Exe-4B Pressure in Fluids and Atmospheric Pressure Selina Physics Class 9th Physics Revised Selina Concise Solutions Chapter-4 .Step By Step Revised Concise Selina Physics Solutions of Chapter-4 Pressure in Fluids and Atmospheric Pressure  with Exercise-4(A) with Numericals and MCQ, Exercise-4(B) with Numericals and MCQ for Class-9. Visit official Website CISCE for detail information about ICSE Board Class-9.

 Board ICSE Publications Selina Publication Subject Physics Class 9th Chapter-4 Pressure in Fluids and Atmospheric Pressure Exe-4(B) Book Name Concise Topics Solution of  Exercise-4(B),  MCQ-4(B),   Numericals-4(B), Academic Session 2021-2022

## Exe-4B Pressure in Fluids and Atmospheric Pressure Selina Physics Class 9th

–: Select Topics :–

Exe-4(A),  MCQ-4(A), Numericals-4(A),

Exercise-4(B),   MCQ-4(B) ,   Numericals-4(B),

Note :-  Before Viewing Selina Concise Physics Solutions of Chapter-4 Pressure in Fluids and Atmospheric Pressure Class-9 Physics . Read the whole chapter carefully and Solved all example of Chapter-4 Pressure in Fluids and Atmospheric Pressure Class-9 Physics.

This post is depend on Pressure in Fluids and Atmospheric Pressure Chapter-4 Concise Physics class 9th. There are solutions of exercise, Multiple Type Question, Very Short Answer Type Question.  these solution  are given of Concise 9th Physics of Chapter-4 (Pressure in Fluids and Atmospheric Pressure).

### Exercise-4 (B) Class-9th Physics Atmospheric pressure in Fluids and Atmospheric Pressure

Page 101

#### Question 1

What do you understand by atmospheric pressure?

The thrust exerted per unit area of the earth surface due to column of air, is called the atmospheric pressure on the earth surface.

#### Question 2

Write the numerical value of the atmospheric pressure on the surface of earth in pascal.

1.013 x 10 5 pascal

#### Question 3

What physical quantity is measured in torr ? How is it related to the S.I. unit of the quantity?

Atmospheric pressure is measured in ‘torr’.

1 torr = 1 mm of Hg.

#### Question 4

Name the physical quantity which is expressed in the unit ‘atm’. State its value in pascal.

At normal temperature and pressure, the barometric height is 0.76 m of Hg at sea level which is taken as one atmosphere.

1 atmosphere = 0.76 m of Hg = 1.013 x 105 pascal

#### Question 5

We do not feel uneasy even under enormous pressure of the atmosphere above as well as around us. Give a reason.

We do not feel uneasy under enormous pressure of the atmosphere above as well as around us because of the pressure of our blood, known as blood pressure, is slightly more than the atmospheric pressure. Thus, our blood pressure balances the atmospheric pressure.

#### Question 6

Describe an experiment to demonstrate that air exerts pressure.

Experiment to demonstrate that air exerts pressure:

Take a thin can fitted with an airtight stopper. The stopper is removed and a small quantity of water is boiled in the can. Gradually the steam occupies the entire space of can by expelling the air from it [Fig (a)]. Then stopper is then tightly replaced and simultaneously the flame beneath the can is removed. Cold water is then poured over the can.

It is observed that the can collapses inwards as shown in fig (b).

The reason is that the pressure due to steam inside the can is same as the air pressure outside the can [Fig (a)]. However, on pouring cold water over the can, fitted with a stopper [fig (b)], the steam inside the can condenses producing water and water vapour at very low pressure. Thus, the air pressure outside the can becomes more than the vapour pressure inside the closed can.

Consequently, the excess atmospheric pressure outside the can causes it to collapse inwards.

#### Question 7

Explain the following :

(i) A balloon collapses when air is removed from it.

(ii) Water does not run out of a dropper unless its rubber bulb is pressed.

(iii) Two holes are made in a sealed tin can to take out oil from it.

(i) Why balloon collapses when air is removed from it

When air is removed from the balloon, the pressure inside the balloon (which was due to air in it) is much less than the atmospheric pressure outside and hence the balloon collapses.

