Slide 1 / 207 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org Slide 2 / 207 QUANTUM PHYSICS AND ATOMIC MODELS www.njctl.org Slide 3 / 207 How to Use this File Each topic is composed of brief direct instruction · There are formative assessment questions after every topic · denoted by black text and a number in the upper left. > Students work in groups to solve these problems but use student responders to enter their own answers. > Designed for SMART Response PE student response systems. > Use only as many questions as necessary for a sufficient number of students to learn a topic. Full information on how to teach with NJCTL courses can be · found at njctl.org/courses/teaching methods
Slide 4 / 207 Table of Contents Click on the topic to go to that section Electrons, X-rays and Radioactivity · · Blackbody Radiation, Quantized Energy and the Photoelectric Effect · Atomic Models · Waves and Particles · Quantum Mechanics · *Theory Unification http:/ / njc.tl/ xy Slide 5 / 207 Electrons, X-rays and Radioactivity Return to Table of Contents http:/ / njc.tl/ xy Slide 6 / 207 Physics by the 19th Century Newtonian Mechanics. Maxwell's Equations - unification of electricity and magnetism. Thermodynamics - heat is another form of energy. Speed of Light measured - and no stationary reference frame (luminiferous aether) found - helped lead to Einstein's Special Theory of Relativity in 1905. Many physicists were comfortable that nearly everything had been discovered and explained. But then, things started popping up. Like the electron. http:/ / njc.tl/ xy
Slide 7 / 207 Cathode Rays About 150 years ago, "cathode rays" were discovered. No one knew what they were. Physicists working on the problem just knew what they could observe. What they saw was that when a high voltage was connected to two plates inside a glass tube, making one plate positive and the other negative, then a shadow was cast on the glass behind the positive plate. Furthermore, the glass surrounding the shadow behind the positive plate glowed. When more air was pumped out of the tube - approaching a vacuum - and the glass was treated with special fluorescent chemicals, the effect was enhanced. http:/ / njc.tl/ xy Slide 8 / 207 Cathode Rays That seemed to mean that something was traveling in a straight line from the negative plate (the cathode) towards the positive plate (the anode) and the positive plate stopped some of it - creating a shadow surrounded by the glow. Since the "rays" seemed to come from the cathode, they were called cathode rays. Click on the website below the diagram to see it in operation: http:/ / njc.tl/ xy Slide 9 / 207 Cathode Rays Light casts a shadow - so it could be light. Or, it could be a stream of invisible particles. http:/ / njc.tl/ xy
Slide 10 / 207 Cathode Rays What two ways could you check to see if the rays were charged particles? One rule - you can't open up the glass tube! Discuss in your group. http:/ / njc.tl/ xy Slide 11 / 207 Apply an Electric Field One idea is to apply an electric field across the path of the rays. If they are charged particles, they will be deflected; if they are waves, they will not be. Also, the direction they are deflected will indicate whether they are negative or positive. + - http:/ / njc.tl/ xy Slide 12 / 207 1 In the below diagram, the blue line indicates the path of the cathode rays in the presence of an electric field. Are the rays: A Waves B Positive particles Answer C Negative particles D Neutral particles + - http:/ / njc.tl/ xz
Slide 13 / 207 Or, apply a Magnetic Field Another idea is to apply a magnetic field (shown with the red X's) across the path of the rays. If they are charged particles, they will be deflected; if they are waves, they will not be. Also, the direction they are deflected will indicate whether they are negative or positive. http:/ / njc.tl/ y0 Slide 14 / 207 2 In the below diagram, the blue line indicates the path of the cathode rays in the presence of a magnetic field. Are the rays: A Waves B Positive particles Answer C Negative particles D Neutral particles http:/ / njc.tl/ y1 Slide 15 / 207 Negatively Charged Particles Both experiments confirmed that cathode rays are streams of negatively charged particles. Also, when different materials were tried for the cathode, the same results occurred - they were independent of the material. In 1897, J. J. Thomson used this experimental procedure of using magnetic and electric fields to learn more about these particles. He and his assistants adjusted the fields so they pushed the particles in opposite directions and canceled out - resulting in the particles going straight through. + - http:/ / njc.tl/ y2
