Pressure

Definition of pressure.

Pressure in solids.

Pressure in liquids.

Pascal's principle.

Buoyancy.

Pressure in gases.

Atmosphere.

Barometer.

Pressure and temperature.

Refrigerator.

Atmospheric pressure and weather.

Buoyancy in air.

Aircraft.

Definition of pressure.

Pressure is the force applied, per unit area.

Pressure is directly proportional, to force.

Higher the force, higher will be the pressure.

Pressure is inversely proportional to area.

Higher the area, lower will be the pressure.

F is the force, in newtons,

A, is the area, in metre squared.

Pressure P, = F divided by ‘A’, newton per meter squared.

The S I unit for pressure, is Pascal.

1 pascal is defined, as 1 newton per meter squared.

1 pascal, is a very small unit of pressure.

1 kilo pascal, = 1000 pascals.

For convenience, we could use kilo pascal, as a unit of pressure.

In the F P S system, pressure was defined as pounds per square inch, or p s i.

For historical reasons, this unit might be still used.

To illustrate pressure, we will take a simple example.

A rectangular cuboid block weighs 60 newtons.

The size of the rectangle, is 3 meters, by 2 meters.

The thickness of the block is .5 meters.

If the block, is resting on the rectangle,

the area is 3, multiplied by 2, = 6 square meters.

Pressure is = force by area.

Pressure is =, 60 divided by 6, = 10 newtons per meter squared, = 10 pascals.

If the block is resting, on a 2 meter edge,

the area is 2, multiplied by .5, = 1 square meter.

Now the pressure is, 60 divided by 1, which is 60 newtons per meter squared,

= 60 newtons.

The same block, with a weight of 60 newtons,

exerts a smaller pressure, over a larger area,

and a higher pressure, over a smaller area.

This is the basic relationship of force and pressure.

Pressure in solids.

Solids have, closely packed atoms, or molecules.

Solids resist pressure.

The amount of resistance, to pressure, that the solid has,

is one measure of the strength of the solid.

If we walk, on a concrete road, we will not leave any foot prints.

If we walk, on a sandy beach, we will leave foot prints.

Sand, is relatively softer than concrete.

It yields more easily, to pressure.

This is why foot prints, appear in sand.

If we walk, on wet mushy soil, we will leave foot prints.

If we walk on snow, we leave footprints.

This is because wet soil and snow are softer than concrete.

If we stand on 2 legs, on sand, we will leave 2 foot prints.

If we stand on 1 leg, what will happen?

The obvious answer, is that, we will leave 1 foot print.

But, if we take pressure into account, we will realise,

that the 1 foot print, is deeper than the 2 foot prints.

Same force, of our weight, is now acting on a smaller area,

resulting in a larger pressure.

The school bag we carry, has a broad strap.

This helps distribute, the force of the bag, and reduce the pressure,

on our shoulders.

If we carry the same bag, with thin straps, it will be very uncomfortable.

If we press, the head of a pin, on our finger, nothing much will happen.

If we press, the pointed edge, of the pin, with the same force,

the skin in our finger, will get pierced.

The pointed edge of the pin, has a much smaller area,

resulting in larger pressure.

The same principle, is used to give an injection.

A sharp needle, is used to pierce the skin.

A syringe injects the medicine.

We can imagine, that it would be very difficult, to give an injection,

to a rhinoceros, which has very thick skin.

A knife can cut an apple.

The sharp edge of the knife, exerts sufficient pressure,

to over come the resistance, of the apple, to pressure.

The same knife, cannot cut a steel ball.

This is because, steel is strong enough, to resist the pressure.

One of the earliest tools, that man invented, was stones,

with a sharp cu edge.

He used these tools, to great advantage, for a variety of tasks,

like cutting wood, and hunting.

Even strong materials, like steel, can be cut.

This is what modern machine tools do.

They use extra hard cu tools, and a large amount of mechanical pressure,

to cut steel.

In a bicycle, we use brakes, to slow down, or stop the bicycle.

The brake shoes, have a larger area, and press, on the rim of the wheel.

This pressure, creates enough frictional force, to slow down the wheel,

and the bicycle.

