Lightning is movement of electrons through air in order to recombine with , and neutralize , positive charge . During some rainstorm , we can model a square - shaped raincloud with side length 900 m as a plane of excess charge and the Earth's surface as a second parallel plane of excess charge with the same size charge density , but opposite sign . The two planes of charge are separated by 400 m . In this rainstorm , a lightning strike occurs when the downward pointing electric field between the Earth's surface and the raincloud layer reaches 3.00 MN / C ( mega Newtons per Coulomb ) , so that the air becomes conducting .

( a ) Just when the lightning strike begins , what is the excess surface charge density ( including sign ) on the Earth's surface ?

( b ) As mentioned , the raincloud covers a square region with side length 900 m . How much excess charge ( including sign ) is located on the raincloud when the lightning strike begins ?

( c ) Just before the lightning strike , a charged raindrop 300 m above the ground begins rising up through the air , instead of falling to the ground . What is the sign of the raindrop's excess charge ? To answer this question correctly , first must identify the direction of raindrop's acceleration , then must use acceleration and electric field directions to determine sign of charge .

D. During this example of a lightning strike, are electrons rising up from the earth into the cloud layer, or are electrons raining down from the cloud layer onto the earth? How do we know?