Chapter 6: Pressure, Winds, Storms, and Cyclones (Physics)

(i) (d) Equal in all three vessels. In connected vessels the water settles at the same level, whatever their shapes.

(ii) (c) M will stick but N will not. On the smooth surface, pressing pushes out the air and none can get back in. The air pressure outside then holds M in place. On the rough surface, air leaks in through the tiny gaps, so N falls off.

(iii) (a) Increase the height H. Pressure in a liquid depends on the depth (height of water above), not on how much water the tank holds.

(iv) (b) P_A = P_B, F_A < F_B. The pressure depends only on the depth, which is the same. The force is pressure × area, and B's base is larger.

(i) (d) (ii) (c) (iii) (a) (iv) (b)

1. T
2. F. Liquids press on the sides as well as the bottom: water spurts out of holes in the side of a bottle.
3. F. The eye is calm, with light winds and clear skies. The strongest winds and rain are in the wall of clouds around it.
4. T. A car with a closed metal body (windows up) carries the lightning's charge around the outside and into the ground.

(i) T (ii) F (iii) F (iv) T

He sinks more when standing. His weight (the force) is the same in both cases. Standing, it acts on the small area of his feet, so the pressure is much greater. Lying down spreads it over a large area, which gives less pressure.

Standing: the same weight on a smaller area gives more pressure.

Total area $=4\times 0.25=1$ m², so

$$\text{Pressure}=\frac{20000\text{ N}}{1{\text{ m}}^2}=20000{\text{ N/m}}^2$$

20000 Pa (N/m²)

• A: $\frac{5\times 700}{7}=\frac{3500}{7}=500$ Pa
• B: $\frac{3\times 700}{3.5}=\frac{2100}{3.5}=600$ Pa

(Here we count only the weight of the people, not of the boats.)

Boat B, by 100 Pa (600 Pa against 500 Pa).

No. Lightning happens because charges build up in clouds. Air is a poor conductor, so the charges cannot flow away gradually. When very large amounts have collected, they jump across the air all at once as a giant spark.

If air and clouds conducted electricity well, the charges would leak away steadily as they formed. No large build-up would occur, so there would be no sudden discharge (no lightning flash).

No: charges would flow away as they formed, so they could never build up enough for a sudden spark.

Yes, both bulge, and equally. The water presses outwards on the walls, and the balloons cover holes in the walls. Both holes are at the same depth, so the pressure on each is the same.

If one hole were lower, its balloon would bulge more, since pressure increases with depth.

Both bulge equally, because liquid pressure acts sideways and is the same at the same depth.

1. Over a warm sea, the sun heats the water. Warm, moist air rises, leaving a region of low pressure below.
2. As this air rises, its water vapour condenses into clouds. This releases heat, which warms the air further, so it rises even faster.
3. Cooler air from the surroundings (at higher pressure) rushes in towards the low-pressure centre. It also gets heated and rises, and the cycle keeps going.
4. Because the Earth rotates, the in-rushing winds start spinning around the centre, forming the calm eye.
5. As long as the storm stays over warm water, it keeps drawing energy. The winds grow stronger until they become a cyclone, with very high speeds and heavy rain.

Warm, moist air rising over a warm sea creates low pressure; condensation releases heat that drives more rising; in-rushing air starts spinning because of the Earth's rotation, and the system keeps strengthening into a cyclone around a calm eye.

B is land. On a summer afternoon the land heats up faster than the sea. The air over the land rises, leaving lower pressure there.

Cooler air from over the sea, at higher pressure, blows towards the land: this is the sea breeze. The trees are bent in the direction the wind blows, from A towards B, so the sea is at A and the land at B.

B is land: the sea breeze blows from sea (A) to land (B) on a hot afternoon, bending the trees towards B.

Balloon activity:

1. Blow up a balloon and hold its neck closed. The air inside is at higher pressure than the air outside.
2. Let go of the neck. Air rushes out of the balloon, towards the lower pressure outside, and you can feel it on your hand.

Another way: light an incense stick near a slightly open door between a warm room and a cool room. The smoke shows air moving in from the cooler, higher-pressure side.

Release an inflated balloon: the air rushes out from the high pressure inside to the lower pressure outside.

A thunderstorm is a storm with heavy rain, strong winds, lightning and thunder, and sometimes hail.

On a hot, humid day:

1. The ground heats the air near it, and this warm, moist air rises quickly.
2. As it rises, it cools. Its water vapour condenses into tall, dark clouds; higher up, the drops freeze into ice particles.
3. Strong winds move up and down inside the cloud. They make ice particles and water droplets rub and collide, which separates electric charges.
4. When enough charge has collected, lightning flashes. The heated air expands violently and makes the sound of thunder. Heavy rain falls.

A storm with lightning, thunder, strong winds and heavy rain. Hot moist air rises fast, forms tall clouds, and the up- and down-draughts separate charges until lightning strikes.

1. Inside a thundercloud, strong winds carry ice particles and water droplets up and down. They rub against and collide with each other.
2. This separates electric charges. Usually the top of the cloud becomes positive and the bottom becomes negative.
3. The negative charge at the bottom of the cloud also attracts positive charges on the ground below, especially on tall objects.
4. Air is a poor conductor, so the charges keep building up. When they become very large, the charge jumps across the air, within the cloud, between clouds, or between the cloud and the ground.
5. This huge spark is lightning. It heats the air so suddenly that the air expands with a loud bang, which is thunder.

Colliding ice and water particles separate charges in the cloud; when enough charge builds up it jumps through the air as a giant spark (lightning), and the suddenly heated air makes thunder.

A strong wind pushes on the whole surface of a banner. On a large banner this adds up to a very large force, which can tear it or bring down its frame.

The holes let some of the wind pass through, so less air pushes against the surface and the force (pressure) on it becomes smaller. The banner is then less likely to tear or fall in a storm.

Holes let wind pass through, reducing the pressure and force on the banner so that it does not tear or blow down.