##### (ii) why Water does not run out of a dropper unless its rubber bulb is pressed

Water is held inside the dropper against the atmospheric pressure because the pressure due to height column of liquid inside the dropper is less than the atmospheric pressure. By pressing the dropper we increase the pressure inside the dropper and when it becomes greater than the atmospheric pressure the liquid comes out of the dropper.

##### (iii) why Two holes are made in a sealed tin can to take out oil from it

There is no air inside a completely filled and sealed can. When a single hole is made to drain out the oil from the can, some of the oil will come out and due to that the volume of air above the oil will increase and hence the pressure of air will decrease. But if two holes are made on the top cover of the can, air outside the can will enter it through one hole and exert atmospheric pressure on the oil from inside along with the pressure due to oil column, and it will come out of the can from the other hole.

#### Question 8

Why does the liquid rise in a syringe when its piston is pulled up?

When syringe is kept with its opening just inside a liquid and its plunger is pulled up in the barrel, the pressure of air inside the barrel below the plunger becomes much less than the atmospheric pressure acting on the liquid. As a result, the atmospheric pressure forces the liquid to rise up in the syringe.

#### Question 9

How is water drawn up from a well by a water pump?

In a water pump, on pulling the piston up, the pressure of air inside the siphon decreases and the atmospheric pressure on the water outside increases. As a result, the atmospheric pressure pushes the water up in pump.

#### Question 10

A partially inflated balloon is placed inside a bell jar connected to a vacuum pump. On creating vacuum inside the bell jar , balloon gets more inflated, how does the pressure change ; increase , decrease or remain same , inside the (a) bell jar and (b) balloon ?

(a) decrease

(b) decrease

#### Question 11  Exe-4B Pressure in Fluids and Atmospheric Pressure

What is the purpose of a barometer?

A barometer is used to measure atmospheric pressure.

#### Question 12

What is a barometer? How is a simple barometer constructed ?

A barometer is an instrument which is used to measure the atmospheric pressure.

##### Construction of a simple barometer:

A simple mercury barometer can be made with a clear, dry, thick-walled glass tube about 1 metre ling. The glass tube is sealed at one end and is filled with mercury completely. While filling the tube with mercury care has to be taken so that there are no air bubbles present in the mercury column. Close the open end with thumb and turn the tube upside down carefully over a trough containing mercury. Dip the open end under the mercury level in the trough and remove the thumb.

##### working of simple barometer:

The mercury level in the tube falls until it is about 76 cm (h =760 mm) vertically above the mercury level. It is the atmospheric pressure acting on the surface of the mercury in the trough that supports the vertical mercury column. The empty space above the mercury column is called the ‘Torricellian vacuum’.

#### Question 13

Explain how is the height of mercury column in tube of a simple barometer is a measure of the atmospheric pressure.

#### Answer 13 Pressure in Fluids and Atmospheric Pressure

In given figure, at all points such as C on the surface of mercury in trough, only the atmospheric pressure acts. When the mercury level in the tube becomes stationary, the pressure inside tube at the point A, which is at the level of point C, must be same as that at the point C. The pressure at point A is due to the weight (or thrust) of the mercury column AB above it. Thus, the vertical height of the mercury column from the mercury surface in trough to the level in tube is a measure of the atmospheric pressure.

The vertical of the mercury column in it (i.e., AB = h) is called the barometric height.

Had the pressure at points A and C be not equal, the level of mercury in the tube would not have been stationary.

#### Question 14  Exe-4B Pressure in Fluids and Atmospheric Pressure

Illustrate with the help of a labelled diagram of a simple barometer that the atmospheric pressure at a place is 76 cm of Hg.

#### Question 15

Why is the barometric height used as unit to express the atmospheric pressure?

It is the atmospheric pressure acting on the surface of the mercury in the trough that supports the vertical mercury column. Hence, barometric height is used as unit to express the atmospheric pressure.

#### Question 16

What is meant by the statement ‘the atmospheric pressure at a place is 76 cm of Hg’? State its value in Pa.

The atmospheric pressure at a place is 76 cm of Hg means at normal temperature and pressure, the height of the mercury column supported by the atmospheric pressure is 76 cm.

