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What we learned last time

What we learned last time. When provided with an electric current, a cathode (negatively-charged metal plate) emits a cathode ray The ray’s identity was unknown to scientists for many years.

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What we learned last time

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  1. What we learned last time • When provided with an electric current, a cathode (negatively-charged metal plate) emits a cathode ray • The ray’s identity was unknown to scientists for many years • When this cathode ray is subject to an electric field, the ray itself is attracted to the positvely-charged metal plate on the outside of the tube • Suggested that the cathode ray has a negative charge

  2. Because the cathode ray came from the metals (which are made of atoms) within the tube, it was reasoned that this beam of negatively-charged particles came from within the atom • Resulted in a new model of the atom • Atoms were no longer seen as indivisible • Negatively-charged particles were over 1,000 times smaller than the hydrogen atom • New model was referred to as the “plum pudding”model by JJ Thomson

  3. Investigating the properties of the electron • Basis of problem: • JJ Thomson was able to prove that electrons were much much smaller than the atom itself • Electrons have a negative charge • What is the numerical value of this charge? • In order to reveal this answer, one of the most ingenious experiments was performed • Robert Milikan’s oil drop experiment

  4. Shoots out little mists of extremely tiny oil droplets Because the drops are so tiny, a microscope was used to observe individual oil droplets After being sprayed out, these oil droplets would pass through the allowed path and fall down to the bottom

  5. While falling down, some of these oil droplets lose their electrons (lose negative charge) • Therefore, the some of the droplets have a positive charge In order to battle the gravitational force, plates were charged so that the oil drop’s path could be manipulated

  6. Strength of the electric plates dictates where the drop will go • Goal is to suspend the oil drop in mid air

  7. When the voltage applied to the plates is just right, the oil drop is suspended in mid air • Electric force balances the gravitational force

  8. Oil drops positively charged because they have lost electrons

  9. Conclusion • Charge of 1 electron = • 1.60 x 10-19coloumbs • 0.000 000 000 000 000 000 1 600 coloumbs

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