very large, this insulating property of air breaks down.
5. The flash. A sudden flow of charges takes place, producing a bright flash of light — that is lightning. It can occur as opposite charges collide within a cloud, between clouds, or between a cloud and the ground. The flash heats the air around it very rapidly, the air expands, and the loud sound produced is thunder.

Holes are made so that wind can pass straight through the banner instead of piling up against it — which keeps the pressure difference across the two faces small, and stops the banner from being torn or blown away.

A banner is a large flat sheet held up in the open. When a fast wind strikes it, the air moving across the front face is at a reduced pressure, because high-speed winds are accompanied by lower pressure, while the sheltered air behind the banner stays at ordinary atmospheric pressure. The difference in pressure between the two faces acts over the whole large area of the banner, so it produces a very large force — enough to rip the cloth, snap the ropes or pull down the frame.

The holes let air flow through from the high-pressure side to the low-pressure side. The two faces are then at nearly the same pressure, the net force on the banner falls sharply, and the banner stays put.