Module 2 : Gaseous Dielectrics
Lecture 5 : Properties of atmospheric air and SF6 , Related ionization Process
Properties of atmospheric air (N2)
The most important, freely available and the cheapest gaseous dielectric is air. The atmospheric air is in fact a mixture of a number of gases. The detailed composition of earth's atmosphere is given in Table 5.1 as given by Goody and Walker in their book Atmospheres.                        
 
Table 5.1 Composition of the earth's atmosphere
Constituent Percent by volume or by number of molecules of dry air

Nitrogen (N2)

78.084

Oxygen (O2)

20.946

Argon (A)
0.934

Carbon dioxide (CO2)

0.031

Neon (Ne)

1.82 x 10-3

Helium (He)

5.24 x 10-4

Methane (CH4)

1.5 x 10-4

Krypton (Kr)

1.14 x 10-4

Hydrogen (H2)
5 x 10-5

Nitrous oxide (N2O)

3 x 10-5

Xenon (Xe)

8.7 x 10-6

Carbon monoxide (CO)
1 x 10-5

Ozone (03)

upto 10-5

Water (average)
upto 1
 

The largest percentage content of atmospheric air is nitrogen (about 78%), which is an electropositive gas. The second largest constituent is oxygen (about 20%), which is a very weak electronegative gas. The content of hydrogen, an electronegative gas too, is so low (5 x 10-5%) that for all practical purposes the air can be considered as an electropositive gas. Majority of the theoretical as well as experimental research work available in literature to study the complicated discharge processes in gaseous dielectrics have been performed on air.
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