Ernest Rutherford: Difference between revisions
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Altilunium (talk | contribs) (Created page with "Ernest Rutherford was the son of James Rutherford, a small business man, and Martha, who had been a schoolteacher before her marriage. His parents, both British born, had emigrated to New Zealand, where Rutherford was born on a farm in the province of Nelson. His earliest physics research dates back to his college days, when he was already aware of current developments from reading the Proceedings of the Royal Society. Most notable among his earliest work, published in...") |
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A year and a half later, in December 1910, Rutherford knew the answer to this puzzle. He wrote to a colleague, "I think I can devise an atom much superior to J.J.'s" At that time, Rutherford had neither much expertise in nor much taste for theoretical physics. It is perhaps more interesting to note that no other theorist had responded to this challenge, nor that this seminal experiment had been repeated anywhere else.
Rutherford's atom consists of a number Z of electron, each with charge -e, and a small sized central body with charge Ze in which practically all the mass of the atom is concentrated. This body is many thousands of times heavier than the electron. Not until October 1912 did Rutherford refer to it as the nucleus ("The atom must contain a highly charged nucleus"). He further assumed that the role of the Z electrons in alpha-particle scattering can be neglected, which is actually a good approximation. So that this process is entirely due to the electrostatic Coulomb interaction between the alpha-particle and the nucleus. The theoretical exercise is then to calculate the path of an alpha particle as it first approaches, then moves away from the nucleus. Rutherford produced a formula that describes the dependence of the orbits on the angle of scattering, on the alpha-particle velocity, and on Z. In particular, the probability for alpha-particle scattering is proportional to Z^2. At that time, the data were not yet sufficient to verify these predictions in all detail, but it did not take long before it was found that his answers worked well.
On 7 March 1911, Rutherford presented his results publicly for the first time. An eyewitness has recalled : "I remember well the occasion on which the idea was first put forward. It was a meeting of the Manchester Literary and Philosophical Society to which all workers in the laboratory were invited. Rutherford's account of his theory, backed by Geiger with some new experimental evidence, created a profound impression." The definitive paper on the subject appeared in the May 1911 issue of Philosophical Magazine. Here Rutherford records a first decent estimate of the radius of a nucleus : about a hundred thousand times smaller than that of an atom. Thus if one imagines an atom blown up to the size of a football field, then the nucleus would be the size of a marble placed at the kick off point. Nuclei are not only heavy. They are also exceedingly small, even by atomic standards. Matter consists largely of emptiness.▼
Manchester in 1912 was the world's foremost center of experimental studies in radioactivity. The discovery of the atomic nucleus is in fact a by-product of such explorations. But at that time, the Rutherford model was still not taken seriously. There was no mention of it in any place. Rutherford's own reticence must have been an important contributing factor. He remained silent about his model during a major international conference (the first Solvay conference) held in the fall of 1911. In his 670 page book, completed a year later, only three pages are devoted to alpha-particle scattering.
Bohr later wrote :
<blockquote>
▲The definitive paper on the subject appeared in the May 1911 issue of Philosophical Magazine. Here Rutherford records a first decent estimate of the radius of a nucleus : about a hundred thousand times smaller than that of an atom. Thus if one imagines an atom blown up to the size of a football field, then the nucleus would be the size of a marble placed at the kick off point. Nuclei are not only heavy. They are also exceedingly small, even by atomic standards. Matter consists largely of emptiness.
This effect -- the large angle scattering of alpha-particles -- though to all appearances insignificant, was disturbing to Rutherford, as he felt it difficult to reconcile it with the general idea of atomic structure then favoured by the physicsists.
</blockquote>
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