Physics 132, Reminders from 3/09/2015 TA & LA Training, for week of 3/09 (Lab 8, Part 1)

1) Recitation: What is "free" about free energy?

(Break--Pass Back graded Lab 7 reports, if ready....)

2) Lab 8, Part 1: Modeling Signal Transmission Along Nerve Axons

Link to documents: NEXUS Physics Labs, 2013-2014

Pre-readings: Resistive electric flow: Ohm's law (2013) & Kirchhoff's principles (2013)

*** In the event that you need replacement parts (resistors, alligator clips, wires, micrograbbers, etc.), these are in a small cardboard box at the font of the room (on the TA desk).

Extra notices for the VERY START of the lab:

1) Point out at the start that we are modeling PASSIVE transport, and showing why passive transport is NOT the method of choice for most mammals (proof by contradiction).

2) Ask your students if they have every actually built a circuit before (even just a bulb, wire, battery circuit), either in middle school or in high school.  I suggest you mix up the groups so that you have at least one person who has some circuit experience in each group.

3) Remind students that the two weeks of this lab are separated by SPRING BREAK--so they should be very careful in documenting what they do this week (maybe even using their cell phones to take photos) so that they will have an easier time remembering their previous work and their next task when they return after the break (when we will also move to room 3312).

a) Motivation/Pedagogy: This is a wonderful lab for modeling why passive transport is NOT the mechanism used by human beings to transport signals on our nerve axons.  (This is a proof by contradiction: we assume passive transport is the correct model, show that it fails under every modification, and therefore active transport must be the correct mechanism.)  Your students will have had varying degrees of exposure to nerve signal transport.  This is an important idea in biology, though, so it is very much worth our time to investigate in this physics class.  This particular lab is adopted from 'cookbook' work created at other universities--I have 'un-cooked' this lab to the extent possible, but it is still a very heavily scaffolded lab.  (In contrast, our next lab, Lab 9, will be extremely non-directed; and where Lab 8 is very deductive in reasoning, Lab 9 will be entirely inductive.)  Physics content-/equipment-wise, this is the students' first exposure to circuits and circuit concepts and their only chance to build circuits in our physics course.  Students may need a review of the circuit diagram symbols and a reminder of Kirchhoff's Loop Rule.

b) Papers: The student document for this lab has been split into two packets--one for Parts 1 and 2 (initial modeling) and another for Parts 3 and 4 (data collection and analysis and further modeling).  Each part of the lab ends with a TA/LA checkpoint.  Use this checkpoint as an opportunity to make sure that each group has gotten the important ideas in that part of the lab before they advance too far into the next part.  There are a TON of questions in this lab.  Not all need to be explicitly responded to in their lab reports.  The  ! and ? symbols are in the margins to help them determine what we want to see in their final lab reports.  Students should be constructing the lab report as they go and should be careful to include all necessary information.  The lab document has some very intentionally-constructed (and clever, I think) pictures to help students engage in the modeling/circuitry/software-usage.  Encourage them to carefully exam the images in the lab document.  I also encourage you to pass out these two packets one at a time--this forces them to start with the modeling (without having extra papers or lab equipment cluttering the tables) and ensures that they have checked with you before they move on to the data-taking (when you can pass them the second packet as well as their equipment AND carefully explain the safety instructions, group-by-group).  It may seem like a waste of time to give safety instructions group-by-group (six times) instead of just giving the instructions once at the start of class but: A) the instructions don't make sense without the equipment to look at; B) once they have the equipment, it gets in the way of using the whiteboard for the modeling; and C) with both a TA and an LA available, it really isn't that hard or time-consuming to give the safety instructions and the equipment to the groups on an as-needed basis (the modeling is difficult enough that all the groups will arrive at the needing-equipment-and-instructions stage at different times).

c) Modeling: Modeling with multiple representations is a VERY important part of this lab.  Part of the reason the lab document has been split is to prevent students from jumping to the data collection and analysis before they have fully engaged in the initial modeling.  Parts 1 and 2 of this lab will take the quite a while to complete.  Encourage the students to be careful and conscientious in their modeling.  As they construct multiple representations of passive signal transmission, also encourage them to regularly shift between representations, connecting the new models they build to the previous representations.  All told, students will examine a drawing of the physical situation, create a circuit diagram (for individual and multiple segments of axon), mathematically and geometrically model the resistors, create mathematical representations of the current and voltage in the circuit, return to the drawing to add arrows representing the current flow, and create a graphical representation of the voltage across the membrane.  Connecting these multiple representations to each other and back to the physical model (the actual nerve) will help the students enhance their own understanding and prepare them to construct the physical circuit in Part 3 and contemplate modifications to the physical system in Part 4.