If we lie down on a soft bed, it yields slightly.

If we stand on the same bed, it yields more.

When we stand, we exert more pressure.

When we carry an open umbrella, in a strong wind,

It tends to blow away.

If we fold the same umbrella, the same force is not felt.

The wind pressure acting on a large area, creates more force.

When the umbrella is folded, there is less area, and consequently less force.

This is the same reason, that a lamp post, is not impacted, by a strong wind.

A billboard with a large area, tends to bend, when there is a strong wind.

In general, solids tend to resist pressure.

In solids, pressure can be related to stress.

Depending on strength of the material, of the solid,

and the amount, and type of pressure or stress applied,

different solids, will respond differently.

Resistance to pressure, or stress, is one of the ways,

to measure the strength of a material.

Resistance to compression, is called as the compressive strength of the material.

For example, Concrete has a high compressive strength.

Resistance to elongation, is called as the tensile strength.

For example, a rubber band, can be easily elongated.

It can be said to have low tensile strength.

Pressure in liquids.

When there is a column of liquid, like water,

the weight of the water itself, exerts pressure.

If we take a tall container, with water,

the pressure at the bottom of the container,

will be more, then at the top of the container.

The pressure would be proportional, to the depth of the water.

If we take two points in the container, the pressure at a lower point,

will be higher, then the pressure, at a higher point.

If we had 3 holes, at different depths, in the container, and filled it with water,

the water at the lowest opening, will flow out with the highest pressure.

Water from the middle opening, will flow at medium pressure.

Water from the top opening, will flow, with the least pressure.

We can calculate, the pressure, in a column of a liquid.

Let ‘rho’ be the density, of the liquid.

Density is expressed in Kg per meter cubed.

Let ‘A’ be the area of the column, in meter squared.

Let ‘h’ be the height of the column.

’g’ is the gravitational acceleration.

The volume of liquid, in the column, will be ‘A’ into ‘h’, meter cubed.

The weight of the liquid, will be volume, into density, ‘rho’, into ‘g’.

So, the weight of the column, in newtons, would be ‘A’ into ‘h’, into ‘rho’, into ‘g’.

The weight of this column of liquid, exerts a downward force.

Pressure exerted, by this column of fluid, would be force divided by area.

This would be equal to ‘A’ into ‘h’, into ‘rho’ into ‘g’, whole divided by ‘A’.

In this equation, the area ‘A’ cancels out.

So, the pressure would be, ‘h’ into ‘rho’ into ‘g’, newtons per meter squared.

In short we can say, pressure ‘p’ equals ‘h’, ‘rho’, ‘g’.

Pressure is directly proportional, to the density of the liquid.

Higher the density, higher will be the pressure.

A column of mercury, will exert a greater pressure, then a column of water.

Pressure is directly proportional, to the height of the column.

More the height, more will be the pressure.

In an apartment building, the apartment in the ground floor,

will receive water at a higher pressure, compared to the apartment,

in the top floor.

There will be a taller column of water, exerting a pressure, on the tap,

in the ground floor.

Pressure is independent, of the area of the column.

Let us take a broad container, and a thin container,

connected to each other, by the pipe at the bottom.

Let us fill this inter-connected container, with water.

We can observe that the water will reach, the same level, in both the containers.

If we pour some more water, the height will increase, by the same amount,

in both the containers.

The pressure is independent, of the shape of the container.

We can take, cylindrical, rectangular and a funnel shaped containers,

and inter-connect them.

We will fill it with water.

Water in all the containers, will have the same level.

The pressure at the given depth, will be the same, for all the containers.

In summary, for a given liquid like water, the pressure will be proportional,

only to the depth.

The tendency of water, to maintain the same level, is true for the oceans also.

Mean sea level, is used as a standard for measuring altitude.

For example, we might say, that the peak of mount everest, is at an altitude,

of 8848 meters above mean sea level.

Dams are used to store water, in a reservoir.

The pressure of water at different depths, can be calculated.

Density of water, is thousand Kg per meter cubed.

Gravitational acceleration ’g’, can be taken as 10 meters per second squared.