76 cm of Hg = 1.013 x 105 pascal

#### Question 17

How will you show that there is vacuum above the surface of mercury in a barometer? What name is given to this vacuum?

The space above mercury is a vacuum. This empty space is called ‘Torricellian vacuum’.

This can be shown by tilting the tube till the mercury fills the tube completely. Again when the mercury column becomes stationary, the empty space is created above the mercury column. If somehow air enters into the empty space or a drop of water gets into the tube, it will immediately vaporize and the air will exert pressure on mercury column due to which the barometric height will decrease.

#### Question 18

How is the barometric height of a simple barometer affected if

(a) Its tube is pushed down into the trough of mercury?

(b) Its tube is slightly tilted from vertical?

(c) A drop of liquid is inserted inside the tube

(a) The barometric height remains unaffected.

(b) The barometric height remains unaffected.

(c) The barometric height decreases.

Page 102

#### Question 19

State two uses of a barometer.

Two uses of barometer:

(a) To measure the atmospheric pressure.

(b) For weather forecasting

#### Question 20  Exe-4B Pressure in Fluids and Atmospheric Pressure

Give two reasons for use of mercury as a barometric liquid.

Two advantages of using mercury as barometric liquid:

(i) The density of mercury is greater than that of all the liquids, so only 0.76m height of mercury column is needed to balance the normal atmospheric pressure.

(ii) The mercury neither wets nor sticks to the glass tube therefore it gives the correct reading.

#### Question 21

Give two reasons why water is not a suitable barometric liquid.

Water is not a suitable barometric liquid because:

(i) The vapour pressure of water is high, so its vapours in the vacuum space will make the reading inaccurate.

(ii) Water sticks with the glass tube and wets it, so the reading becomes inaccurate.

#### Question 22

Mention two demerits  of a simple barometer and state how they are removed in a Fortin barometer.

In a simple barometer, there is no protection for the glass tube but in Fortin’s barometer, this defect has been removed by enclosing the glass tube in a brass case.

In a simple barometer, a scale cannot be fixed with the tube (or it cannot be marked on the tube) to measure the atmospheric pressure but Fortin’s barometer is provided with a vernier calipers to measure the accurate reading.

#### Question 23

Draw a simple labelled diagram of a Fortin barometer and state how it is used to measure the atmospheric pressure.

To measure the atmospheric pressure, first the leather cup is raised up or lowered down with the help of the screw S so that the ivory pointer I just touches the mercury level in the glass vessel. The position of the mercury level in the barometer tube is noted with the help of main scale and the vernier scale. The sum of the vernier scale reading to the main scale reading gives the barometric height.

#### Question 24

What is an aneroid barometer? Draw a neat and labelled diagram to explain its construction and working.

A barometer calibrated to read directly the atmospheric pressure is called an aneroid barometer. It has no liquid, it is light and portable.

##### Construction aneroid barometer

Figure above shows the main parts of an aneroid barometer. It consists of a metallic box B which is partially evacuated. The top D of the box is springy and corrugated in form of a diaphragm as shown. At the middle of diaphragm, there is a thin rod L toothed at its upper end. The teeth of rod fit well into the teeth of a wheel S attached with a pointer P which can slide over a circular scale. The circular scale is initially calibrated with a standard barometer so as to read the atmospheric pressure directly in terms of the barometric height.

##### Working aneroid barometer

When atmospheric pressure increases, it presses the diaphragm D and the rod L gets depressed. The wheel S rotates clockwise and pointer P moves to the right on the circular scale. When atmospheric pressure decreases, the diaphragm D bulges out due to which the rod L moves up and the wheel S rotates anti-clockwise. Consequently, the pointer moves to the left.

#### Question 25  Exe-4B Pressure in Fluids and Atmospheric Pressure

State two advantages of an aneroid barometer over a simple barometer.

Android barometer has no liquid and it is portable. It is calibrated to read directly the atmospheric pressure.

#### Question 26

How is the reading of a barometer affected when it is taken to (i) a mine, and (ii) a hill?

(i) In a mine, reading of a barometer increases.

(ii) On hills, reading of barometer decreases.

#### Question 27

How does the atmospheric pressure change with altitude? Draw an approximate graph to show this variation.

The atmospheric pressure decreases with an increase in the altitude.

