Problem # 50 Pressure [Solved]

Questions:(1) What’s the definition of pressure and more specifically the atmospheric pressure? (2) Explain how the atmospheric pressure at a given location can affect the performance of an athlete? (3) Do we need to indicate the pressure at which a chemical reaction takes place? Justify your answer. (4) Why usually we don’t feel that pressure on us?

Answers:

We live with many natural phenomena and take them as granted, without asking ourselves about their nature and how they work. One of such phenomena is the Atmospheric Pressure.

1/ A pressure is the force acting normally on a unit area of a surface or the ratio of force to area.

Imagine two persons of the same weight. One is wearing high heels shoes, another is wearing flat shoes. By accident, both of them walk on your feet; where do you feel more pain?

You feel more pain on the foot stepped on by the high heel shoe, because at that point of your foot, the pressure or the force acting on your foot is distributed on a small area, the surface of the high heel shoe. You feel less pain on the other foot, because the pressure or the force acting on that foot is distributed on a larger area of the flat shoe.

The atmospheric pressure is the force exerted by the weight of the air [composition of dry air: N₂(78.08%), O₂(20.95%), other gases (0.97%)] above any point of the Earth’s surface. It is expressed in atmosphere unit, atm., or Pascal unit, Pa:

1 atm = 101,325 Pa = 760.0 mmHg

The atmospheric Pressure is equal to 1 atm at sea level.

2/ As the altitude increases, the air become less dense than air nearer to the sea level; in other words the quantity of oxygen decreases with increasing altitude. An athlete, whose lungs and body are used to higher quantity of O₂ at lower altitude will experience difficulty at higher altitude where the oxygen is in lower quantity; this may affect his/her performance. That is why, in some cases, athletes need some acclimatization period.

3/ It is recommended to indicate the pressure and the temperature (if different from room temperature) at which a chemical reaction takes place. Because Temperature and Pressure are two factors that affect the speed of the reaction. Nevertheless the Pressure affects gaseous reactions more than reactions in liquid or solid state. That is why this recommendation is more critical for gaseous reactions.

4/ Two reasons can explain this:

(a)    Our body is hit by air molecules from all directions with the same force; that is why we do not feel it, because the forces in all directions are balanced. But if this balance is disturbed, for example by a lorry passing nearby, we are pushed away because of a higher pressure coming from the air displaced by the lorry on the other side.

(b)   The other reason is that the air within our bodies (in our lungs, stomach, ears, etc...) is exerting the same pressure outwards, so there is no difference of pressure and we feel nothing.

But this balance of pressure can be disturbed; for example when we change altitude quickly, for instance for people in an airplane that takes off or descends for landing, some people may feel pain in their ear drums (tympanic membrane) due to unbalanced pressure on both sides of the ear drum.