c) In a Rutherford experiment, helium nuclei also known as alpha particles, which consist of two protons and two neutrons were accelerated through an electric potential difference, and then directed towards a very thin Gold foil. Although most of these charged particles pass through the foil, almost unaffectedly, some were observed to be deflected sharply, almost backwards. Rutherford (1909 -1911) was thereby led to the conclusion that Gold atoms consist mostly of void, but have strongly and positively charged central regions, namely nuclei.
In such an experiment assume that an alpha particle with a positive charge 2e is accelerated through a potential difference of ?V and then happens to strike, head on, a Gold nucleus of positive charge Ze. The energy W it would then acquire would be (by definition of electric potential) W=2e?V. The distance of closest approach d, in such a special collision is expected to give an approximate value for the radius of gold nucleus. First, show that the alpha electric potential energy W, at d (in MKS unit system), becomes
W = 2Ze² / 4???d?
and thereby, one can write for the closest distance d, as,
d = Ze / 4????V