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## Homework Statement

An unknown element has a nucleus with charge 14.00

*e*and a radius of about 3.60×10-15

*m*. How much voltage must be used to accelerate a proton (radius 1.20×10-15

*m*) so that is has sufficient energy to

__just__penetrate the unknown element?

Assume that the potential is that for point charges.

## Homework Equations

V = kQ/r

## The Attempt at a Solution

I tried calculating the two potential differences caused by the incoming proton and the positively charged atom, then adding them separately, but it seems that this isn't the right answer/ right approach.

V = (8.99e9*(14*(1.6e-19))/3.6e-15) + (8.99e9(1.6e-19)/1.2e-15) = 6.79e6 V

Any pointers would be very much appreciated!