week14-1 - The Existence of Atoms: Evidence from Chemistry...

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The Existence of Atoms: Evidence from Chemistry Why believe in the existence of something we have never seen? Observations can provide convincing evidence. Much early evidence of atoms came from chemistry , the study of the differences in substances and how they can be combined to form other substances. Since certain substances were retrievable, early scientists were also tempted to believe that certain elements were made up of tiny indivisible atoms .
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Cathode rays, Electrons, and X-rays By the end of the nineteenth century, chemists were using the concept of atoms to explain their properties. Physicists were less convinced. The discovery of cathode rays was the beginning of atomic physics. Two electrodes are sealed in a glass tube. As the tube is evacuated, a glow discharge appears in the gas between the electrodes. With further evacuation, the discharge disappears, and a glow appears on the end of the tube opposite the cathode.
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An invisible radiation seemed to emanate from the cathode to produce the glow on the opposite wall of the tube. The invisible radiation was called cathode rays . If the north pole of a magnet is brought down toward the top of a cathode-ray tube, the spot of light is deflected to the left across the face of the tube. This indicates the cathode rays are negatively charged particles.
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J. J. Thomson used both electric fields and magnetic fields to deflect the beam. The combined effect allowed him to estimate the velocity of the particles. With the deflection produced by the magnetic field alone, this allowed him to estimate the mass of the particles. We now call these particles electrons .
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Thomson’s discovery provided the first known subatomic particle. The mass of an electron is 9.1 x 10 -31 kg. The charge of an electron is 1.6 x 10 -19 C. The electron was the first possible candidate for a building block of atoms. The study of cathode rays led Roentgen to discover yet another type of radiation.
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He noticed that a fluorescent material would glow when placed near his covered cathode-ray tube.
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This note was uploaded on 04/23/2011 for the course PHYS 214 taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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week14-1 - The Existence of Atoms: Evidence from Chemistry...

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