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Question

The top of the atmosphere is about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface of the earth, the field is about 100Vm1. Still, we do not get an electric shock as we step out of our house into the open house, because (assume the house to be a steel cage so that there is no field inside)

A
There is a potential difference between our body and the ground
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B
100Vm1 is not a high electric field so that we do not feel the shock
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C
Our body and the ground forms an equipotential surface
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D
The dry atmosphere is not a conductor
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Solution

The correct option is D The dry atmosphere is not a conductor
Dry atmosphere is not a conductor. Only when top of the atmosphere increases to a very large value and there is moisture in the air, the lightning strikes the ground. Since, 400 kV is not very high voltage so, it will not strive the ground.

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Answer the following:

(a) The top of the atmosphere is at about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface of the earth, the field is about 100 Vm−1. Why then do we not get an electric shock as we step out of our house into the open? (Assume the house to be a steel cage so there is no field inside!)

(b) A man fixes outside his house one evening a two metre high insulating slab carrying on its top a large aluminium sheet of area 1m2. Will he get an electric shock if he touches the metal sheet next morning?

(c) The discharging current in the atmosphere due to the small conductivity of air is known to be 1800 A on an average over the globe. Why then does the atmosphere not discharge itself completely in due course and become electrically neutral? In other words, what keeps the atmosphere charged?

(d) What are the forms of energy into which the electrical energy of the atmosphere is dissipated during a lightning? (Hint: The earth has an electric field of about 100 Vm−1 at its surface in the downward direction, corresponding to a surface charge density = −10−9 C m−2. Due to the slight conductivity of the atmosphere up to about 50 km (beyond which it is good conductor), about + 1800 C is pumped every second into the earth as a whole. The earth, however, does not get discharged since thunderstorms and lightning occurring continually all over the globe pump an equal amount of negative charge on the earth.)

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