d) Predicted progress in Week 1: We expect ALL students to complete the first packet (Parts 1 and 2) and to start building and testing the first circuit in the second packet (Part 3), in addition to starting their lab report.  Next week, we expect all students to finish building and testing the circuits (Part 3) and to complete the investigations of modifications to the nerve (Part 4), as well as to complete their lab report.

e) Reminder of safety instructions: The resistors can get HOT, so have the students close the switch only when taking data (opening the switch again immediately after the data is taken) and handle the resistors carefully.  The students should be careful to avoid a short-circuit--build the circuits carefully and have a TA check them (at least the first one) before the students close the switch and take data.  Students should be especially careful to make sure that the alligator clips do not touch each other--a short-circuited battery will get HOT VERY QUICKLY and will also cause the burnt metal smell in the air--disconnect the battery immediately if you notice this and then let the battery cool before you touch it and before you try to use it again (a hot battery does not function properly).  NO ADDITIONAL EQUIPMENT, beyond what is present in each plastic bin, is needed for these labs--students should not 'borrow' equipment from other bins; the more equipment they have the more likely they are to cause a short-circuit!  Encourage the students to treat the micrograbbers gently--depress the plunger to attach/remove the grabber; never yank the micrograbbers.  The LabPro (ULI) needs to be plugged into a power strip (it will sing when it is first powered-up), linked to the computer with a USB cable, and connected to the circuit they are testing with the voltage probe (with micrograbbers).

f) Check the circuits the students build carefully.  The membrane resistor should be larger than the axon resistor.  The trays for the resistors include both the color code and the value for each of the resistances.  There is also a resistor chart at the front of the room.  If a circuit does not appear to be working, check that the wires are all placed in the correct holes, that the wires are all pushed into the holes fully, and that the circuit the students have created is correct and complete (the pictures in their lab document can guide their circuit-building and their use of the Logger Pro software).  If you haven't yet done so, review with the students the construction of the circuit/bread board--the central two columns of holes (red line and blue line) are electrically connected in a vertical line; the other holes in the board are connected in horizontal rows, and the dip in the breadboard is an electrical break between the rows, as are the red and blue lines.  Students should ensure that the START of their circuit is correct--including screwing the red and black knobs tight on the wires and making sure that the high voltage leads through an axon resistor before first crossing a membrane resistor to the low voltage/ground (extra cellular fluid). 

g) Ask the students regularly to connect the multiple representations/different models together--which part of the circuit is the axon?  Which part the membrane?  How do the different parts of the physical circuit match up to the circuit diagram?  To the drawing of the nerve?  Is the data they collect in Part 3 consistent with the graphical model they predicted in Parts 1 and 2?  Remind students to include in their report discussions of the exclamation points (!) in all parts (1 through 4) of the student document AS WELL AS a discussion of how this relates to things they have learned in their other biology/chemistry classes.  Diagrams, drawings, pictures and other representations of their models should also be included in their report.  (If any are beautiful and great in color, have the students also email the report to you and then forward it to me!)  Graphs produced in Logger Pro, with the data fit functions, should be included in the report--one for each circuit they build.

h) Please have students put their equipment away neatly when they have finished taking data.  This includes taking apart their circuits, sorting and straightening their resistors (there should be 10 of each kind, replacement resistors are on the TA table at the front of the room), placing all equipment neatly in the plastic bin, and placing the bins neatly in the center-back cabinet.  "All equipment" means: 3 trays of different resistors, two circuit boards, 1 battery, 3 banana plug wires, two alligator clips, one switch, and one voltage probe (with micrograbbers).  Have the students leave the LabPro (ULI) on their group's lab table.

i) Organize papers and clean up tables after recitation/lab.  Place sorted papers in the back of the room by the printer.

That's all for now.  Good luck!

~KIM