Pressure at a depth of 10 meters,

will be equal to ‘h’, ‘rho’, ‘g’.

Pressure ‘p’ equal to 10 into 1000, into 10,

is equal to 100 thousand newtons per meter squared.

We can express this as 100 kilo pascals.

The pressure at a depth of 20 meters,

will be equal to 200 thousand newtons per meter squared.

We can express this as 200 kilo pascals.

In these calculations, we ignore air pressure.

There are sluice gates, located at the bottom of the dam.

When a sluice gate is opened, water flows out at high pressure.

This is used to drive turbines, to generate electricity.

Water exerts pressure, equally in all directions.

The water, in the reservoir of the dam,

not only exerts, a downward pressure, but also a lateral pressure.

This lateral pressure, is greater at a larger depth.

This is the reason, that dams are constructed, with a much wider base.

The wide concrete base, is designed to be strong enough,

to resist the high lateral pressure, of the water, at the bottom of the dam.

Pascal’s principle.

Let us take the example, of the two inter connected containers,

with different areas.

Let the areas be “A1” and “A2”.

”A1” is the smaller area, and “A2” is the bigger area.

Using a piston, if we apply a force, “F1”, to the water in the thinner container,

the water level will go down.

The water level in the broader container, with a car in a platform, will go up.

What will be the force “F2”, in the broader container?

Pressure is the same.

Pressure in the smaller container, is “F1” divided by“A1”.

Pressure in the broader container, is “F2” divided by “A2”.

”F2”, divided by “A2”, is = “F1”, divided by “A1”.

”F2” is = “F1” into “A2” divided by “A1”.

”F2” will be much greater, than “F1”,

since, ”A2” is greater than “A1”.

Larger the ratio of “A2” by “A1”, greater will be the amplification of force.

Devices which use Pascal’s principle, for mechanical work,

are called, hydraulic machines

In a garage, a hydraulic Jack, is used to used to lift a car, parked in a platform.

The car is heavy.

We can use Pascal’s principle, to lift this car.

The platform is lifted, by a large cylinder, with a hydraulic fluid, like oil.

By applying a smaller force “F1”,

on a thinner cylinder,

we can lift a larger weight, of the car.

This principle, is widely used in many hydraulic applications.

Though the smaller force “F1”, exerts a larger force, in the bigger cylinder,

the work done, will be the same.

The piston, applying the force “F1”, goes down a longer depth,

compared to the increase in the height of the platform.

But, by repeatedly applying a smaller force, we will be able to exert a larger force,

using Pascal’s principle.

For example, when a car tyre has a puncture, we need to lift the car.

It would not be physically possible, for us to do so.

But, we can use a hydraulic jack, and by applying smaller forces,

through a repeated pumping action, we can lift the car.

Archimedes principle.

Relative to solids, atoms in a liquid, are less densely packed together.

Liquids, tend to yield to pressure.

If we place a pebble, in a cup of water, the water will yield,

and the pebble, will sink to the bottom of the cup.

Archimedes, was reportedly given the problem, of finding the density,

of the king’s gold crown.

He could easily find, the weight of the crown.

But, he found it very difficult, to find the volume of the crown.

This was because, the crown had a intricate and complex design.

He said to have thought about this problem, in his bath tub.

He noticed that when he got into the tub, water was displaced, from the tub.

He correctly surmised, that the volume of water displaced, was exactly equal,

to the volume of his body.

This gave him the idea, that he could now measure the volume of the crown.

He was so thrilled, with this finding, that it is said, he ran naked,

through the streets, shouting “Eureka”.

When a solid body is immersed in a liquid, it displaces a volume of liquid,

equal to its own volume.

This principle, has been extended, to explain the concept of buoyant force.

Buoyancy.

If we suspend, a steel ball, from a spring balance, we can find it’s weight.

If we submerge, the steel ball, in water, the weight of the ball, will reduce.

This is because, water exerts a buoyant upward force, on the steel ball.

The weight of the steel ball in the water, will be,

it’s normal weight minus, the buoyant force acting on it.

If we place a cork, in a cup of water, it does not sink, it floats.

In general, if the density of the object, is greater than the density,

of the liquid, the object will sink.

