PHY 2049L
Lab 7

Alternating Current LR Circuit

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Updated: 2/2/2013

Guiding Question:

You must bring an extra copy of the cover page for your Lab 6 report so you can use the abstract from that report while doing Lab 7. That abstract will need to specify which "thin" box you used and the values of L and r that you measured because you will need that information to do anything in Lab 7.

Preparing for the lab:

We will be doing "Lab 36" with the addition of an extra step where we use the oscilloscope to measure the phase angle between current and voltage from what we see on the screen.

Answer the pre-lab questions on LON-CAPA after reading the background information on pages 359 through 362 in the lab manual.


Be sure to record the letter that identifies which "thin" box you used, both on your data sheet and in your report abstract. You need it in the abstract so it is available when you do the next lab.

Your abstract for this lab must identify which "thin" box contained the inductor you used and specify the values (and standard deviations) of L and r you determined in this lab. You will have to use this information in the next lab to (1) find the correct box to use and (2) supply the value of L and r needed in the calculations and analysis for that next lab.

Note that the inductor is between the 13-14 connectors for the "thin" box, while the resistors we will use are all in the "thick" box. Also note that we will use a different set of resistors than specified in the manual; we will use ones that are approximately 330 ohm (7-8), 470 ohm (5-6), 680 ohm (3-4), and 995 ohm (1-2). The actual values of the resistors in your box are written on a label on the "thick" box. Be sure to record the actual values in your data table as well as the identity of the box you used at your lab station. You will want to be sure that you are using the same lab station for the next lab.

You can use the sample standard deviation (sigma_{N-1}) rather than alpha in your data analysis. If you do, just change the column heading from alpha to sigma.

Information covered in lecture:
We will be doing this lab at the same time we are covering these topics in class. Specifically, we will sometimes introduce the complex-valued impedance early in the week and work examples relevant to both this lab and the next one about the time that you do this lab. You will not find this material in our textbook or the lab manual, but the complex value of the impedance Z = R + iX is the reason we plot the corresponding voltages (found from deltaV = I*Z) on perpendicular axes. I will give a short introduction to some differences between DC and AC circuit responses at the start of lab and remind you that part 1 of this lab is basically a repeat of the demos on phases that are done in lecture.


There are lots of cables for this experiment and the next one. I suggest using the longest ones (100 cm) as measurement leads connected to the multimeter (set for AC volts), the next longest ones (65 cm) to connect the wave generator to the circuit, and the shorter ones to finish the circuit. If you place the boxes carefully, you will find that the shortest cables (25 cm) are perfect for jumpering between them.

meter, boxes containing R and L, scope with generator and freq counter

Photo of the equipment as you will find it at the start of the lab.

You should recognize the wave generator and frequency counter from the last lab. The "thick" box containing resistors was used for the Kirchhoff lab, while the "thin" box containing the inductor (and capacitors and other assorted items) was used for the RC lab. There is also a compass and protractor for use in doing a graphical solution of the law of cosines, which will also show the change in phase angle as the resistance increases.

The generator will be wired up this week much as it was last week, but you should pay attention to these details because it will not be connected for you next week. There is also a very important detail that you must fix before doing the experiment: the button indicated by the blue arrow in this picture.

closeup of wiring of the scope, generator, and freq counter

Arrow showing the button for the "sine" function selector.

Our last measurement in the previous lab used a triangle wave at 1000 Hz. You will usually find that the generator is still set for that, as this one is. Be sure to push in the button on the far right to select the sine function for this week's lab, and monitor this next week as well.

Procedure Changes

This lab has had a significant addition to it made at the suggestion of a student a few years ago. As described in a handout, we will start by using the oscilloscope to measure the phase angle between the current in the circuit (as determined from the voltage on the resistor R) and the voltage produced by the generator. We will then do the experiment described in the lab manual, where we will use rms voltages to calculate that angle in two different ways.

Other details are given on a separate page.


The set of cables shown in the picture below contains the ones you will have available for this lab. The set above the meter stick includes a red-black pair of 100 cm leads and a red-black pair of 65 cm leads. One of these pairs is usually used to make connections to the multimeter while the other is used to connect to the power supply.

Full set of 9 cables used in circuit labs

The set below the meter stick includes a red-black pair of 40 cm leads, an extra 40 cm lead that is black with red connectors, and a red-black pair of 20 cm leads. The shortest cables are used for connections between nearby points on a resistor box.

Old Setup

wave generator, freq counter, and boxes containing R and L

Photo of the equipment when we did not use the oscilloscope.


 ?? Contact me if you have any questions.