РефератыИностранный языкThThe Atom Essay Research Paper The AtomAP

The Atom Essay Research Paper The AtomAP

The Atom Essay, Research Paper


The Atom


AP Physics Period 2


In the spring of 1897 J.J. Thomson demonstrated that the beam of glowing


matter in a cathode-ray tube was not made of light waves, as “the almost


unanimous opinion of German physicists” held. Rather, cathode rays were


negatively charged particles boiling off the negative cathode and attracted to


the positive anode. These particles could be deflected by an electric field and


bent into curved paths by a magnetic field. They were much lighter than


hydrogen atoms and were identical “what ever the gas through which the discharge


passes” if gas was introduced into the tube. Since they were lighter than the


lightest known kind of matter and identical regardless of the kind of matter


they were born from, it followed that they must be some basic constituent part


of matter, and if they were a part, then there must be a whole. The real,


physical electron implied a real, physical atom: the particulate theory of


matter was therefore justified for the first time convincingly by physical


experiment. They sang success at the annual Cavendish dinner.


Armed with the electron, and knowing from other experiment that what was


left when electrons were stripped away from an atom was much more massive


remainder that was positively charged, Thomson went on in the next decade to


develop a model of the atom that came to be called the “plum pudding” model.


The Thomson atom, “a number of negatively electrified corpuscles enclosed in a


sphere of uniform positive electrification” like raisins in a pudding, was a


hybrid: particulate electrons and diffuse remainder. It served the useful


purpose of demonstrating mathematically that electrons could be arranged in a


stable configurations within an atom and that the mathematically stable


arrangements could account for the similarities and regularities among chemical


elements that the periodic table of the elements displays. It was becoming


clear that the electrons were responsible for chemical affinities between


elements, that chemistry was ultimately electrical.


Thomson just missed discovering X rays in 1884. He was not so unlucky


in legend as the Oxford physicist Frederick Smith, who found that photographic


plates kept near a cathode-ray tube were liable to be fogged and merely told his


assistant to move them to another place. Thomson noticed that glass tubing held


“at a distance of some feet from the discharge-tube” fluoresced just as the wall


of the tube itself did when bombarded with cathode rays, but he was too intent


on studying the rays themselves to purse the cause. Rontgen isolated the effect


by covering his cathode-ray tube with black paper. When a nearby screen of


florescent material still glowed he realized that whatever was causing the


screen to glow was passing through the paper and intervening with the air. If


he held his hand between the covered tube and the screen, his hand slightly


reduced the glow on the screen but in the dark shadow he could see his bones.


Rontgen’s discovery intrigued other researchers beside J.J. Thomson and


Ernest Rutherford. The Frenchman Hernri Becquerel was a third-generation


physicist who, like his father and grandfather before him, occupied the chair of


physics at the Musee Historie in Pairs; like them also he was an expert on


phosphorescence and fluorescence. In his case, particular of uranium. He heard


a report of Rontgen’s work at the weekly meeting of the Academie des Sciences on


January 20, 1896. He learned that the X rays emerged from the fluorescence


glass, which immediately suggested to him that he should test various


fluorescence materials to see if they also emitted X rays. He worked for ten


days without success, read an article on X rays in January 30 that encouraged


him to keep working and decided to try a uranium slat, uranyl potassium sulfate.


His first experiment succeeded-he found that the uranium salt emitted


radiation but misled him. He had sealed a photographic plate in black paper,


sprinkled a layer of uranium salt onto the paper and “exposed the whole thing to


the sun for several hours.” When he developed the photographic plate “I saw the


silhouette of the phosphorescent substance in black on the negative.” He


mistakenly thought sunlight activated the effect, much as a cathode ray releases


Rontgen’s X rays from the glass.


The story of Becqueerel’s subsequent serendipity is famous. When he


tried to repeat his experiment on Feb. 26 and again on February 27 Paris was


covered with clouds. He put the uncovered photographic plate away in a dark


drawer, with the uranium salt in place. On March 1 he decided to go ahead and


develop the play, “expecting to find the images very feeble. On the contrary,


the silhouettes appeared with great intensity. I thought a t once that the


action might be able to go on in the dark.” Energetic, penetrating radiation


from inert matt

er unstimulated by rays or light: now Rutherford had his subject,


as Marie and Pierre Curie, looking for the pure element that radiated, had their


backbreaking work.