This is because, the weight of the object, is greater than the buoyant force.

If the density of the object, is less than, the density of the liquid,

then the object, will float in the liquid.

This is because, the buoyant force, is greater than the weight of the body.

The buoyant force, is = to the weight of the displaced liquid.

If “rho” is the density, of the liquid,

”V” is the volume, of the displaced liquid,

and ”g” is the gravitational constant,

the buoyant force, is given by,

”rho” into “V”, into “g”.

If we place an ice cube, in a cup of water, it submerges significantly,

but, it does not sink.

We will find most of the ice, is under water, and the tip of the ice, is over the water.

The density of ice, is slightly lower, than the density of water.

This causes the ice, to submerge, but still float in the water.

The buoyant force acting on the ice cube, is able to balance

the weight of the ice cube.

In the arctic, the ocean freezes, during winter.

Only a layer of ice, on the top of the ocean freezes.

This layer floats on the water.

The ocean under it, is still liquid water.

This helps, all the marine life living in the ocean, under the frozen ice,

to survive the winter.

If we place a hollow steel cup, in a tub of water, it floats.

The density of steel, is greater than, the density of water.

How does the steel cup float?

Let us revisit, the formula for buoyant force.

Buoyant force = “rho” into “V”, into “g”.

”V”, is the volume, of the liquid displaced.

If somehow, the cup is able to displace more than it’s own weight of water,

the cup will float.

This is exactly, what the hollow steel cup does.

Since the cup is hollow, the empty space is filled with air.

The density of air, is much lighter, than the density of water.

The overall volume of the cup, includes the hollow space.

Density = weight by volume.

So, the overall density of the cup, becomes lower than the density of water.

This principle, is widely used, in all sailing vessels, from a fisherman’s boat,

to a large cruise ship, or even an aircraft carrier.

Boats and ships, are designed to displace more water, than their own weight,

and the weight, they carry.

The empty steel cup, floats in the tub, with very little submersion.

Let us load this cup, with a small pebble.

The cup will submerge slightly, but still float.

Let us add, more pebbles, one at a time.

Each time, we add a pebble, the cup will sink, to displace as much water,

as the pebble weighs.

Finally, if we add enough pebbles, to overcome, the buoyant force,

the cup with the pebbles will sink.

When a ship is fully loaded with cargo, it will submerge more.

When the cargo is unloaded, the ship will move up.

Submarines fill compartments with water, to sink, and stay out of sight.

When they want to surface, they expel the water, become lighter,

and float to the surface.

The buoyant force, acting on a floating vessel, can be calculated.

We can take a simple rectangular vessel, with an area of “A”, square meters,

and a height of “h” meters, and float it in water.

If it sinks by “x”, meters,

the volume of water displaced =,

”A”, into “x” meter cubed.

The buoyant force = “rho” into “A”, into “x”, into “g”.

To calculate, the buoyant force, in any floating ship,

we just need to measure the volume of water displaced.

Pressure in gases.

In solids and liquids, atoms are closely packed together.

This is not so in gases.

In gases, atoms freely move about.

Water in a container, will only occupy the space at the bottom of the container.

Gas in a container, will occupy all the space in the container.

Solids and liquids, resist pressure.

Gas succumbs to pressure, and gets compressed.

We can take a cylinder with a piston.

The cylinder has air in it.

If we push down the piston, the air will get compressed.

With increase in pressure, the volume of the gas decreases.

When we decrease the pressure, the gas will expand, to occupy,

all the space that is available.

If we take a closed container, and pump the air out of it,

the remaining air, will expand, to occupy all the space, in the container.

Gas always expands, to fill the whole container.

If we pump out all the air, a vacuum is created.

This is what is done, in light bulbs.

The vacuum prevents the heated element, from reacting with oxygen, in the air.

If we light a incense stick, in a room, the fragrance will spread through out the room.

Smoke from a candle, will spread, to occupy the whole room.

This is a general property, of all gases.

When air is compressed, the compressed air, exerts pressure.

We use a bicycle pump, to compress the air, in the tube of the tyre.

The bicycle pump, acts like a manually operated compressor.

