8th Class Science Force and Pressure Atmospheric Pressure

       Atmospheric Pressure

Our atmosphere contains large volume of mixture of gases. These gas exert pressure on the objects form all directions. This pressure is called atmospheric pressure. The atmospheric pressure is maximum at the sea level and minimum on the top of the mountains. This is because the air volume is maximum on the sea level and minimum at the top of the mountain. Thus, as we go up the pressure goes on decreasing. The air pressure on the surface of earth is 101.3 kPa and on the surface of sea-level is 760 mm of Hg.

We can demonstrate the existence of atmospheric pressure with the help of the following activity:

Take a glass jar and fill it with the water up to the brim. Place a sheet of paper on its brim so that no air molecule remains inside. Now keeping the paper in its position, invert the jar in downward direction. You will observe that, the paper does not fall off, though the jar is full of water and exerting pressure in downward direction. This is because of the fact that, the atmospheric pressure acting on the paper in the upward direction, holding the paper sheet in its position. The upward pressure on the sheet is more than the downward pressure, hence it is not falling.

           Pressure Inside Our Body

Our body contains liquid in the form of water and blood. These blood flow through the blood vessels and exert pressure on the wall of the vessels. This pressure is called blood pressure. Our body is balanced by the atmospheric pressure and blood pressure and hence we do not get crushed. Our body is so balanced by these pressures that we never feel discomfort. But if we go up high on the mountain where the pressure is very low, we start feeling uncomfortable and uneasy. Many people even start nose bleeding at higher altitude. This is because, due to the difference in the pressure at higher altitude the blood vessel in our body burst and bleeding starts.

            Application of Atmospheric Pressure

• While taking cold drinks from the bottles, we use straw. The lower end of the straw is dipped into the bottle and we suck at the upper end with our mouth, the pressure inside the straw decreases. The pressure on the surface of the cold drinks is equal to the atmospheric pressure, which pushes the drinks up into our mouth. Hence, we are able to take the cold drinks.
• The injection syringe works due to the existence of atmospheric pressure. When the nozzle is dipped into the liquid and its piston is pulled, the pressure inside the syringe is lowered. The greater atmospheric pressure on the surface of the liquid pushes the liquid inside the syringe and the syringe gets filled.
• The working of dropper is also due to the existence of atmospheric pressure. We press the rubber bulb keeping its nozzle dipped into the liquid, the air in the glass nozzle and the bulb escapes out of the tube. Due to this, the pressure inside the tube become lesser and the air pressure on the surface of the liquid becomes more. So when we release the bulb, it pushes the liquid inside the tube and the liquid raises in the tube. When we remove the dropper from the liquid and press it again, the liquid comes out of the nozzle in the same way and on the same principle.
• Ink pens are not allowed at higher altitude. Because at higher altitude the atmospheric pressure is less and pressure of ink inside the pen is more, due to which ink oozes out of the pen and spoils the cloth.

       David tries to float in the air with the help of a glider. After several attempt he remains unsuccessful. He tries to find the reason responsible for it, and comes to conclusion that, it has been the air pressure of the region responsible for his failure. The air pressure of the region has been _____?______than 101.3kPa.

(a) Less

(b) More

(c) Equal

(d) Infinite

(e) None of these

Answer: (b)

Explanation

The air pressure of the region was more than 101.3 kPa.              

       Mary went to the restaurant and took a cold drink with the help of straw. She realised that, it was easy for her to drink with the help of straw, rather than directly from the bottle. Which one of the following is the possible reason for this?

(a) Mouth pressure                                       

(b) Sucking pressure

(c) Atmospheric pressure

(d) All of these

(e) None of these

Answer: (c)

Explanation

We can drink with the help of straw because of difference in the  atmospheric pressure.