A tiny sphere of mass 8.60 µg and charge −2.80 nC is initially at a distance of 1.44 µm from a fixed charge of +8.53 nC.
(a) If the 8.60-µg sphere is released from rest, find its kinetic energy when it is 0.500 µm from the fixed charge.
(b) If the 8.60-µg sphere is released from rest, find its speed when it is 0.500 µm from the fixed charge.
At least what I believe are relevant equations: PE1 + KE1 = PE2 + KE2
PE = k(q1q2/r)
KE = mv2/2
The Attempt at a Solution
The sphere is released at rest, so KE1 = 0 --> KE2 = PE2 - PE1 = ΔPE
ΔPE = ke(q1q2/r0-r1) = 8.99e-09(2.80e-09C*8.53e-09C)/(1.44e-06m - 5.0e-07m) = 0.23 J
This answer continues to come back wrong. I am supposed to be getting 0.28 J, but I'm not sure why.
The second part should be easy if I get the PE correct.
KE = PE = v(sqrt(m/2)) --> 0.28J/(sqrt(8.6e-09kg/2)) = 4269 m/s
...but that is wrong too. It should be 8070 m/s.