Q. In cosmic rays 0.15 protons $cm^{- 2}\sec^{- 1}\ $ are entering the earth’s atmosphere. If the radius of the earth is 6400 km, the current received by the earth in the form of cosmic rays is nearly.
A) 0.12 A
B) 1.2 A
C) 12 A
D) 120 A
Explanation
(a) Surface area of earth, $A = 4\pi r^{2}$
Charge entering the earth per sec per unit area
$J = 0.15 \times 1.6 \times 10^{- 19}Cm^{- 2}s^{- 1}$
$= 0.15 \times 1.6 \times 10^{- 19} \times 10^{4}Cm^{- 2}s^{- 1}$
$\therefore Current,\ i = JA = J4\pi r^{2}$
$= 0.15 \times 1.6 \times 10^{- 19} \times 10^{4} \times 4 \times 3.14 \times \left( 6.4 \times 10^{6} \right)^{2}$ on solving
$i = 0.12\ A$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.