In 1897 J.J. Thomson discovered the electron. While he could not find all the se
ID: 1987937 • Letter: I
Question
In 1897 J.J. Thomson discovered the electron. While he could not find all the separate properties of the electron, he very accurately found the charge to mass ratio, Q/m, using a fairly simple set up, and the same physics we have learned in class. Thomson knew electric and magnetic fields could deflect charged particles, and used these concepts to prove that cathode rays were actually negatively charged particles which were later named electrons. There are several set ups which can mimic Thomson’s discovery, and you will be working out the details of two of them. Note that you will not be solving for a number for the charge to mass ratio, but rather a formula that could be experimentally tested. Your equations will have a few parameters an experimentalist could tweak to get results. With the charge to mass ratio known, 1.758820150*1011 C/kg, find some values for your parameters that would produce the correct results.The first experiment involves a vacuum tube with a cathode and anode at one end, and a phosphor plate at the other (the the electrons strike the phosphor it glows, signifying where the electron hit). The cathode/anode pair provides an initial voltages source to accelerate the electrons. The electrons then pass through a small slit in order to focus them into a thin beam. This bean then passes through two parallel plates (creating an electric field), and past the midpoint of two Helmholtz coils (two large current carrying coils of radius R, separated by a distance R). If the voltage across the plates and the current across the Helmholtz coil are tuned just right, there should be no deflection of the electron beam, and the phosphor plate should simple glow at its center. Using the concepts and equations discussed in class, find an expression for the charge to mass ratio, Q/m.
The second experiment is similar to the mass spectrometer we discussed in class. An electron beam is first accelerated by a voltage source, and then passes into a region of uniform magnetic field. The experiment is performed in a vacuum tube which is filled with a small amount of phosphor gas so the arc of the electron bean can be seen and measured. Find a new expression for the charge to mass ratio, which this time will also include the radius of the arc the electron beam would make.
Hint: Your final expressions should not contain the velocity of the beam, as it cannot be directly measured. Think about work and energy to find a way to remove velocity from the equations.
Now pretend you are giving your equations to an experimentalist. Provide some voltages, currents, magnetic fields, etc. for each of your equations to give no deflection, and hence the proper charge to mass ratio.
Please show all of your work. After you have derived your expressions, briefly explain in a paragraph or two what concepts you used and why you chose the formulas used. After J.J. Thompson found the charge to mass ratio, he concluded the electron must have an extremely small mass. Based on the charge to mass ratio given (pretend for this part you do not know the charge and mass of an electron separately), try to give some reasons why Thompson made his assumptions.
Explanation / Answer
Joseph John Thomson (J. J. Thomson, 1856-1940; see photo at the Science Museum, London) is widely recognized as the discoverer of the electron. Thomson was the Cavendish professor of Experimental Physics at Cambridge University and director of its Cavendish Laboratory from 1884 until 1919. For much of his career, Thomson worked on various aspects of the conduction of electricity through gases. In 1897 he reported that "cathode rays" were actually negatively charged particles in motion; he argued that the charged particles weighed much less than the lightest atom and were in fact constituents of atoms [Thomson 1897a, 1897b]. In 1899, he measured the charge of the particles, and speculated on how they were assembled into atoms [Thomson 1899]. He was awarded the Nobel Prize for physics in 1906 for this work, and in 1908 he was knighted. His Nobel lecture is reproduced below. The case of the electron raises several interesting points about the discovery process. Clearly, the characterization of cathode rays was a process begun long before Thomson's work, and several scientists made important contributions. In what sense, then, can Thomson be said to have discovered the electron? After all, he did not invent the vacuum tube or discover cathode rays. Discovery is often a cumulative process. The credited discoverer makes crucial contributions to be sure, but often after fundamental observations have been made and tools invented by others. Thomson was not the only physicist to measure the charge-to-mass ratio of cathode rays in 1897, nor the first to announce his results. (See Pais 1986.) But Thomson did carry out this measurement and (later) the measurement of the particles's charge, and he recognized its importance as a constituent of ordinary matter.