#### Question 28

State two factors which affect the atmospheric pressure as we go up.

Factors that affect the atmospheric pressure are:

(i) Height of air column

(ii) Density of air

#### Question 29

Why does a fountain pen leak at high altitude?

A fountain pen filled with ink contains some air at a pressure equal to atmospheric pressure on earth’s surface. When pen is taken to an altitude, atmospheric pressure is low so the excess pressure inside the rubber tube forces the ink to leak out.

#### Question 30

Why does nose start bleeding on high mountains?

On mountains, the atmospheric pressure is quite low. As such, nose bleeding may occur due to excess pressure of blood over the atmospheric pressure.

#### Question 31

What is an altimeter? State its principle. How is its scale calibrated?

An altimeter is a device used in aircraft to measure its altitude.

Principle: Atmospheric pressure decreases with the increase in height above the sea level; therefore, a barometer measuring the atmospheric pressure can be used to determine the altitude of a place above the sea level.

The scale of altimeter is graduated with height increasing towards left because the atmospheric pressure decreases with increase of height above the sea level.

#### Question 32  Exe-4B Pressure in Fluids and Atmospheric Pressure

What do the following indicate in a barometer regarding weather :

(a) gradual fall in the mercury level,

(b) sudden fall in the mercury level,

(c) Gradual rise in the mercury level?

(a) It indicates that the moisture is increasing i.e., there is a possibility of rain.

(b) It indicates the coming of a storm or cyclone.

(c) It indicates that the moisture is decreasing i.e., it indicates dry weather.

MULTIPLE  CHOICE TYPE 4(B) Atmospheric pressure in Fluids and Atmospheric Pressure Chapter-4 Solutions

Page 102

#### Question 1

The unit torr is related to the barometric height as:

(a) 1 torr =l cm of Hg

(b) 1 torr = 0.76 m of Hg

(c) 1 torr = 1 mm of Hg

(d) 1 torr = 1mof Hg

(c) 1 torr = 1 mm of Hg

#### Question 2

The normal atmospheric pressure is :

(a) 76 m of Hg

(b) 76 cm of Hg

(c) 76 Pa

(d) 76 N m-2

(b) 76 cm of Hg

#### Question 3

The atmospheric pressure at earth’s surface is Pand inside mine is P2. They are related as :

(a) Pl =P2

(b) Pl > P2

(c) Pl  < P2

(d) P2= 0

(c) P l  < P2

### NUMERICAL 4(B) Atmospheric pressure in Fluids and Atmospheric Pressure Chapter-4 Solutions

Page 102

#### Question 1

Convert 1 mm of Hg into pascal. Take density of Hg = 13.6 x 103 kg m-3 and g = 9.8 m s-2 .

We know that, the pressure is given by the formula,

δP = hρg

Putting the given values in above equation, we get,

δP = 0.001 x 13.6 x 1000 x 9.8

δP = 133.28 Pa

Therefore, 1 mm of Hg is equal to 133.28 pascals.

#### Question 2  Exe-4B Pressure in Fluids and Atmospheric Pressure

At a given place, a mercury barometer records a pressure of 0.70 m of Hg. What would be the height of water column if mercury in barometer is replaced by water? Take density of mercury to be = 13.6 x 103

#### Question 3

At sea level, the atmospheric pressure is 76 cm of Hg. If air pressure falls by 10 mm of Hg per 120m of ascent, what is the height of a hill where the barometer reads 70 cm Hg. State the assumption made by you.

Assumption: Atmospheric pressure falls linearly with ascent.

#### Question 4

At sea level, the atmospheric pressure is 1.04 x 105 Pa. Assuming g = 10 m s-2 and density of air to be uniform and equal to 1.3 kg m-3, find the height of the atmosphere.

Pressure = hdg
h = Pressure / (dg)
= (1.04 × 105 Pa) / (1.3 kg/m³ × 10 m/s²)
= 8000 m
= 8 kilometers
Height of the atmosphere is 8 kilometers

#### Question 5

Assuming the density of air to be 1.295 kg m-3, find the fall in barometric height in mm of Hg at a height of 107 m above the sea level. Take density of mercury = 13.6 × 103 kg m-3.