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3.
Due to downward electric field at ground level, earth has a negative surface charge density over the surface of earth. The negative surface charge density of the earth is - 10-9 C
m-2
4
5.
Total charge on the surface of the earth is - 5 x 5 C The top of the stratosphere has an equal positive charge i.e. + 5 x 105 C ....the potential difference between the earth's surface and that of the stratosphere should be 5 x 106 V However, the observed value (4 x 105 V) is much lesser. The explanation for this difference is the fact that the intervening air becomes more and more conducting as we go up. Due to the downward electric field, the positive ions continuously flow into earth and the negative ions ascend. The current flowing per unit area has been estimated to be about 3 x 10-12 Cs-1. In other words, about + 1800 C of charge is being pumped into earth per second. As such, the negative charge ( = - 5 X 105 C) on the earth would get neutralised in a few moments. However, it does not happen at all. It is because of thunderstorms and lightning flashes occuring world wide. Steady condition of earth's atmosphere. It can be explained on the basis of the follwing experimental observations : On the average, about 4 x 104 = 40000 thunderstorms occur per day world wide. In other words thunderstorm occurs every two seconds in the world some where. It lasts for about one hour. During each thunderstorm, positive charges are carried upwards to a height of about 6 km above of the surface of earth, while the negative charges collect at about 2 - 3 km above the earth.
Thus, the top of a cloud gets positively charged and its bottom negatively charged. Due to separation of charges, a potential difference of the order 2 x 10? V to 108 V is created between the earth and bottom of the cloud. (The bottom of the cloud may carry a total negative charge between - 20 C to - 30 C). Such a big potential difference across the 3 km thickness of atmosphere creates an electric field,
3.
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