PHY 2049L
Lab 3

Kirchhoff's Rules

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Updated: 12/27/2009

Prelab

Answer the pre-lab questions for Laboratory 34 on LON-CAPA after reading the description of the lab. Note: Allow yourself some extra time for this one, 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 ask 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.

Comments

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.)

Apparatus

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.

Cables

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.

Procedure

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 set up "circuit zero", which is just the first half of the circuit for part 1 of the lab, and measure the current at two places in the circuit. There is no place to record what you measure in the data tables. I would record it on the same worksheet I used to figure out what to expect from circuit zero. All you need to know from this extra step is that you know how to hook up the ammeter and that you and the circuit are working as expected.
  
  
• Part 2 of the lab will use a two-loop circuit just like in part 1. The change will be to use a much smaller resistance for R₃ in the second loop rather than add a third loop.

Contact me if you have any questions.