User:Brian P. Josey/Notebook/Junior Lab/2010/11/08: Difference between revisions
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==Set-up== | ==Set-up== | ||
[[Image:2010-11-15 14.05.49.jpg|thumb|right|The bulb containing helium surrounded by the Helmholtz coils.]] | |||
[[Image:2010-11-15 14.06.14.jpg|thumb|right|The Gelman and HP power supplies.]] | |||
[[Image:2010-11-15 14.06.39.jpg|thumb|right|The SOAR power supply for the heater.]] | |||
[[Image:2010-11-15 14.07.08.jpg|thumb|right|The final set-up including the meters.]] | |||
Equipment: | Equipment: | ||
Revision as of 09:03, 21 November 2010
 Project name
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e/m RatioThis week, my partner, Kirstin, and I did the e/m ratio experiment. This experiment it used to determine the ratio of the charge of an electron to its mass. Combined with the Millikan oil drop experiment, two fundamental physical quantities can be determined, the mass and the charge of the electron. This quantities are significant in that they are very useful in quantum mechanics, and more importantly, the charge of the electron is the fundamental charge and the smallest amount of charge that can exist on its own. (Some other subatomic particles have fractional charge, but they cannot exist on their own.) Historically, this experiment was first conducted by J. J. Thomson in 1897 using cathode ray tubes. This experiment, however, is a little more updated and has a different approach than the original groundbreaking experiment. In place of the cathode tubes, we have a glass tube full of a very dilute helium gas. This tube is surrounded by a Helmholtz coil that can supply a nearly uniform magnetic field throughout the whole tube. In this tube, we will release electrons from a heater plate, focus them into a nearly coherent beam, and apply a varying magnetic field. This varying magnetic field will change the trajectory of the electrons, which emit light from the collisions with the helium, so that it forms a complete circle. We then measured the dimensions of the circular path, and the voltages used to free the electrons to determine the ratio, e/m. Set-up    Equipment:
Connections:
MultimeterWe also used three different meters, one ammeter and two voltmeters, to monitor the currents and potentials that we fed into the experimental set up. They were:
ProcedureAfter we set up all of the equipment, we began the experiment. First we turned on the heater for the electron gun and let it warm up for two minutes, after which, we raised its initial voltage to 6.3 V as measured from the voltmeter. This voltage was kept constant for the duration of the experiment. We then raised the acceleration voltage of the electrons and focused the beam so that it appeared as a single line firing to the right. After the generating the electron beam, we set out to measure the current passing through the Helmholtz coils and the voltage of the electrons. By adjusting the current going into the Helmholtz coil, we were able to change the path of the electrons by bending it. A higher current resulted in a higher magnetic field acting on the electrons, and a tighter coil. Adjusting the accelerating potential on the electrons would also change their pattern, a higher voltage resulting in a tighter circle. We then made twenty measurements. For the first ten, we kept the accelerating voltage constant near 200 V, and adjusted the current of the Helmholtz coil. The last ten measurements were conducted by maintaining a constant current, at 1.125 A, and adjusting the voltage. To measure the radius of the beam, we aligned the beam with its reflection on the ruler behind the bulb, and recorded the marking that they crossed. While we measured the radius on both the left and right sides, we will only used the radius on the right. The reason for this is that the one on the left is more likely to change depending on the interactions that it goes through with the helium in the tube. It was only measured to serve as a check for the experiment, weeding out any discrepancies. Data and ResultsHere is a summary of the data that I collected from this experiment: {{#widget:Google Spreadsheet |
key=0AjJAt7upwcA4dFpiSk1kcXc2SFZTS3hSQkNXekVYa0E | width=600 | height=325
}} In the procedure, we are supposed to measure the ring radius twenty times. The first ten times, I kept the acceleration voltage constant at 200 V, and then varied the current feeding into the Helmholtz coils. For the second ten times, I kept the current into the Helmholtz coil constant at 1.25 A, and varied the acceleration potential in steps of 10 V. For the radius of the ring, it was unclear on which one I needed, so I have two values for each measurement. The "right" value is the radius of the ring on the right side where the electrons came out of the gun, and the "left" value is on from the left side, where the electrons curl back into themselves. | |