We can also have electrically driven air compressors.

The pressure of compressed air, can be measured, in newtons per meter squared.

It can also be expressed, as kilogram per centimeter squared.

The F P S unit for pressure, is pounds per square inch, or P S I.

The tyres in our bicycles and cars, is filled with compressed air.

If there is a small hole, in the tube of the tyre, the compressed air, will leak out.

We then have a flat tyre.

A tyre with compressed air, acts like a cushion.

It absorbs the force of bumps in the road.

A rubber ball, a tennis ball, a football, are other examples,

where we use compressed air.

The pressure of the compressed air, keeps the ball in shape.

All gases, when subjected to pressure, gets pressurised.

Atmosphere.

The Earth is surrounded, by a ocean of air.

The air surrounding our globe, extends for hundreds of kilometers.

In general, this blanket of air, is called as the atmosphere.

Air is a fluid.

Like other substances, air also has mass.

The density of air, is however very low, compared to solids and liquids.

Though the density is low, air still has mass.

The column of air, surrounding the Earth, exerts a downward pressure.

This pressure, is called as the atmospheric pressure.

Standard atmospheric pressure, is widely used in science.

Atmospheric pressure, is measured and standardised.

It is = 101,325 pascals, or 101,325 newtons per meter squared.

This can be expressed as about, 101.3 kilo pascals.

This works out to about, 1.03 kg per centimeter squared.

We might be surprised, that such a force, is being exerted,

on the table that we are using.

Air like water, exerts pressure, in all directions.

When a balloon is filled with air, its air pressure, will be the same,

at all points in the balloon.

This is true for atmospheric pressure also.

Atmospheric pressure is exerted, over the table, and under the table.

The net pressure, due to the atmosphere, on the table, is zero.

We do not feel atmospheric pressure, because our body, is designed to resist it.

We can sense, atmospheric pressure indirectly.

We use a straw, to drink a beverage.

How does the beverage, climb up the straw?

When we suck on the straw, the air is drawn out.

A vacuum is created in the straw.

The atmospheric pressure, acting on the beverage, pushes it up the straw.

This principle, is used in measuring atmospheric pressure.

When there is a column of water, the pressure varies according to the depth.

Atmosphere exhibits a similar behaviour.

When the altitude increases, atmospheric pressure decreases.

The pressure on top of Mount Everest, is much lower than, at the ground level.

This is the reason, Everest mountaineers carry oxygen cylinders, to breathe.

At high altitudes, the column of air, on top, is less than, the column of air,

at the ground level.

This is one of the reasons, the atmospheric pressure decreases, as altitude increases.

The atmospheric pressure, near the Earth, is higher.

This pressure, results in compressing the air.

The density of air, near the ground, is higher.

The density of air, decreases, with increase in altitude.

At higher altitudes, there is a smaller column of air.

This smaller column of air, is also less dense.

Pressure and density of air, are interrelated.

At higher altitudes, both pressure and density, of the atmosphere decreases.

The atmosphere extends, for hundreds of kilometres above the Earth.

Scientists have classified, the different layers of atmosphere.

There is no definite boundary, between the layers.

Just like latitudes and longitudes, these imaginary layers,

help to understand the atmosphere.

The ranges given below are approximate.

From the surface, to a height of 12 km is called the troposphere .

From 12 to 50 km, is called the stratosphere.

From 50 to 85 km, is called the mesosphere.

From 85 to 675 km, is called the thermosphere.

Above that, is called the exosphere.

Most of weather conditions, happen in the troposphere.

Commercial aviation, also takes place in the troposphere.

A Jet aircraft, flying at an altitude of 10 km,

will have very less ambient pressure.

This is the reason, passenger aircraft, are artificially pressurised.

It helps us breathe normally, during the flight.

The international space station, orbits at an altitude of about 350 km.

This is in the thermosphere layer.

Due to the low pressure,in the thermosphere,

astronauts wear space suits, to breathe and live,

when they go for a space walk.

Barometer.

A barometer is an instrument, to measure atmospheric pressure.

It comprises of a tube, with a vacuum created in it.

One end of the tube, is sealed.

