30. The earth’s surface has a negative surface charge density of 9 2 10 Cm   The

Question

The earth’s surface has a negative surface charge density of

9 2 10 Cm  

The potential difference of

400kV

between the top of the atmosphere and

the surface results (due to the low conductivity of the lower atmosphere) in

a current of only

1800A

over the entire globe. If there were no mechanism

of sustaining atmospheric electric fields, how much time (roughly) would

be required to neutralize the earth’s surface? (This never happens in

practice because there is a mechanism to replenish electric charges,

namely the continual thunderstorms and lightning in different parts of the

globe). (Radius of earth =

6

6.37 10 m 

.

Answer 1

Surface charge density of the earth is

9 2 10 Cm     .

Current over the entire globe is

I 1800A  .

Radius of the earth is

6

r 6.37 10 m   .

Surface area of the earth is given by:

2 A 4 r    

2

6     A 4 6.37 10 14 2    A 5.09 10 m

Charge on the earth surface is given by:

q A  9 14 q 10 5.09 10     

5    q 5.09 10 C

Now if time taken to neutralize the earth’s surface is

t

, then

Current,

q

I

t



q

t

I

 

5

5.09 10 t 282.77s

1800



  

Therefore, the time taken to neutralize the earth’s surface is

282.77s.

31. Choose the correct alternative:

a) Alloys of metals usually have (greater/less) resistivity than that of their

constituent metals.

Ans: Alloys of metals usually have greater resistivity than that of their