PHYSICS [3 Mark Question Answer]

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.

Atmospheric pressure , P = 76 cm Hg
Rate at which pressure falls = 10 mm of Hg per 120 m of ascent = 1 cm of Hg per 120 m of ascent
Let h be the height of the hill.
Pressure at hill , P' = 70 cm Hg
Total fall in pressure = P - P' = (76 - 70) cm Hg = 6 cm Hg
Now , fall in pressure is 1 cm Hg for every 120 m increase in height
Thus , if the fall in pressure is 6 cm Hg , increase in height shall be (6 × 120)m = 720 m
∴ Height of the hill = 720 m

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.

The atmospheric pressure decreases with an increase in the altitude.

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.

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.

Pressure exerted by thrust is inversely proportional to area of surface on which it acts. Thus, larger the area on which the thrust acts, lesser is the pressure exerted by it. Example: If we stand on loose sand, our feet sink into the sand, but if we lie on that sand, our body does not sink into the sand. In both the cases, the thrust exerted on the sand is equal (equal to the weight of the body). However, when we lie on sand, the thrust acts on a large area and when we stand, the same thrust acts on a small area.

Laws of liquid pressure:
(i) Pressure at a point inside the liquid increases with the depth from its free surface.
(ii) In a stationary liquid, pressure is same at all points on a horizontal plane.
(iii) Pressure is same in all directions about a point in the liquid.
(iv) Pressure at same depth is different in different liquids. It increases with the increase in the density of liquid.
(v) A liquid seeks its own level.

The reason is that when the bubble is at the bottom of the lake, total pressure exerted on it is the atmospheric pressure plus the pressure due to water column. As the gas bubble rises, due to decrease in depth the pressure due to water column decreases. By Boyle's law, PV = constant, so the volume of bubble increases due to decrease in pressure, i.e., the bubble grows in size.

Take a can or large plastic bottle filled with water. Place it on a horizontal surface. Make a series of holes in the wall of the vessel anywhere below the free surface of the liquid. The water spurts out through each hole. This shows that the liquid exerts pressure at each point on the wall of the bottle



Liquid exerts pressure at all points in all directions