- Computers are needed for lab this week
- I set all meters to a "neutral" setting (I use hFE) before each lab section. That way students don't start lab with a meter already in the correct setting
- In the pre-lab, I walk everyone through connecting wires to the meters and connecting the resistor to the battery (without meters). I spend time going over the connection of the voltmeter and ammeter, and talk about connecting the circuit to the power supply.
- Each setup has five wires – that's all they'll need for this experiment!
- Tell students that if they think they need more wires, they're doing something wrong and they need to talk with you!
- Discuss which ports on the meter to plug wires into (and to Never use the '20A' terminal!)
- Start with two wires in voltmeter: red in V terminal, black in Com
- Then plug one (short) wire in the ammeter: place in A terminal, which reminds students that they must open the circuit to insert the ammeter in series
- Common mistake: they will put the free wire from the circuit in the V terminal; it must go in Com
- Power supply:
- red wire in red + terminal, black wire in – terminal. Don't use the terminal labeled GND; the circuit won't work!
- Keep Current knob so that white mark is vertical. Don't change the current limiter setting, or the circuit won't work
- Make sure Voltage knob is turned all the way off (counter-clockwise) before turning on power switch for the first time
- I put a sticky note on each meter with V or A. I tell students that the sticky notes will be in place this week and next to help them sort out the differences between the meters, but the notes will be removed in the third week. No sticky notes will be in place for the lab practical
- Since this is the first circuit they assemble, I watch them carefully as they make the connections. The instructions have them check with the instructor before turning on the power; hopefully, they actually read the instructions
- KaleidaGraph is used to analyze the data. Note that students must plot I vs. V
so that KaleidaGraph plots all three curves on one graph!
- They print their data table from KaleidaGraph this week. It's important to check that they have labeled their columns properly, not just left them as A, B, etc.
- Students don't measure resistance directly this week; they do so next week. Don't tell them about this ability just yet!
- Notation is important this week: the instructions distinguish between Rset (the pair measured together) and R1 + R2 (the individual resistances, added up). Students should stick to this notation, or chaos ensues
- Jeff has an Excel spreadsheet to check student calculations: Enter the slope and standard error from their KaleidaGraph plot, and it calculates the resistance and uncertainty of Rset and R1 + R2. A graph also shows the overlap between the uncertainties of these values
- Since R comes from the inverse-slope, a different procedure is needed to calculate the uncertainty in R (the % uncertainty). Students will know this if they bother to flip to page 3 of the instructions! The % uncertainty will again be calculated in the Lens Optics experiment later in the semester
- Suggest to students that they work through the numbers in the example given. The majority (Spring 2019) did the calculations wrong (they can't take a percentage properly!)
- When talking with students about sources of error (all students should find that R1 + R2 < Rset; the spreadsheet will check their results), be sure to ask them what assumptions are made about this experiment. The key phrase appears on pg. 2 of the instructions in boldface, italics and double-underlined: "… at constant temperature …"
- Update Spring 2023: Going forward, the color code section has been moved to an appendix in the instructions and getting resistance from color codes will no longer be tested on the lab practical. The following is retained for historical interest.
- The third stripe (indicating the power of 10) is red (102 Ω) on all resistors used in this lab. Some of these aging resistors are discolored, as well as the white stripe (which looks yellow on R2 of set D). Tell students not to worry; if they find that they picked the wrong color, they can go back and change it. During Spring 2019, I looked at all my student ohmmeter measurements (from the "Systematic Error" lab) and found that their measured resistances were within 3% of the color code resistance; getting one color wrong resulted in a 4.5% error. Here are the colors of each resistor:
| Rset |
R1 Colors |
R1 (kΩ) |
R2 Colors |
R2 (kΩ) |
| A |
Orange, Orange, Red, Silver |
3.3±10% |
Orange, White, Red, Gold |
3.9±5% |
| B |
Red, Violet, Red, Silver |
2.7±10% |
Orange, Orange, Red, Gold |
3.3±5% |
| C |
Orange, Blue, Red, Silver |
3.6±10% |
Red, Violet, Red, Gold |
2.7±5% |
| D |
Orange, Orange, Red, Gold |
3.3±5% |
Orange, White, Red, Gold |
3.9±5% |
| E |
Red, Red, Red, Silver |
2.2±10% |
Orange, Blue, Red, Gold |
3.6±5% |
| F |
Red, Red, Red, Gold |
2.2±5% |
Red, Violet, Red, Gold |
2.7±5% |
| G |
Orange, Orange, Red, Gold |
3.3±5% |
Orange, Blue, Red, Gold |
3.6±5% |
| H |
Orange, Orange, Red, Gold |
3.3±5% |
Red, Violet, Red, Gold |
2.7±5% |
|
