PHY 2049L
Lab 4

Kirchhoff's Rules

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Updated: 2/2/2013

Guiding Question:

Preparing for the lab:

We will be doing "Lab 34" in the lab manual, although there will be some significant changes from that procedure. We will only do a two-loop circuit in Part 2 (rather than the three-loop circuit shown in the manual) and the resistance values will be those shown in the circuit diagrams given on a separate page here on the web and in a handout that will be available in class or in the lab the week before.

Answer the pre-lab questions for Laboratory 34 on LON-CAPA after reading the description of the lab on pages 339 to 341 of the lab manual. You should also review what is learned in lecture from Section 25.3 of Wolfson.

Allow yourself some extra time for this pre-lab, as you have to set up and solve three equations in three unknowns to answer question 6 before you can answer question 7. Your instructor may have asked you to turn in your solution to these equations on a separate sheet of paper, stapled to your version of the problem from LON-CAPA. You should show your work in a clear and organized fashion whether you turn it in or not.

You should also set up and solve the equations for the circuits we will measure in the lab. These are given on a handout (pdf) and are also shown in detail on a separate page where the notation in the diagrams is explained. These circuits also make excellent review problems for the lecture course.


The main challenge here is getting things wired up correctly to reproduce the circuit diagram you are given. I will give the specifics for what connects to what with a handout in the lab (the same info is on a separate procedure page), but the main way to be sure things are hooked up correctly is to have worked out in advance what to expect for the currents in a simple one-loop circuit (circuit zero) and the circuit for part 1. If you know what to expect for the currents in these two cases, you will know that you have everything hooked up correctly and that the power supplies are providing a properly regulated voltage under load. See comments on the procedure page, or listen at the start of the lab, to understand why this is very important.

(Note that each setup has slightly different resistors, so you will need to do the final calculations for the lab report with the specific values from the resistor box at your lab station. This is less hassle than you might expect, since it is actually quite trivial to repeat the same algebra with slightly different numbers for the various coefficients.)


Cenco power supply, resistor box, plus familiar items from last lab

The small silver box on the top left is the Cenco power supply that will be used to supply a regulated 10 V for the first EMF in this lab, the second EMF of 5 V will be supplied by the Kenwood supply we have used before. The resistors we will use are arranged in the left-hand column of the wooden resistor box. Be sure to record which box you used so you can be sure you use the same one for later experiments.

We will use the BK Precision multimeter two different ways in this lab. We will use it as a voltmeter (same button settings as shown in photos for an earlier lab) to set and spot-check the voltage from the Cenco and Kenwood supplies, and then we will use it as an ammeter. You convert it to a DC ammeter by pushing in the "A" button after disconnecting all cables. You then select the 2000 mA range and connect the cables to measure a current AFTER having your plan checked by the lab instructor. At that point you can change the range setting to get the maximum number of sig figs.


The set of cables shown in the picture above is the full set available for any particular electrical circuit lab. You will not need all of them for this lab, but they will all be available so you can choose the ones you want. 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.


A separate section shows the circuits we will set up along with the approximate values of the resistors you will use. As it shows, there are two changes in the lab procedure.

You should just cross out the last line in the data and calculation table 2, because there are only 3 currents. Be sure your lab report includes your worksheets showing your solution for circuits 1 and 2 with the actual resistors you used.


 ?? Contact me if you have any questions.