Question 1 Why holes are made in hanging banners and hoarding?

Question 2 Why does the air filled in a bicycle tube keep it tight?

Question 3 State few observations which tell us that air exerts pressure?

Question 4 Why when air is filled into a balloon, the balloon gets inflated?

Question 5 Why do the leaves of trees, flags and banners flutter when the wind is blowing?

Question 6 Why is it easier to row a boat if there is wind coming from behind you?

Question 7 When you fly a kite, the wind coming from your back helps you? Explain?

Question 8 Define the term expansion of air and contraction of air?

Question 9 How will you show that air expands on heating?

Question 10 Why does hot air rise upwards?

Question 11 Why smoke always rise up?

Question 12 Why a house must have windows as well as ventilators?

Question 13 Why is it difficult to ride a bicycle against the direction of a wind?

Question 14 Define the term atmospheric pressure?

Atmosphere

Air is a mixture of colourless gases.

The layer of air above the earth is called atmosphere.

 

The amount of water vapour in the atmosphere depends on temperature and humidity. The atmosphere exerts a pressure called atmospheric pressure due to the weight of air contained in it. The atmospheric pressure decreases with height. All the phenomena such as winds, storms, and cyclones, etc., occur in the earth’s atmosphere.

Air Exerts Pressure

The continuous physical force exerted on an object (or against an object) by something in contact with it, is called pressure. Air exerts pressure in different ways under different situations.

For example: 

(1) Air enclosed in a container (like a balloon or bicycle tube) exerts pressure,

(2) Air in the atmosphere around us exerts pressure, and

(3) Moving air (called wind) exerts pressure.

Air is made up of tiny particles called molecules which move around quickly in all directions. 

If we put air in a closed container then the fast moving air molecules collide’ with the walls of the container and exert a force on the walls of the container from inside. This force produces pressure.If we put more air into the container, then the container will have more air molecules in it.Due to more air molecules, the number of collisions of air molecules with the walls of the container will increase and hence the pressure exerted by air will also increase.

Activity: Air exerts pressure

1) Take a rubber balloon and fill air into it with mouth.On filling air, the balloon gets inflated (it expands and becomes bigger in size). When we put air in the balloon, then the number of air molecules in the balloon increases. These air molecules cause collisions with the rubber walls of balloon and create an air pressure. The air pressure acting on the thin rubber walls of the balloon from inside causes the balloon to expand and get inflated. Just like balloon, a football also gets inflated when air is filled in it. The football gets inflated because the air filled in it exerts pressure.

2) The bicycle tube is a rubber tube . When we fill air into the bicycle tube, then the air molecules inside the tube collide with the walls of the tube and exert air pressure. This air pressure exerted from inside, inflates the bicycle tube, So, it is the pressure exerted by air filled in a bicycle tube which keeps the tube tight and makes the bicycle tyre feel hard. If we go on filling more and more air into bicycle tube with a pump, then ultimately the air pressure in the bicycle tube will increase too much due to which the bicycle tube may even burst. 

The atmosphere contains a tremendous amount of air. Air has weight, so the atmosphere consisting of tremendous amount of air has enormous weight. The weight of atmosphere (or air) above us exerts a pressure on the surface of earth and on all the objects on the earth.

 

Atmospheric Pressure

Atmospheric pressure is the air pressure which is exerted by the weight of air present in the atmosphere above us. As we go to higher altitudes, the atmospheric pressure goes on decreasing . Atmospheric pressure acts in all directions.

Tin can is a vessel made of a thin and flexible metal sheet. An empty tin can has air inside it as well as outside it. So, the air pressure (or atmospheric pressure) acting on the walls of an empty tin can from inside and outside is equal and opposite, and balance each other. Due to this, an empty tin can (which contains air) does not get crushed by the large atmospheric pressure exerted by the air around us. 

Activity: Air around us exerts pressure

1) Take a tin can and put some water in it. Heat the tin can by using a burner to boil the water so that steam is formed.

2) This steam expels all the air from inside the tin can so that when water is boiling, there is no air inside the can, the whole can is filled with steam.

3)A tight cork is now fitted in the mouth of the tin can and heating is stopped. We now pour cold water from a tap on the hot tin can.

4) On pouring cold water, the tin can collapses inwards as if a large force acting on it from outside has crushed it.

When the water is boiling, steam is formed in the tin can. Steam has pressure. The steam pressure acting from inside the tin can balances the air pressure (or atmospheric pressure) acting on all the sides of the tin can from outside and hence the tin can does not get crushed.

When a tight cork is put on the mouth of the tin can containing steam and then cold water is
poured over it, then the hot steam inside the tin can gets cooled, condenses and forms water and water vapour at very low pressure. On cooling, the pressure inside the tin can decreases too much and it cannot balance the large air pressure acting on the tin can from outside.

So, the large air pressure outside the tin can crushes the tin can inwards.

