ATMOSPHERIC ELECTRICITY IN PHYSICS FULL INFORMATION

 Atmospheric electricity in Physics Full Information 

The earth and the atmosphere surrounding it show very interesting electric phenomena. The earth has a negative charge spread with approximately uniform density over its surface. The average surface charge density on the earth is little less than one nanocoulomb per square metre. There is a corresponding electric field of about 100Vm^-1 in the atmosphere above the earth. This field is in the vertically downward direction. This means, if you look at a flat desert, the electric potential increases by about 100 V as you move up by 1 m. The potential keeps on increasing as one goes higher in atmosphere but the magnitude of the electric field gradually decreases. At about 50 km from the earth's surface, the field is negligible. The total potential difference between the earth's surface and the top of the atmosphere is about 400 kV.

The atmosphere contains a number of ions, both positively charge and negatively charged. The main source of these ions is cosmic rays which come from outside the earth, even from outside the solar system. These rays come down to the earth and ionize molecules in the air. Air contains dust particles which become charged by friction as they move through the air. This is another source of the presence of charged ions in air. Because of the electric field in the atmosphere, positive ions come down and negative ions go up. Thus, there is an electric current in the atmosphere. This current is about 3.5 ｘ10^-12 A over a square metre area parallel to the earth's surface. When the total surface area of the earth is considered, 1800 A of current reaches the earth.

The density of ions increases with height over the earth;s surface. Also, the density of air decreases and the ions can travel larger distance between collisions. Both these factors contribute to the fact that ''conductivity of air'' increases with altitude. At about 50 km above the earth's surface, the become highly conducting. We can draw an equivalent picture by assuming that at about this height there is a perfectly conducting surface having a potential of 400 kV and current comes down from this surface to the earth.

If 1800 A of current flows towards the earth, the entire negative charge of the earth should get neutralised in about half an hour and the electric field in the atmosphere should reduce to zero. But it is not so. So, there must be some mechanism which brings negative charge back to the earth, so that the 400 kV potential difference is maintained. This situation is like that of a battery. The current provided by a battery discharges it. There is a source of emf which maintains the potential difference across the battery's terminals. So, what is the source that charge our atmospheric battery. The answer is thunderstorms and lighting. 

Because of  the difference in temperature and pressure between different parts of the atmosphere, air packets keep on moving in a rather systematic fashion. As the upper atmosphere is cool (temperature is around -10 ℃ at a height of 3-4 km and -20℃ at a height of 6-7 km ), water vapour condenses to form small water droplets and tiny ice particles. A parcel of air with these droplets and ice particles forms a thunderstorm. A typical thunderstorm may have an average horizontal extension of about 7-8 km and a vertical extension of about 3 km. A matured thunderstorm is formed with its lower end at a height of about 3-4 km above the earth's surface and the upper end at about 6-7 km above the earth's surface. 

The upper part of a thunderstorm contains excess positive charge and the lower part contains excess negative charge. The density of negative charge in the clouds in the lower part of the storm is very high. This negative charge created a potential difference of 20 to 100 MV between these clouds and earth. Note that this potential difference is much larger than the 400 kV between the earth and the top of atmosphere and is opposite in sing.