PurposeDetermine the charge to mass ratio (e/m) of an electron.Design your own e
Weir, Mike, Executive Producer has reference to this Academic Journal, PHwiki organized this Journal PurposeDetermine the charge to mass ratio (e/m) of an electron.Design your own experimental procedures. Decidewhat measurements to take,the range of values you will measure,how to obtain the uncertainties of measured in addition to computed quantities,how to analyze the data graphically, in addition to what error propagation procedures to per as long as m.Gain experience in writing a report in the as long as m of a scientific paper.Background1892: Hendrik Lorentz introduces the as long as ce acting on a charged particle moving through a magnetic field (the Lorentz as long as ce, or magnetic as long as ce):The cross product means:Only the components of the vectors v in addition to B that are perpendicular to each other contribute to the as long as ce.The as long as ce is perpendicular to both the vector v in addition to the vector B.qThe charge here is assumed to be positive.For negative charges the as long as ce would be downwards.(points into the page)
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qThe as long as ce continually changes the direction of the velocity, but not its magnitude.The direction of the as long as ce changes also in addition to its magnitude also remains the same.(points into the page)circular pathqrFor Circular Motion (Radius r)Derive expression as long as e/m(v,B,r)= Getting the electron up to speed in addition to knowing what that speed is.IfilamentfilamentelectronsMany electrons return because they hit the plate.(Note: return path of electronsis opposite to conventional current.)acceleratingvoltage(150V-300V)filamentVoltage(6V)Some electrons miss the plate in addition to flythrough the hole in the plate. electron beam comes out of hole.The filament current heats up the filament electrons get enough energy to exit the filament.Electrons get accelerated by the as long as ce in the electric field (created by the accelerating voltage).Speed increases as electrons approach platevvvE
Getting the electron up to speed in addition to knowing what that speed is.vvvelectron has potential energy = eVacc in addition to kinetic energy = 0electron has potential energy = 0 in addition to kinetic energy = ½ mv2Conservation of energy:Generating the Magnetic FieldA pair of Helmholtz coilsN turns of wire in each coil.IIBMagnetic field in thecenter of the Helmholtzcoil pair.aIIPower SuppliesBe as long as e you switch on the power supply: Identify all components according in addition to verify that all connections are as shown in the lab manual. Identify on the high voltage power supply the dial that controls the accelerating voltage: Turn that dial all the way counterclockwise into the zero position. Identify on the low voltage power supply the two dials that control the current through the Helmholtz coil: Turn both dials (voltage control in addition to current control) counterclockwise into the zero position. Identify on the low voltage power supply the heater voltage dial. This is a step dial. Verify that it is set to 6 Volts.
High Voltage SuppliesOlder modelNewer modelLow Voltage Power SuppliesOlder modelNewer modelHeater voltage:must be dialedto 6VHelmholtz coilcurrent in addition to voltage regulationSwitching on the Power SuppliesNow switch on both power supplies. Check in addition to make sure that the Helmholtz coil current is zero in addition to the accelerating voltage is also zero.Look at the tube: You should see the filament glow orange/yellow.Carefully dial up the accelerating voltage on the high voltage power supply to about 200V (never exceed 300V).Regulating the current in the Helmholtz coil:There are 3 knobs that control voltage in addition to current through the Helmholtz coil:Current adjustment knob as long as Helmholtz coils on the e/m apparatus.Current adjustment knob on low voltage power supply.Voltage adjustment knob on low voltage power supply. Next page: how to use these three knobs.
Regulating the Current in the Helmholtz Coil: First turn DC VOLTAGE ADJUST to zero (all theway counterclockwise)Then turn DC CURRENTADJUST all the way clockwise (max. current).Then turn current adjust knobOn the e/m apparatusall the way clockwise (max. current).The current meter should STILL READ ZERO.Regulating the Current in the Helmholtz Coil: Carefully turn DC VOLTAGEADJUST clockwise. Watch the current meter!When current meter reaches2 Amps: Stop!!Leave at this setting!!From now on only regulatethe current with this knob onthe e/m apparatusTotally clockwise = 2A Totally counterclockwise = 0ASwitching Off Power SuppliesBe as long as e you switch off the power supply:Identify on the high voltage power supply the dial that controls the accelerating voltage: Turn that dial all the way counterclockwise into the zero position.Identify on the low voltage power supply the two dials that control the current through the Helmholtz coil: Turn both dials (voltage control in addition to current control) counterclockwise into the zero position.Now switch off.
Hints When measuring the radius of the electrons circular path, take advantage of the mirror-scale behind the tube: – Move your head to the left or right until the mirror image of the beam overlaps the actual electron beam. Then read off the scale. – Repeat the procedure as long as the left side. – Determine diameter from the two values (radius is half of that).Read off right sideRead off left sideImportant as long as the Longevity of ApparatusNever exceed 300V on the accelerating voltage. Always monitor this voltage on the attached volt meter when changing that voltage.Never exceed 2A on the Helmholtz coil current. Always monitor this current with the attached current meter when changing that current.Always dial down accelerating voltage in addition to Helmholtz current to zero be as long as e switching off the power supplies.Lab ReportLab report must be written in the as long as m of a scientific paper (see example in the back of the lab manual).Each person must write their own lab report (no group report as long as this experiment).You have 1 week to complete in addition to submit the lab report.Late lab reports will get a deduction of 5% per day late (e.g., if your report is 3 days late you will only get credit as long as 85% of the score that you would have gotten if you submitted on time).
Weir, Mike Executive Producer
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