Moving air is called wind. Moving air (or wind) exerts a force or pressure on the objects on which it strikes. The force (or pressure) exerted by the moving air (or wind) is due to the kinetic energy possessed by it.

(1) It is due to the pressure exerted by moving air (or wind) that the leaves of trees, flags and banners flutter when the wind is blowing.

A long strip of cloth used to display advertisements is called a banner and a large board used to display advertisements is called a hoarding.

Holes are usually made in hanging banners and hoardings so that high speed wind may pass through them easily without damaging them or bringing them down with its huge force or pressure.

(2) It is easier to ride a bicycle in the direction of blowing wind because the blowing wind exerts a force (or pressure) on us in the same direction in which our bicycle is moving and makes our bicycle move faster. We find it very difficult to ride a bicycle against the direction of wind. This is because in this case the blowing wind exerts a force on us in the direction opposite to the motion of our bicycle. And since the wind opposes our motion, we have to pedal very hard to keep the bicycle moving against the wind.

(3) While rowing a boat in the lake, we find it easier to row the boat forward if the wind is coming from behind us. The blowing wind exerts pressure on the boat in the direction of their motion and helps it move forward. It becomes much more difficult to row a boat against the direction of blowing wind because in this case the wind exerts a pressure opposite to the direction of motion of boat. And since the wind opposes the motion of boat, we have to apply much more force to row the boat against the direction of wind.

(4) When we are flying a kite, then the wind coming from our back side helps. This is because the wind coming from our back side, strikes the Kite and exerts pressure on the kite to make it fly higher and higher. The pressure of blowing wind will make the kite fall to the ground.

If the wind blows in the same direction in which the object is moving , it makes the movement of object easier. If the wind blows in opposite direction to the motion of the object, it makes the movement of the object difficult.

 

Expansion of air means increase in the volume of air, and the term contraction of air
means decrease in the volume of air. Air expands on heating and contracts on cooling.When air is heated, its volume increases and it occupies a bigger space. And when air gets cooled, then its volume decreases and it occupies a smaller space.

Air expands on heating

When air is heated, its volume increases and it occupies a bigger space. We say that air expands on heating.

Activity : Air expands on heating

(1) Take a boiling tube. The empty boiling tube contains air. Take a balloon stretch the mouth of the balloon and fix it tightly over the neck of the boiling tube.Make the joint of balloon with the neck of boiling tube air-tight by wrapping some adhesive tape over it.

(2) Take some hot water in a beaker. Place the boiling tube (having rubber balloon fixed to its neck) in the hot water in the beaker. After 2-3 minutes we will see that the balloon gets inflated (as if someone has filled air in it).

(3) When the boiling tube is placed in hot water, the air present in boiling tube gets heated, expands and its volume increases. The increased volume of air fills the balloon due to which the balloon gets inflated. It is the ‘expansion of air on heating which inflates the balloon when the boiling tube is placed in hot water.

If we take out the boiling tube from hot water and allow it to cool, then after some time, the balloon will get deflated . This is because the air present in boiling tube and balloon contracts on cooling due to which its volume decreases and balloon gets deflated.

Hot Air is Lighter Than Cold Air

Hot air is lighter than cold air.When air gets heated at a place, it expands, becomes lighter and rises up in the sky.

Activity : Hot air is lighter than cold air

(1) Take two paper bags (or paper cups) of exactly the same size. Hang the two paper bags in the inverted position on the two sides of a light wooden stick with the help of short threads. Tie a piece of thread in the middle of the stick. The wooden stick with two paper bags tied to its two ends will hang like a common beam balance. The wooden stick is perfectly horizontal showing that the two paper bags contain an equal mass of the same air.

(2) Put a burning candle below the open mouth of the left side paper bag. We will see that after some time, the left side of the wooden stick goes up showing that it has become lighter than the right side.

When a burning candle is placed below the left side paper bag, the air above the candle flame gets heated. The hot air, being lighter, rises up and fills the left side paper bag(by displacing the heavier, cold air from it). Since the left side paper bag now contains hot air (which is lighter than cold air present in right side paper bag), therefore, the left end of wooden stick becomes lighter and moves up.

A white, grey or black substance (formed of small particles) emitted from a burning substance is called smoke. Smoke always rises up. Since warm air is lighter (than cold air), therefore, smoke contained in warm air always rises up. The air inside a house gets warmed by the body heat of all the persons living in the house and also by the burning of fuels (LPG, etc.) in the kitchen.

We should buy a house having windows as well as ventilators. The windows are for bringing in fresh air from outside into the house. The ventilators (located near the roof of house) take out the warm and stale air from inside the house. The presence of ventilators in the house increases our comfort.

When air at a place gets heated and rises up ,then the pressure at that place is lowered (or reduced).The cold air from the surrounding areas (being at higher pressure) rushes in to fill the space vacated by hot rising air.