Physics 241 Lab – Matt Leone Week 2: Cathode Ray Tube [email protected] (email preferred), PAS 376, o. 520-621-6819 Office Hours: M & W 11:00-11:50, or by appointment. Consultation Room (PAS 372): F 12:00-12:50 General Comments: • Don’t forget that there is a lab practical at the end of the semester is worth two lab report grades. Basic circuit building and oscilloscope operation are essential techniques for passing so be sure BOTH lab partners practice using the equipment in EVERY lab. • You must submit to Turnitin.com before turning in your paper in class! • (again) ALL labs count for a grade, so 12*100 = 1200 points possible on lab reports (another 200 for the practical), but if you miss more than 2 you automatically fail the course! • 1200 report points + 200 practical points = 1400 total points. NO LABS ARE DROPPED. Lab 2 – Summary: There are some main goals to this lab: 1. Learn how charged parallel plates can be used to accelerate a charged particle or how they can be used to deflect a charged particle. In this case, the particle being accelerated and deflected is an electron. 2. Discuss the important deflection equation, ! Dx=L * l2d"Vx"Vz or ! Dy=L * l2d"Vy"Vz. Note that we will call ! L * l2d the geometric constant because we can’t know each of these variables separately without opening the CRT (vacuum glass). Some students prefer to just write one variable for the geometric constant, ! Dx= g"Vx"Vz because they find this less confusing. 3. Test the above deflection equation experimentally in three separate ways. Use graphs of your data to provide for experimental support for your theoretically derived deflection formula. 4. Check the magnetic term in the Lorentz force equation, ! r F = qr E + qr v "r B . This equation says that a charged particle feels a force from an electric field AND from a magnetic field, BUT the force from the magnetic field depends on its velocity. You will quickly test the magnetic effect of the Lorentz force equation qualitatively with your CRT and a small magnet. Lab 2 – DEMO: 1. Hook up wires: “yellow, yellow, brown, blue, red.” and turn on. 2. Change ΔVZ = ΔVB + ΔVC and get electron beam to appear on scintillation screen. To change accelerating voltage, change Δ VB and then focus beam to dot using ΔVC. 3. Hook up wires from CRT correctly to little box. The little box separates charge on the deflection plates so you can have ΔVX, ΔVY or both. Make electron beam wiggle. 4. Use DMM to get ΔVX causing beam deflection. 5. Get average and standard deviation from three numbers: 4, 6 and 14.Lab 2 – Matt’s theory discussion on board 1. Pic of how CRT works. 2. Baseball diagram relating F, E, ΔU and ΔV. 3. Discuss the deflection equation. 4. Discuss the Lorentz force equation. 5. Taking data and making graphs: take enough data points to obtain good line of best fit in graph (so make graphs in lab). Graphs should o Be hand made and somewhat neat (use rulers but it doesn’t have to be perfect). o Include title, labeled axes, units on axes, line of best fit, and slope of line of best fit. o When someone says “graph first thing vs. second thing” that means first thing on the y-axis and second thing on the x-axis. Lab 2 – Suggestions for Procedure 1. Experiment 1: a. Use tape on screen to mark position of electron beam when there is NO DEFLECTION. Be sure to record ΔVZ. b. Adjust ΔVX and mark DX on the tape for several ΔVXs (be sure to record ΔVXs as well). c. Remove tape and measure DXs with ruler. d. Create data table from these measurements: ΔVX vs DX. e. Create graph of DX vs ΔVX by hand. [GRAPH 1] f. Measure the slope of the line of best fit. Since ! Dx= g"Vx"Vz, the slope will equal ! slope =g"Vz so find g. Remember y=mx+b with b=0 in this case. [RESULT 1 numerical – some points awarded for accuracy – use SI units] 2. Experiment 2: Redo experiment 1 in Y-direction. [GRAPH 2] & [RESULT 2 numerical] 3. Experiment 3: {WARNING – DELICATE MEASUREMENTS NEEDED} g. Begin with no deflection using ΔVZ about as large as you can get it while in focus. Mark the zero deflection position on the tape. h. Adjust ΔVY so that the beam is deflected by about 1 cm. If your apparatus is rather fuzzy, try ½ cm. This is your first data point of ΔVZ vs DY. mark it on your tape. Be sure to record Δ VY. i. Slowly lower ΔVZ and refocus so that deflection is more. Mark position on tape. Do this for several ΔVZs. Remove tape and measure DYs with ruler. Be sure to record your ΔVZs as well! j. Create data table from these measurements: ΔVZ vs DY. k. Create graph of DY vs ! 1"Vz by hand. [GRAPH 3] l. Measure the slope of the line of best fit. Since ! Dy= g"Vy"Vz, the slope will equal ! slope = g"Vx so find g. [RESULT 3 numerical] 4. Use basic statistics to report a final experimental value for the geometric constant, g. Report in the standard way: [RESULT 4 numerical] [REPORTED VALUE] = [AVERAGE] +/- [STANDARD DEVIATION] 5. Experiment 4: qualitatively test the magnetic field term in the Lorentz force equation using a small magnet. [RESULT 5 observational] 6. CLEAN UP LAB STATION.Lab 2 – Report Guidelines This week’s lab is one big experiment with a couple parts, which is very different than your first lab, and so your report will reflect this. In this lab, results will mean statistical values (with units) that you calculated from your measurements AND graphs made by hand which you attach. This week, I am being very detailed in what I would like to see for your benefit. (Ellipsis means you should already know what to write). 1. Title - … 2. Goals - … 3. Theory – a. Describe in words the physics behind the operation of the cathode ray tub. Basically explain the two different effects on the electron that can be caused by charged plates. b. Examine the deflection equation. Explain how the deflection equation works. What happens to the deflection when ΔVX is increased and WHY? What happens to the deflection when ΔVZ is increased and WHY? c. Derive the deflection equation. CANCELLED!! I have decided to let you answer extra problems instead of performing this very tedious exercise. I believe this is better for you because the problems tie the ideas together better for you than this derivation would and so helps you more in your test preparation. 4. Procedure Your main goal was to obtain three
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