But no one understood what produced the lines. At best, mathematicians


and spectroscopists who liked to play with wavelength numbers were able to find


beautiful harmonic regularities among sets of spectral lines. Johann Balmer, a


nineteenth-century Swiss mathematical physicist, identified in 1885 one of the


most basic harmonies, a formula for calculating the wavelengths of the spectral


lines of hydrogen. these collectively called the Balmer series.


It is not necessary to understand mathematics to appreciate the


simplicity of the formula Balmer derived that predicts a line’s location on


spectral bad to an accuracy of within on part in a thousand, a formula that has


only on arbitrary number: lambdda=3646(n^2/n^2-4). Using this formula, Balmer


was able to predict the wavelengths of lines to be expected for parts of the


hydrogen spectrum not yet studied./ They were found where he said they would be.


Bohr would have known these formula and numbers from undergraduate


physics especially since Christensen was an admirer of Rydberg and had


thoroughly studied his work. But spectroscopy was far from Bohr’s field and he


presumably had forgotten them. He sought out his old friend and classmate, Hans


Hansen, a physicists and student of spectroscopy just returned from Gottigen.


Hansen reviewed the regularity of line spectra with him. Bohr looked up the


numbers. “As soon as I saw Balmer’s formula,” he said afterward, “the whole


thing was immediately clear to me.”


What was immediately clear was the relationship between his orbiting


electrons and the lines of spectral light. Bohr proposed that an electron bound


to a nucleus normally occupies a stable, basic orbit called a ground state. Add


energy to the atom, heat it for example, the electron responds by jumping to a


higher orbit, one of the more energetic stationary states farther away from the


nucleus. Add more energy and the electron continues jumping to higher orbits.


Cease adding energy-leaving the atom alone-and the electron jump back to their


ground states. With each jump, each electron emits a photon of characteristic


energy. The jumps, and so the photon energies , are limited by Plank’s constant.


Subtract the value of a lower-energy stationary state W2 from the value of a


higher energy stationary state W1 and you can get exactly the energy of light as


hv. So here was the physical mechanisms of Plank’s cavity radiation.


From this elegant simplification, W1-W2=hv, Bohr was able to derive the


Balmer series. The lines of the Balmer series turn out to be exactly the


energies of the photons that the hydrogen electron emits when it jumps down from


orbit to orbit to its ground state.


Then, sensationally, with the simple formula, R=2pi^2me^4/h^3, Bolar


produced Rydberg’s constant, calculation it within 7 percent of its


experimentally measured value. “There is nothing in the world which impresses a


physicist more,” an American physicist comments, “than a numerical agreement


between experiment and theory, and I do not think that there can ever have been


a numerical agreement more impressive than this one, as I can testify who


remember its advent.”


“On the constitution of atoms and molecules” was seminally important to


physics. Bexzides proposing a useful model for the atom, it demonstrated that


events ensts that take place on the atomic scale are quantized: that just as


matter exits as atoms and particle s in a state of essential graininess, so


also does process. Process is discontinuous and the “granule” of mechanistic


physics was therefore imprecise; though a good approximation that worked for


large-scale events, it failed to account for atomic subtleties.


Bohr was happy to force this confrontation between the old physics and


the new. He felt that it would be fruitful for physics. because original work


is inherently rebellious, his paper was not only an examination of the physical


world but also a political document. It proposed, in a sense, to begin a reform


movement in physics: to limit claims and clear up epistemological fallacies.


Mechanistic physics had become authoritarian. It had outreached itself to claim


universal application, to claim that the universe and everything in it is


rigidly governed by mechanistic cause and effect. That was Haeckelism carried


to a cold extreme. It stifled Neils Bohr as a biological Haeckelism and stifled


Christian Bohr and as a similar authoritarianism in philosophy and in bourgeois


Christianty had stifled Soren Kierkegaard.


Bibliography


Rodes, Richard. The Making of the Atomic Bomb. New York: Ssimon and Schuster,


1986.


“Nuclear Wapon.” The Enclopedia Britannica. Encylopedia Britannica In.


Chicago


V8; 1991, p 820-821.


317

Сохранить в соц. сетях:
Обсуждение:
comments powered by Disqus

Название реферата: The Atom Essay Research Paper The AtomAP

Слов:1717
Символов:11652
Размер:22.76 Кб.