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November 26, 2014

November 26, 2014

Posted by **rick** on Wednesday, January 30, 2008 at 7:51pm.

- physics -
**Damon**, Wednesday, January 30, 2008 at 8:10pmThey attract, so the potential energy will go down as they get close. (the result will be negative)

the force is a constant *1/r^2 = k/r^2

(You can do the charges and Coulomb's law)

So the real question is what is the integral from r = infinity to r = R (which is given as 5.11*10^-11) of 1/r^2 dr ?

int from infinity to R of dr/r^2 = -1/r at R - (-1/r) at infinity

= -1/R

now use that with k Q1 Q2 /r^2

to get

- k (charge of electron)^2 / R

for the change in potential energy coming in from infinity.

Of course that loss of potential went somewhere, so the thing must have that amount of kinetic energy, which is probably the next problem.

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