A charge falls from infinity to within r of another charge, find velocity.

[User1247]

Homework Statement

Velocity of an electron that falls to r from infinity?
An electron falls from infinity to r=10^-8m from a charge q1=4.8x10^-19C. What is the velocity of the electron?

Homework Equations

U=q1V
V=kq2/r

The Attempt at a Solution

Potential energy change U. coulomb's constant k=9x10^-9, electron charge q2=1.602x10^-19C, electron mass m=9.11x10^-31kg, kinetic energy Ek=½mv²

U=k(q1)(q2)/r=6.912x10^-21J
set U=Ek => v=(2U/m)^0.5=1.23x10^5m/s

Anyone see where I went wrong? Thanks.

 

[Doc Al]

U=k(q1)(q2)/r=6.912x10^-21J

Redo this, paying attention to the power of ten.