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Download Physics 2 Homework 21 2013 In 1909 British physicist
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Physics 2 Homework 21 2013 In 1909 British physicist and chemist, Ernest Rutherford, performing (together with Hans Geiger and Ernst Marsden) experiment on scattering of heavy and positively charged alpha particles by atoms of gold, found, to his great surprise, that some of the particles were reflected back. He said that ”It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you...” Ernest Rutherford (1871-1937). This indicated that positive charge of an atom as well as most of the atom’s mass was concentrated in a small volume, which did not agree with the plum pudding model. Rutherford put forward another model which is called “planetary model”of atom. According to this model the electrons are revolving around a small positively charged nucleus similar to planets revolving around the Sun. But this model produced even more questions than the “plum pudding model”. The main question was: “Why don’t the negative electrons fall to the positive nucleus?”. As long as we try to think of electron as of a small hard ball there is no answer. The explanation can be done in the frame of quantum mechanics, the theory which describes the motion of microscopically small particles such as electrons, protons and neutrons. According to quantum mechanics, the electron is an object which possesses the properties of both particle and wave. One of the important properties of such an object is that it does not “like” when we try to confine it in a small volume. As we try to „squeeze‟ it, its energy increases greatly. That is why, in spite of strong attraction force, the electron does not fall on the nucleus, but surrounds it as a small cloud. The atomic nucleus consists of positively charged protons and neutral neutrons. Both proton and neutron have close masses: ~1,673x10-27kg (proton) and ~1,675x10-27kg (neutron).The electron’s mass more than 1000 times smaller: 9.1 x 1031 kg. Problem: 1. An alpha particle is moving to the atomic nucleus of gold (let us assume that we have a bare gold nucleus with charge +1.26x10-17C) at a speed 107m/s. How close it will go to the nucleus before it is pushed back? Please consider the nucleus and the alpha particles as point charges. 1 2. Find the velocities of the alpha particle and the gold atomic nucleus after collision when they are far away from each other. The mass of the gold nucleus is 3.27x10-25kg, the mass of alpha particle is 6.64x10-27kg. (before the collision the gold nucleus was at rest in “our” reference of frame) 2