37_jjthomson - Lesson 37 Thomson's Plum Pudding Model...

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Lesson 37: Thomson's Plum Pudding Model Remember back in Lesson 18 when we looked at trapping moving particles in a circular path using a magnetic field? In this lesson we look at how using this type of set up eventually helped physicists start to figure out the nature of the atom. Early Theories about Atoms In 1803 John Dalton presented his theory that the elements of the periodic table were made of atoms . The main reason he was proposing this idea was to be able to explain the chemistry that he was studying. Dalton believed that atoms were solid pieces of matter that could not be broken down any further. This is why his model is often referred to as the Solid Sphere Model . Although Dalton's model was able to explain the way he saw chemical reactions working, he was unable to really prove that matter was made up of these atoms. During the late 1800's experiments with cathode ray tubes (CRTs) were starting to give the first glimpses into what an atom might be. A cathode ray tube is just a vacuum tube with two electrodes at the ends. When a really high voltage is applied, mysterious cathode rays moved from the negative electrode to the positive electrode. Sometimes you could see a glow at the opposite end of the tube when the tube was turned on. William Crookes used a really high quality cathode ray tube in 1885. In his tubes he usually placed a mask , an object that would block some of the cathode rays from getting to the other side of the tube. This resulted in a shadow . He believed that the the results of his experiment suggested that the particles coming off the cathode were negatively charged. In 1895 Jean Baptiste Perrin was able to verify this by showing that cathode rays shot into a metal tube caused the metal tube to gain a negative charge. J.J. Thomson's Cathode Ray Tube The real breakthroughs started when J.J. Thomson began his research. He performed a series of three experiments. First Experiment His first experiment was to see if the negative charges could be separated from the cathode rays using a magnetic field. At the time, it was not known that the cathode rays were the negative charges. 8/28/2008 © studyphysics.ca Page 1 of 6 / Section 15.1 The negative electrode is referred to as the cathode . That's the reason this is called a cathode ray tube. Illustration 1: A classic "Crookes" tube. cathode anode mask shadow Did You Know? Besides winning the Nobel Prize in Physics himself, seven of his research assistants and even his own son, George, all won Nobel Prizes in Physics. One of Thomson's students was Ernest Rutherford.
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When Thomson failed to separate them, he figured out that the negative charges were what the
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This note was uploaded on 12/02/2011 for the course PHYSICS 235 taught by Professor Staff during the Fall '08 term at Rutgers.

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37_jjthomson - Lesson 37 Thomson's Plum Pudding Model...

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