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HELP!!! Isaac physics. Essential Pre-Uni Physics H9.7

An alpha particle (charge of 3.2×10 −19 C and mass 6.7×10 −27 kg) is fired at 1.8×10 7 m/s directly towards a gold nucleus which has a charge of 1.26×10 −17 C. Assuming that the recoil of the gold nucleus is negligible, calculate the distance of the closest approach.
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You'll be using the formula for electric potential energy of point charges shown in the video hint.

the α is closest to the gold nucleus when all of its initial kinetic energy has been converted to electrical potential energy

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