Slide 16 / 207 3 What would happen if the electric and magnetic fields were turned on at the same time? A If the electric force was stronger, the particles would deflect up. B If the magnetic force was stronger, the particles would deflect down. C If the forces were equal, the particles would go straight. D All of the above. Answer + - http:/ / njc.tl/ y3 Slide 17 / 207 Learning about Negatively Charged Particles From measuring the magnitudes of both fields, they computed the velocity of the particles, as shown below: ΣF= ma + F B - F E = 0 F E qvB = qE F B - vB = E v = E/B Since they could measure E and B, they could find v. http:/ / njc.tl/ y4 Slide 18 / 207 4 What is the velocity of a charged particle that passes undeflected through a magnetic field of 4.0 T and an electric field of 4.0 x 10 3 N/C? Answer http:/ / njc.tl/ y5
Slide 19 / 207 5 What is the velocity of a charged particle that passes undeflected through a magnetic field of 8.0 T and an electric field of 6.0 x 10 3 N/C? Answer http:/ / njc.tl/ y6 Slide 20 / 207 6 What perpendicular electric field will allow a particle whose velocity is 610 m/s to pass straight through a magnetic field of 2.0 T, and not deflect in either direction? Answer http:/ / njc.tl/ y7 Slide 21 / 207 7 What perpendicular magnetic field will allow a particle whose velocity is 920 m/s to pass straight through an electric field of 8.0 x 10 3 N/C, and not deflect in either direction? Answer http:/ / njc.tl/ y8
Slide 22 / 207 8 Then, the electric field was turned off, but the magnetic field was left on. What would the experimentalists see happen? A The particles kept going straight. B The particles deflected upwards. Answer C The particles deflected downwards. D The effect depended on the velocity. http:/ / njc.tl/ y9 Slide 23 / 207 Discovery of the Electron Now, let's see how J. J. Thomson and his team used all of these experiments to find an interesting property of these negatively charged particles. They balanced the electric and magnetic fields so that the particles went straight through the vacuum tube. This enabled the team to calculate the velocity of the particles. Then, they turned off the electric field. The particles were deflected downwards, in a semicircular orbit and struck the side of the tube. http:/ / njc.tl/ ya Slide 24 / 207 Learning more about charged particles They measured the radius of the semicircular orbit, by seeing where the particles struck the tube and made it fluoresce. From these measurements they could determine the ratio of the particle's charge to its mass: q/m. v F B r http:/ / njc.tl/ ya
Slide 25 / 207 Calculating Charge to Mass Ratio The particles had a velocity v=E/B before the electric field was turned off. The magnetic field now exerted a perpendicular force on the electrons, which caused the semicircular orbit. Substituting in the set values for E and B, and Thus, the charge the measured r: to mass ratio was determined. http:/ / njc.tl/ ya Slide 26 / 207 The Electron Thomson found that this particle had a very low mass for its charge. In fact, its mass per charge was 1800 times less than the previous lowest amount measured for a particle. Before this work, physicists were speculating that the Hydrogen atom was the smallest fundamental particle. This led Thomson to propose that this negatively charged particle was new - and he called them "corpuscles." The name "electron" was taken from George Johnstone Stoney's work in 1874, and proposed again by George F. Fitzgerald - and the name stuck. Furthermore, since the electron was so much lighter than the hydrogen atom, it was concluded that it must be part of an atom. http:/ / njc.tl/ ya Slide 27 / 207 9 Which of the following is not true about cathode rays? A They orginate from the negative electrode. B They travel in straight lines in the absence of electric or magnetic fields. C They are electrons. Answer D They depend on the material from which they are emitted. http:/ / njc.tl/ yb
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