The other end, is open, and placed inside a reservoir, of mercury.

The atmospheric air pressure, presses down, on the reservoir, of mercury.

This pushes up, the mercury in the vacuum tube.

This is similar to what happens, when we drink a beverage, with a straw.

A barometer, uses mercury, because it is a heavy liquid.

The height to which the mercury rises, is the indicator, of the atmospheric pressure.

It measures the difference, between atmospheric air pressure, and the vacuum.

Pressure in the vacuum, can be assumed to be zero.

So, the pressure measured, reflects the atmospheric pressure.

Atmospheric pressure, can be expressed in millimeters of mercury.

Standard atmospheric pressure, is 763 mm, of mercury.

For historical reasons, this unit of measurement, is still used.

This also is referred to, as one standard atmosphere.

One atmosphere, is equal to 101.3 kilo pascals.

Pressure and temperature.

By nature, molecules in a gas, are loosely packed.

They freely move about.

When the temperature is increased, gases expand.

The density of the gas reduces.

It becomes lighter.

Hot air, is lighter than cold air.

Hot air tends to rise up, in the atmosphere.

When a gas, in a closed container, is heated,

It is unable to expand.

The pressure of the gas, inside the container, increases.

In closed containers, pressure of the gas increases, with increase in temperature.

When we heat water, which is a liquid,

it turns into steam, which is a gas.

Heat can change the state of matter, in a substance.

Heat imparts energy, to the molecules, in a liquid.

We can observe the turbulence, in a boiling liquid,

due to increased energy of the molecules.

When further heat is applied, molecules escape the liquid, to become a gas.

The temperature at which, a liquid turns into gas, is called as the boiling point.

Liquids, can be vaporised, by heating it, to it’s boiling point.

When a gas is cooled, the reverse happens.

It becomes a liquid.

Steam when cooled, condenses and becomes water.

Gases, if sufficiently cooled, can be liquefied.

The boiling point, of a liquid, varies with ambient air pressure.

When air pressure is reduced, the boiling point, is also reduced.

Under lower pressure, liquids turn more readily, into gas.

Under lower pressure, liquid molecules, find it more easier,

to become a gas.

When air pressure is increased, the boiling point, also increases.

Under higher pressure, liquid molecules, find it more difficult,

to become a gas.

When a gas is pressurised, it tends to become a liquid.

If sufficiently pressurised, gases can be liquefied.

The boiling point of propane, is minus 42 degrees Celsius.

Under normal room temperature, propane is a gas.

When propane is pressurised, to the equivalent of 14 atmospheric pressure,

it becomes a liquid.

It’s boiling point, under this high pressure, has increased to 38 degree celsius.

Even under normal room temperatures, pressurised propane gas, is in a liquid state.

Propane is a fuel, which we use for cooking.

It will be inconvenient to transport it, as a gas.

It is transported, and supplied to house holds, in a liquid form.

We call it liquefied petroleum gas, or LPG.

The cylinder holding LPG, is designed to withstand the heavy pressure,

that the gas is in.

In gases, temperature and pressure, are closely interrelated.

Refrigerator.

The relationship between temperature, air pressure, and the state of the matter,

has been harnessed, for many practical and useful applications.

The refrigerator or fridge is one example.

A refrigerator uses a coolant, called as the refrigerant to transfer heat,

from inside the refrigerator, to the room outside.

Depending on the pressure, and temperature, the refrigerant can be liquid or a vapour.

It uses a mechanical compressor, to compress the refrigerant.

It uses a evaporator, to vaporise the refrigerant.

The refrigerant goes through a vapour compression cycle, with multiple steps.

Low pressure refrigerant vapour, enters the compressor .

The compressor, compresses the vapour.

This process also results in heating the vapour.

The super heated vapour, under high pressure, passes through a condenser.

A condenser is a array of tubes, which we can find, at the back of the fridge.

This causes the vapour, to cool down, by exposure to room temperature.

The cooling causes the vapour to liquefy.

The liquid under pressure, is passed through a vaporiser.

The vaporiser, is like a aerosol can.

The evaporator has a pin sized hole, in a tube.

When the liquid passes, through the pin sized hole, the pressure suddenly drops.

