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Pressure- Physics Guide for Class 8

Information about Pressure

Title

Pressure

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Liquid Pressure
  • Atmospheric Pressure
  • Variation of Air Pressure
  • Importance of Atmospheric Pressure
  • Force and Pressure

Pressure

  • Careful observations and analysis reveal that the effect of a force also depends on the surface area over which the force acts.
  • It is a common experience that it is difficult to carry a school bag, when it is tied to narrow thin string; it becomes easier to carry the same school bag when it is tied to 'broad straps'. This implies that if the same force (say, the weight of an object) acts over a smaller area, its effect is felt more.
Thus the overall effect of a force depends on:
  1. its magnitude
  2. the area over which it acts.
We, therefore, need to define a physical quantity that takes both these factors into account. Physicists have now defined such a quantity and named it as pressure.
It is pressure which is a measure of the effect of force over a given area. When we apply force in a direction perpendicular to a given surface area, we call it as thrust
The thrust, acting, on a unit area of a surface, is called pressure.
  • Pressure = Thrust/Surface area over which it acts (contact area) 

It follows that the pressure, due to a given force, would vary according to the area over which the force is acting. We can therefore, increase or decrease pressure, without any change in force, by changing the surface area over which :he force acts. To understand this, let us perform an activity. 

Activity 1
Take some moulding clay. Spread a thick [3 to 5 cm] layer of this moulding clay on the desk. Place a wooden block on the surface of the moulding clay (length-wise). Now, place a book on it for sometime. Remove the wooden block and book. Measure the depression produced in the moulding clay with the help of a scale.
Repeat the above steps keeping the wooden block (breadth-wise/thickness-wise) on the surface of the moulding clay. What do we observe?
  • It is the same force that is acting on the clay in all the three cases. However, the effect produced is different because of the difference in the contact area. We, thus, realise that for a given applied force, when the contact area is less, its pressure, on the surface in contact, becomes more.

Applications

The decrease in the pressure of a given force, through an increase in the surface area over which the force acts, finds many applications in our day to day life. We list below some such applications:
  1. Buses and trucks usually have double wheels on the rear side.
  2. High rise buildings and dams have a wide base.
  3. Tanks and bulldozers are fitted with caterpillar tracks rather than wheels.
  4. Railway tracks are laid on large sized wooden/iron sleepers.
There are many situations where we need to have a larger pressure due to a given force. In such cases, we decrease the area over which the force acts.
For example,
  1. It is easier to cut with a sharp knife than with a blunt knife.
  2. Nails, pins and spikes have pointed ends so that they can be driven into the surface with minimum effort. 

Liquid Pressure

  1. It is easy to observe that a liquid exerts pressure, due to its weight, on the bottom of its container. This is much the same way as a solid does on the surface supporting it.
  2. The pressure, exerted by a stationary liquid (kept in a container) at any point inside the liquid, is known as hydrostatic (liquid) pressure
Activity 2
Take a transparent pipe (plastic/glass). The length of the pipe should be 15-20 cm and its diameter may be about 6 cm. Stretch a rubber balloon/sheet over one end of the pipe. Hold the pipe in vertical position as shown in the figure. Now, pour some water in the pipe. What do we observe? The rubber balloon bulges out. Note the height of water column in the pipe. Pour some more water. Observe again the bulge in the rubber balloon. What do we infer? Pressure, exerted by the water column at the bottom of the container, increases with an increase in the height of its column. 

Activity 3
Take a tin can, some coloured water, a sharp pin/nail and some cellotape. Make holes with the pin/nail at four different points, along a vertical line in the tin can, as shown in the figure. These holes should be equidistant. Cover the holes with cellotape. Place the can on a stool and fill it with coloured water. Now, remove the tapes from the holes and observe the streams of water coming out of these holes. We observe that the stream, from the lower holes travel a larger distance. Why? What do we infer?

The emerging water goes out farther from the lower holes; this is because the pressure of water increases with an increase in the 'depth' of the hole. Hence the water pressure, at a point, increases with the height of water column above it.
We thus observe that, for a particular liquid, the pressure, exerted at any point, is directly proportional to the height of liquid column above that point (or depth of that point below the surface); however, this pressure is different for different liquids. 

Activity 4
Make a number of holes at the same level in a tin can using a sharp (pin/nail). Repeat the steps of the earlier activity. When tapes are removed, water is seen to emerge out from all these holes with equal force. Mark the points, on the floor, where the water has fallen. Join these points to form a closed figure. What do we observe? The closed figure is (nearly) a circle, with the (centre of the) can approximately at its centre. This illustrates that the liquid pressure is transmitted equally in all directions, and is same at a given horizontal level. 

