question_answer 37)

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, corresponding to an the field 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 step 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 1 m2. 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 occuring continually all over the globe pump an equal amount of negative charge on the earth.]  

Answer:

(a) Our body and the earth surface become equipotential (at equal potential). It means there is no potential difference between the earth and our body. Hence no current flows through our body and therefore we do not experience an electric shock. (b) Yes. The aluminium sheet and the earth form a capacitor with the insulating slab as dielectric. The down pour of the atmospheric charge raise the potential of the sheet of aluminium. When we touch the aluminium sheet, charge will flow to the earth through our body. This flow of charge constitutes on electric current and we will experience a shock. (c) No doubt the atmosphere continuously gets charged due to lightning, thunderstroms but simultaneously it gets discharged through normal weather zones. This keeps the system balanced. (d) Electrical energy of the atmosphere appears as light, sound and heat energies during thunder storms and lightning.