7 Trillion-Trillion Air Molecules Strike Your Face Every Second


The Sunday Nation


29 May 2005


What happens when air is heated? Answer: It rises. Why does it rise? Because when the temperature is increased, air expands thereby reducing its density (mass per unit volume). As you might have guessed by now, the next question is: Why does air expand when heated?

Air molecules are constantly moving in all directions. They are not bound together as in the case of solids and liquids. As they move, they knock against each other and against any obstacle in their path.

The speed of the molecules depends on the temperature of the gas. At zero degrees celcius, air molecules will be moving at an average of about 490 metres per second. That is equal to 1,760 kilometres per hour!

As the temperature is increased, the molecules move faster. For example, at 25 degrees celcius the velocity of air molecules is about 1,800km/h. Clearly, even though the molecules have higher velocities in hotter air, the relationship between velocity and temperature is not straightforward. For a simulation of this, visit the internet site www.jillionlimited.com and click on the “Articles” link.

All the same, the important point to note is that the molecules move faster when the temperature is increased. As their speed increases, the volume occupied also increases, thus the density reduces causing the hot air to rise.

Now, air is made up many different gasses, the main ones being nitrogen (78 percent) and oxygen (21 percent). The density of nitrogen under normal atmospheric temperature (20 degrees celcius) is about 1.25 grams per litre and that of oxygen is about 1.43g/l. One gram of nitrogen contains about 680 billion-billion molecules, therefore one litre of the gas has approximately 680 x 1.25 = 850 billion-billion molecules.

Now all these molecules are flying in all directions at speeds of about 1,800km/h. Thus an interesting question arises: can the air particles escape from the Earth’s gravity?  The simple answer is no. For an object (big or small) to escape from the Earth without continuous addition of energy, it must take off from the surface at about 40,000km/h. Even at very high altitudes where the temperature can reach 90 degrees celcius, the molecular velocity is only slightly over 2,000km/h. So we are not likely to lose our air!

Atmospheric pressure arises as a result of the molecules striking and bouncing off an obstacle. At sea level, the pressure is approximately equal to a one-kilogram weight sitting on an area of one square centimetre.

In terms of molecular collisions, this corresponds to 70 trillion-trillion molecules striking one square metre every second. The total surface area of the human face is about a tenth of a square metre, therefore it is struck by about 7 trillion-trillion molecules every second. But there is no need to worry; the particle can’t do much harm – we are designed to withstand the bombardment.

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