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# Fall 2016 Physics 131 Reminder from 10-28 Training

last edited by 3 years, 7 months ago

Reminders from 10/28 TA Training (Lab 4, Part 1) for week of 10/31

Here are the reminders from our TA training on Friday.  Please read through it all and let me know if you have any questions.

1) Recitation: Pressure, Gases, & Fluids

http://umdberg.pbworks.com/w/file/94614581/GasPhETtutorialv2.0.pdf

2) Lab 4, Part 1: Random vs. Directed Motion:

a) Make sure to have students consider the system schema and FBD for a bead on an inclined slide before they begin.  (This could be a bit tricky--especially for representing 'thermal' forces--individual collisions with water molecules.  More than anything, we just want them to see that the force of gravity COULD cause the objects to accelerate, reaching a terminal speed down the incline due to a balance between gravity and the viscous resistive force (and other forces, too, but they may not think of buoyant force--no one included it at our training!).  If they are having particular trouble, encourage them to model for the large bead.)  Also have them watch the tracked video of various-sized beads in water on an incline (in the 'My Documents' folder of the desktop computers, entitled "Overlay Progressive Lines 1-5umTilted2Cropped") and describe what they are seeing and what characteristics of the beads differentiate their exhibited motion.  If students do not find a satisfying reason why the different-sized beads exhibit different motion, this is something they should think about and be ready to discuss in week 2!

b) When students are ready to gather data, their equipment (inclining block of wood & blue trays for pipettes and chamber slide) are in the plastic bins in the right-center cabinet in the back of the room.  Extra eyedropper pipettes are also in this cabinet.  The bead solutions are in the block in the fridge and drying chamber slides are on top of the fridge.  Each group needs two pipettes (one for each solution) and one chamber slide.  Remind students to treat the chamber slides gently.  Instruct them on using the wooden block to incline the microscope (long edge of the wooden block lying flat along the table, microscope on the topmost or second-to-topmost stair).  [If the bead solutions are running low, you (as TA or Helper, but don't let students do this) can refill the small vials from the large centrifuge tubes of stock solution--these are in the refrigerator and are clearly labeled.  Shake them up (or vortex them) and draw the fluid from the bottom of the tube using an 'eyedropper' 1mL pipette.  Then seal off all the tubes and place them back where they belong.  If the large centrifuge tubes are running low, please let Omar know ASAP.]

c) We expect that, in this first week of the lab, students will be able to: i) collect all three videos, ii) harvest the data from all three videos using ImageJ, and iii) start getting the spreadsheets for these videos into the proper form for <r^2> vs. t plots and log-log plots (though they may not actually make the plots until next week).

d) Students should be reminded to note the average frames-per-second for EACH video, as recorded by VirtualDub.  They should also be noting the magnification and resolution for the video (so that they can use the correct calibration slide to get distance-to-pixel information).  You should consider having students show you their "good" videos when they think they have gotten them--that way you can look for classic "bad video" problems, such as insufficient "trackable" beads (due to bead density too high, mostly stuck beads, etc.) or flow due to convection currents (though flow due to inclined plane is expected).  There is no point in analyzing a bad video, so if they need to take one or more videos again until they get a good video, they should do so!

e) If they are having trouble adjusting the frame rate, try the following:

1. Check that the "Video"->"Capture Pin" is set for Medium or Low resolution (High resolution files are too big to analyze and VERY slow to record).
2. Check that the "File"->"Set Capture File" is saving to the computer itself, not to a flashdrive.  If saving to a flashdrive is somehow faster, wipe the computer (as I demonstrated in training)!  Don't forget to wipe the "Downloads," "My Pictures," and "My Videos" folders.
3. Check that the "Capture"->"Stop Conditions" total video time is set to a reasonable amount--short times for the first two videos and about 100 seconds for the long video of the 2um beads (the third video).
4. Default the "Video"->"Capture Filter" settings in all of the tabs.
5. Adjust the light on the microscope (brighter for fast/high frame rates and darker for slow/low frame rates).
6. Enter the "Video"->"Capture Filter" settings and tick the "Automatic Adjustment" box--VirtualDub should be able to do a good job of adjusting to make the image a more normal brightness, automatically fixing the frame rate to account for the changed lighting.  Then re-enter "Capture Filter" and un-check this box (as leaving it ticked all the time tends to cause problems).

f) Students should be analyzing (ImageJ and Excel) on multiple computers, starting ASAP after collecting their first good-quality video.

g) Please encourage your students to clean up after themselves (throwing out their 1 mL 'eyedropper' pipettes, discarding any wet paper towels, replacing their solution vials in the appropriate portion of the block in the fridge, and flushing their chamber slides using the DI water by the sink and leaving the slides upside down on a paper towel on the fridge to dry--you, as TA, should tap these upside down chambers to speed their drying).  We also need to keep all the lab documents--these same documents will be used for both this week and next week's lab sections!  [In the event that a chamber slide is broken--though this should NOT happen--please save all of the parts and get them to Omar ASAP.]

h) Remind your students to take their data (Excel files, Word documents, etc.) with them when they leave the lab--the computers will be wiped before Lab 4, Part 2!

Okay!  That's everything I can think of!

Good luck!

~KIM