Properties of liquid pressure

Careful observations show that the pressure, exerted by a liquid, has the following characteristics: 
  • Liquid pressure, on the bottom of the container (due to weight of liquid column), does not depend on the area of the bottom. 
  • Liquid pressure, at any point inside the liquid, depends upon the density of the liquid and the height of liquid column above that point.
  • Liquids exert (an equal) pressure on all the walls of the container. 
  • An external pressure, applied on a liquid in a closed container, is transmitted uniformly throughout the liquid. 

Atmospheric Pressure

We all know that there is air all around us. It is the earth's gravitational pull that holds this air around us.
This envelope of air, around the earth, is known as atmosphere.
It extends up to nearly 1,000 km above the surface of earth. The weight of this huge mass of air exerts a pressure, at all points and at all objects, on the earth. We call this pressure as the atmospheric pressure or air pressure
We now know that pressure is thrust per unit area. We imagine a unit area at a place on earth.
Let the height of the atmosphere above that place be H. The weight of an air column, 'contained' in a cylinder, of height (H) having a base of unit area, is the atmospheric pressure at that place.

Why is it that we do not feel this large atmospheric pressure acting on us all the time?
We do not normally feel it because there is also a pressure inside our bodies that is almost same as this external air pressure. This internal pressure cancels the effect of this outside pressure and saves us from getting crushed under it. 

Activity 5
  • Pour some hot water into a (good quality) plastic bottle, carefully.
  • Close the lid of the bottle. Shake it for half minute.
  • Now, pour out the water and (quickly) close the lid of bottle very tightly. 
We observe that the walls of the bottle are seen to get deformed and may get crushed inwards. This happened because the hot water heats up the air in the bottle and causes it to expand. A good part of the air inside the bottle, therefore, escapes out. When the lid is now (quickly) closed, there is less air inside the bottle. The pressure of the outside air, therefore, becomes (considerably) more than the pressure of air inside. It is this (large) difference in pressure, acting inwards, that can deform and crush the bottle.

Variation in Air Pressure

  • As we move upwards through the atmosphere, the height of air column, above us, would decrease. This would result in a decrease in air pressure at higher altitudes.
  • In fact, when we move towards higher altitudes, breathing can become difficult. Sometimes bleeding from nose may also occur.
  • Most climbers, who attempt to scale high range mountains, (like Mount Everest), need to carry oxygen cylinders with them. For this very reason, aircrafts have 'pressurised cabins'. The air pressure in these is increased to a (sufficient) value that safeguards the passengers and the crew.
  • Air pressure also varies with temperature and time at a given place. We have already learnt that due to uneven heating of earth's surface, air pressure can change rapidly.
  • At a hotter place, the warm air there is lighter than the cooler air in the surrounding regions. Hence, air can rush in from (the neighbouring) high pressure surrounding area to this lower pressure area. This phenomenon can result in land breezes, sea breezes, winds and storms.

Importance of Atmospheric Pressure

We make use of atmospheric pressure in our day to day life while performing very many simple tasks.
For example:
  1. When we drink liquid with a straw, the air pressure inside the straw decreases (due to our sucking). The air pressure, acting on the surface of liquid, then becomes greater than the pressure inside the straw. This forces the liquid to move up inside the straw. The syringe also works in a similar way. 
  2. We, sometimes, use rubber suckers for installing hooks in a room. When we press the sucker the air between the air sucker and the wall gets forced out. Hence, the air, pressing on it from outside, holds it firmly against the wall. If we wish to pull the sucker off the surface, the force, applied by us, has to be large enough to overcome this effect of the atmospheric pressure.

Force and Pressure

Force and pressure are two different concepts. At times, we tend to use these word! interchangeably. This needs to be avoided. Let us bring out the differences between the two by making a Concept Map (Flowchart).

Important Points

  • In case of solids, the force can be applied in any direction with respect to the surface. However, in case of fluids (liquid/gases) at rest, the force must be applied at right angles to the liquid surface. This is because fluids, at rest, cannot sustain a tangential force. We, therefore, usually speak in terms of pressure in their case.
  • Pressure always acts normal to the surface and it is always compressive in nature. We, therefore, need only its magnitude for its complete description.
  • The pressure, exerted by a given liquid, increases with depth. It is for this reason that submarines are always built with very thick and heavy metals. They have to withstand an enormous water pressure when they go deep down, near to ocean floors.
  • Pressure applied at any point in a liquid is transmitted equally in all directions. Hydraulic jack (used for lifting a car), the car hoist (used for lifting the car for washing at service stations) and hydraulic brakes (used in cars for applying brakes to their wheels) are all based on this principle of 'equality of transmission of pressure' in liquids.
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