Pressure in Fluids Ques Ans ICSE class 9

(Q.1)  What are atmosphere and atmospheric pressure?

(Ans) There is an envelope of air around the world which is known as the atmosphere.
Atmospheric pressure is the pressure exerted by this air. It is due to the weight of the air above us.

(Q.2)  What is the atmospheric pressure in cm of mercury column and in Pascal?

(Ans) The atmospheric pressure at 0^oC and at sea level is 76 cm of the mercury column or 1.013 x 10⁵ Pa.

(Q.3)  How is barometric pressure related to the height?

(Ans) Barometric pressure is inversely proportional to height above the sea level, i.e., the barometric pressure decreases with increase in height or vice-versa.

(Q.4)  What is aneroid barometer?

(Ans) Aneroid barometer is a device which is used to measure the pressure exerted by the atmosphere without the use of liquids.

(Q.5)  Give two factors which affect the atmospheric pressure.

(Ans) Two factors which affect the atmospheric pressure are:
Temperature: An increase in temperature of air results in the decrease in the atmospheric pressure.
Moisture: An increase in moisture in the air results in the decrease in the atmospheric pressure.

(Q.6)  What is Torricellian vacuum?

(Ans) It is an empty space which is created in the tube of the simple barometer by the flow of mercury in the downward direction.

(Q.7)  Calculate the pressure exerted by vertical column of water of height 55 cm in SI system. Consider the density of water to be 1000 kg m-3, and acceleration due to gravity = 9.8 ms-2.

(Ans)

(Q.8)  What do you understand by the term lateral pressure? How is it different from pressure?

(Ans) Lateral pressure is a pressure that a fluid exerts on the sides of the container while, the pressure exerted by the fluid at the base of the container is simply known as pressure.

(Q.9)  Define the SI unit of pressure.

(Ans) The SI unit of pressure is Pascal. One Pascal is the pressure exerted by a force of 1 N on an area of 1 m².

(Q.10)  P is the pressure at some point in a liquid. State whether pressure P is a scalar or vector quantity.

(Ans) Pressure is a scalar quantity because at any fixed level inside the liquid, the pressure is exerted equally in all the directions. It implies that a definite direction is not associated with liquid pressure.

(Q.11)  What is a manometer?

(Ans) A manometer is an instrument used for measuring the pressure acting on the column of a liquid. In a manometer, a funnel is connected to a U-shaped glass rod through a rubber tube. It is immersed in the liquid whose pressure is to be measured. The pressure is measured as the difference in the height of liquid in the two limbs of the U tube.

(Q.12)  Name the factors on which the pressure exerted by liquid at a point depends.

(Ans) Depth of the point below the free surface of liquid, density of liquid and acceleration due to gravity are the factors on which pressure at a point in liquids depends.

(Q.13)  Define thurst.

(Ans) Force� perpendicular to�any surface is called thrust.

(Q.14)  Why the cutting edge of a knife should be as sharp as possible?�

(Ans) The cutting edge of a knife should as�sharp as possible� so as to� have greater pressure even for a very �small applied force for quick action.

(Q.15)  Why does a truck or motor-bus has much wider tyres?

(Ans) A truck or motor bus has wider tyres so that the pressure acting on the road due to weight of the truck may be small.

(Q.16)  What is a hydrometer? What is the principle of hydrometer?

(Ans) Hydrometer is a direct reading instrument used to measure the density or relative density of a liquid.

It is based on the principle of floatation.

(Q.17)  Why do we not feel the tremendous atmospheric pressure surrounding us?

(Ans) We do not feel the tremendous atmospheric pressure surrounding us because the pressure at which our blood contains dissolved oxygen is slightly more than the pressure of the atmosphere. Therefore, it counter-balances the atmospheric pressure.

(Q.18)  Why does our nose start bleeding when we travel in a non- pressurised aeroplane?

(Ans) The pressure drops at a very fast rate when the aeroplane suddenly gains height as compared to the pressure due to the dissolved oxygen in our blood. Due to this difference in pressure, the fine capillaries within the nose bursts and it starts bleeding.

