How fast does the negatively charged body move in an electric field in the following case:2 oppositely charged parallel plates with an electric...

The electrical field between the two plates is 80 V/m. The
potential difference between them is 40 V. Using this, we can derive the distance
between the plates which is 40/80 = 0.5 m.

The sphere has a
mass of 50*10^-6 kg and a charge of 2120 electrons. The charge of each electron is -1.6
* 10^-19 C. 2120 electrons would give the sphere a charge of 2120*1.6*10^-19
C.

The electric field which is given in V/m can be
converted to N/C.

So the field of 80 N/C exerts a force of
2120*1.6*10^-19*80 N on the sphere.

Force =
mass*acceleration. The mass of the sphere is 50*10^-6, this gives its acceleration as 
2120*1.6*10^-19*80/50*10^-6.

Starting from rest, the speed
acquired in a distance of 0.5 m is sqrt(2*a*s), where a is the acceleration and s is the
distance.

The velocity with which the sphere strikes the
positive plate is :
sqrt(2*0.5*2120*1.6*10^-19*80/(50*10^-6))

=> 2.2396
* 10^-5 m/s

The required velocity with which
the sphere strikes the positive plate is 2.2396*10^-5
m/s.