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【正文】 ? ? ? ? （ ） Where m is the mass of the particle, and υ its speed. From Newton’s second law,this change of momentum must be equal to the force acting on the particle, integrated over the whole time that the particle is in the field of the nucleus. Therefore, 02 sin ( / 2 )p m F d t? ?? ? ? ? ? （ ） Figure the direction of F a particular position of the particle, defined by through angle φ, as shown, by symmetry, it can be seen that the integral in () is given by 00 c o sI F dt F dt??????? (since the integral of the ponent parallel to the χaxis, F sinφ, must be zero, by symmetry ). Fig Change in momentum of an α particle during interaction with through nucleus. A change of variables for integration enables () to be rewritten: ? ? ( ) / 2( ) / 22 s in / 2 c o s ( / )m F d t d d??? ? ? ???? ? ??? ? （ ） (see Fig for the changed limits of integration). Finally, (dt/dφ) is equal to 1/ω where ω is the angular speed of the particle about the origin. Since the force acting on the particle is radial, the angular momentum of the particle is the same for any value of φ, and ω must be given by the equation 2mr m b??? Therefore 2( / ) /dt d r b??? Coulomb’s law gives 2202 / 4F Ze r??? so that substituting in () and integrating through right hand side gives an expression for Θ in terms of υ and b 220c ot( / 2) ( 2 / )m Z e b?? ??? （ ） or, in terms of the kiic energy T of the particle 20c ot( / 2) ( 4 / )T Z e b???? （ ） This gives an equation for the scattering angle in terms of the kiic energy and impact parameter of the particle and of the charge on the nucleus, Ze. 介紹 雖然希臘哲學(xué)家德謨克利特曾推測了在公元前一世紀(jì)原子的存在和道爾頓的原子理論 1807 年奠定了原子的存在 ,在 20 世紀(jì)之交以前。Introduction Although the Greek philosopher Democritus had postulated the existence of atoms in the first century BC and Dalton’s atomic theory of 1807 laid the basis for the existence of atoms before the turn of the twentieth century. Indeed, at that time an influential school of German physicists led by Ernst Mach considered the atomic model to be merely a useful picture with no basis in reality. THE EXISTENCE OF ATOMS The situation was dramatically changed by an explosion of experimental investigatio n over the fifteen years between 1897 and 1912. in the 1870s, technical improvements in the construction of vacuum pumps had made possible the investigation of electrical phenomenon in evacuated tubes and the discovery of invisible rays which traveled between an electrically negative electrode (cathode) and an electrically positive electrode (anode) in such a tube. These rays came to be known as cathode rays. At first there was considerable controversy over their nature, but a series of experiments carried out by . Thomson in 1897 demonstrated conclusively that the cathode rays consisted of a stream of negatively charged particles, presumably emitted by atoms in the cathode (Fig. ). Thomson’s measurements of the deflection of the rays by electric and magic fields enabled the speed of the particles to be measured and also the ratio of the charge of a particle to its mass. By the turn of the century, the chargemass radio of these particles, which came to be called electrons, could be measured to quite high precision. However, to give absolute values of the charge and mass, experiments of a different type were required. The most successful were investigations where macroscopic particles such as oil drople
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