I have gotten the answers for part a and b of this problem but keep getting c and d wrong.

Question

a) A +10 C charge is held fixed 6 m from a +4 C charge. What potential energy is stored in this charge configuration? 

b)If the second charge is released from rest, what is the new potential energy between the charges after the second charge has traveled 2 m? 

c)What is the final KE? 

d)If the second charge has a mass of 0.002 kg, what is its final velocity? 

Answer 1

The correct result from part (a) is the total energy of the system. This quantity must be conserved. Note the kinetic energy is zero. The correct result from part (b) is the final potential energy of the system. If you subtract this from the total energy calculated in part (a), you will get the final kinetic energy of the system, which also happens to be the kinetic energy of the one moving charge in part (c). So the answer to (c) is the difference between the answers to (a) and (b), which you already have. For part (d), let K be the kinetic energy that you find in part (c).

K = mv²/2 ⇒

v = √(2K/m)

You have K and m, so just plug in the numbers in correct units and get v.