(Q.18)  Why does the astronauts wear special space suits?

(Ans) Astronauts wear special space suits because as they go higher and higher, the outside pressure drops down at a faster rate as compared to the pressure due to the dissolved oxygen in their blood. The difference in pressure can cause the bursting of their bodies.

(Q.19)  Why does the fountain pen leak at higher altitudes?

(Ans) Fountain pen leaks at higher altitudes because the fountain pen has ink in their tube and some of the air at the ground level is at atmospheric pressure. The pressure outside the pen drops as it is carried to higher altitudes. Due to this, the air inside the tube exerts pressure and forces the ink out.

(Q.19)(a)Why are the walls of the dam thicker at the bottom and thinner at the top? (b) Why do the deep sea divers wear specially designed suits? (Ans) (a) The pressure exerted by the water on the base walls is high because pressure increases with increase in the depth. This is the reason the walls of the dams are thickened from the base so as to withstand the huge pressure. �(b) Liquids exert pressure and the pressure exerted by liquid is directly proportional to the depth of the liquid. Due to this reason, deep sea divers wear specially designed suits which prevent their bodies from getting �crushed due to the huge water pressure deep inside the sea.
(Q.20)  (a) Define the term fluid. (b) State the difference between the pressure exerted by solids and liquids.

(Ans) (a) Any substance which has no fixed shape and size and can flow is known as fluid. All liquids and gases are called fluids.

(b) A solid body exerts pressure on another surface due to its weight. Similarly, a liquid has weight and exerts pressure on the bottom as well as on the walls of the vessel in which it is enclosed. However, unlike a solid liquid exerts pressure in all directions.

(Q.21)(a) State Pascal’s law of transmission of pressure in liquids. (b) Name two machines which are based on Pascal’s law. (Ans) (a) Pascal’s law states that ‘Pressure exerted anywhere in a confined, incompressible liquid is transmitted equally in all directions, irrespective of the area on which it acts. (b) Hydraulic press and hydraulic brakes are two machines that are based on Pascal’s law.

(Q.22)  Why fluid exert pressure ?How is the pressure transmitted in a fluid?

(Ans) As fluids have weight , they exert pressure on the base and walls of the container they are enclosed .
In a confined mass of fluid, pressure applied on the fluid in any particular direction is transmitted undiminished (i.e. equally)in all directions.

(Q.23)  Why are the straps of school bags made wider?

(Ans) A student carries a school bag on his/her shoulder. The straps of the bag are made wider so that pressure due to weight of the bag falls over a wider area of the shoulder. Due to less pressure, student carrying the bag feels more comfortable.

(Q.24)  What are the various factors which affect the barometric height?

(Ans) The factors which affect the barometric height are:�
Presence of moisture in mercury: When the mercury is damp, the moisture in it evaporates. Resulting vapours exert a pressure on the mercury column, thereby reducing its height.
 Presence of dissolved metals in mercury: When the metals like tin, zinc, lead, etc. are dissolved in the mercury, then its apparent density decreases. The height of the mercury column therefore has to increase so to balance the atmospheric pressure.
Vertical height from the sea level: The vertical height of the air exerting pressure decreases on higher altitudes which results in the fall of air pressure. Thus, the barometric height decreases.
Presence of moisture in air: With increase in the moisture present in the air, the density of the air decreases which further results in the decrease in the barometric height

(Q.25)  State different laws of liquid pressure.

(Ans) Different laws of liquid pressure are listed below:

(i)Pressure increases with an increase in the depth of the liquid.

(ii)Pressure increases with an increase in the density of the liquid.

(iii) Liquid pressure is identical at all points in a horizontal plane at a known depth for an immobile liquid.

(iv)Pressure is same in all directions, about a given point within the liquid.

(v) A liquid seeks its own level.

(Q.26)  What is a hydraulic press? How is it based on Pascal’s law? Also define mechanical advantage and obtain an expression for it.

(Ans) Hydraulic press is based upon the principle of transmission of fluid pressure. In it, a small force can be used to balance a large force by making the area of the piston supporting the force larger than the area of the piston on which the effort is applied.