- Computers are needed for lab this week
- The sticky notes have been removed from the meters this week. Hilarity ensues.
- This week, students will be using a dual-power supply (set buttons for "Series") or two singles connected in series. You'll need to show each group how to use their power supply when they get to Experiment 4
- Don't expect them to remember what they learned previously about how to connect a voltmeter and ammeter!
- We use cheap power supplies, so it might be difficult for students to get exactly 5.0v, 10.0v, etc. Tell them to get close, take a current reading, record I and V and then move on
- Instructor Demo: Connect the small wire coil
to an Ω-meter and measure its resistance. While still connected to the
meter, dip the resistor in the liquid Nitrogen, and observe the change. Do
the same with the carbon resistor.
- Measure the wire coil first. It has long wires, so you don't need to dip the gators connected to the ohmmeter into the LN2 – much easier to then switch to the carbon resistor! Here are my measurements from Spring 2020:
-
| |
Rroom temp (kΩ) |
RLN2 (kΩ) |
| Wire Coil |
3.75 |
0.48 |
| Resistor |
0.550 |
0.836 |
- Liquid Nitrogen temp = 77 K = -321 °F
= -196 °C (in case anyone asks, liquid Oxygen temp = 90 K)
- Don't leave LN2 unattended in lab; I did that once, and students had a pen cap explode on them!
- Some rubber bands are generally left on the bench. Freeze one, then smash it. Hilarity ensues. More ¼" rubber bands can be found in the Majors' stockroom, shelf L6 (with the stuff to make motors for Phys104)
- Jeff will take out the thermometer gun (it generally lives in the locked cabinet of the Majors' stockroom). You can show students the temperature of various objects, and how it freaks out when pointed at the dewar. Measure the 'room temperature' of a resistor before they start running current through it (~74°F), and then again when they have 50-volts across the resistor (~90°F)
- They can easily calculate the resistance in their data from Experiment 4 using KaleidaGraph's Formula Entry. Make sure they click the 'Expand/Collapse Column Header' button (it looks like an up & down arrow to the right of the column title) to show the column numbers (C0, C1, etc). They need to run their formula twice since the current is in two columns (Ihigh, Ilow); they only need to change the column number for the appropriate current
- They'll need guidance interpreting the residual pattern seen in their graph from Experiment 3. A second-order polynomial is fit to each set of residuals to accentuate the variation between their data points and the expected value from the best fit line. Ask them what the residuals plot should look like if all their data (from the Ohm's Law experiment) was exactly on the best-fit line (answer: horizontal line through 0).
- In general, the residuals pattern deviates greatly from the 0-line for R1 and R2, while the residuals from Rset vary much less. Therefore, whatever affected the resistances had a greater affect on the individual resistors. Be sure to point out that the magnitude of the residuals are very small
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