This causes some of the liquid, to evaporate.

The latent heat for the evaporation, is drawn from the remaining liquid.

This causes the liquid portion of the refrigerant to cool down.

This cool refrigerant, is passed through coils.

A fan blows air, across the coils, to the inside of the fridge.

This lowers the temperature, in the fridge, and cools the inside space.

The refrigerant completely evaporates, during this process.

The vaporised refrigerant, now goes to the compressor.

The cycle repeats.

The coils with cool refrigerant, absorb the heat, from the fridge.

The condenser transfers the heat, carried from inside the fridge,

to the ambient air in the room.

The same principle for cooling, is used in freezers, and air conditioners.

Atmospheric pressure and weather.

Many factors are involved, in the making of weather conditions.

Atmospheric pressure, is one of them.

The air in the atmosphere, is not static.

It is constantly on the move.

We can feel the breeze, of wind movement.

We can observe, the movement of clouds.

Sunlight causes the Earth to heat up.

Sunlight also, heats up the seas and oceans.

Earth heats up, much faster than water.

The air close to the Earth, also heats up.

Hot air, is lighter, than cold air.

The hot air, rises up in the atmosphere.

This causes air pressure, to drop.

The air over the sea, which is cooler, comes in, and equalises the pressure.

The resultant wind movement, can be felt, in coastal areas, as sea breeze.

Differential heating, of Earth and water, is one of the factors,

responsible for wind movement.

Wind movements are called as air currents.

The Earth does not receive, uniform sunlight.

The amount of sunlight received, varies with the latitude, of the place.

The regions near the equator, is called as the tropics.

Tropical regions, receive more sunlight.

Regions in the north of the tropics, and the south of the tropics,

receive less sunlight.

The region near the north pole, is called as the arctic region.

The region near the south pole, is called as the antarctic region.

The arctic and antarctic regions, receive the least amount of sunlight.

We can now visualise, that the Earth and the seas, are ge heated,

at differential rates, depending on the latitude.

This differential heating, causes differential air pressure, across the Earth.

Northern hemisphere tilts towards the sun, during northern summer.

This causes, differential heating, in the northern and southern hemisphere.

This results in differential air pressure, in the northern and southern hemisphere.

Differential air pressures, causes continuous air currents, throughout the globe.

When these currents, pass over the oceans, they pickup moisture.

The air currents with the moisture, are the source for rain and snow precipitation.

Extreme weather , such as storms, cyclones and hurricanes,

are also caused by differential air pressure conditions.

Scientists who study the weather, are called meteorologists.

Air pressure, is one of the important factors, that meteorologists study,

to understand and predict weather.

Buoyancy in air.

We know that liquids exert, a buoyant force.

This force enables some objects to float, in the liquid.

Air is a fluid.

Air also has buoyant properties.

Helium is lighter than atmospheric air.

A balloon filled with helium, will float in air.

Hot air, is also lighter than normal air.

The first flight of man, took place in 1783.

Two french men, made the first flight, using a hot air balloon.

They went up 150 meters, and the flight duration was 25 min.

This was mankind’s first step in aviation.

Hydrogen and helium balloons, were used for transport, for a brief period.

These balloons were called air ships.

Air ships could carry many passengers, and could cross the atlantic ocean.

Design of air ships, used the same principle of buoyancy as ships designed for seas.

They displaced more air, then their own weight.

These air ships, were very large balloons, filled with light gas.

It had a attached basket like compartment, hanging below, to carry passengers.

After the invention of aircraft, airships went out of use.

Aircraft.

Modern aircraft use a different principle, from air ships, to fly.

When the velocity of air increases, the pressure decreases.

This is called as the Bernoulli’s principle.

The wing of an aircraft, is designed with a curved surface at the top,

and a flat surface at the bottom.

When air moves over the top surface, it speeds up.

This causes a lower pressure, on the top of the wing.

The differential pressure, between the top and bottom of the wing, creates a lift force.

The wings creates the lift force, for the aircraft.

A jet engine provides the forward thrust, for flight.

As the aircraft moves forward, the wings provide the lift, for the aircraft.