PHY 2049L
Lab 5

RC Time Constant (DC)

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Updated: 2/2/2013

Guiding Question:

Be sure you record with "thin" box you used, because your abstract will not provide a fully acceptable answer to the guiding question unless it identifies the box used as well as the values of C and R for the capacitor and inductor you measured in that particular box.

Preparing for the lab:

We will be doing "Lab 33" as described in the lab manual.

Answer the pre-lab questions on LON-CAPA after reading the background information on pages 329 and 330 in the lab manual. There is also relevant information in Section 25.5 of Wolfson.


According to the user manual, the BK Precision multimeter we use has an internal resistance of 10 M ohm when used on a specific range setting. This has been confirmed by direct measurement at the 20 V (DC) setting we will use for this experiment.

Connectors 1-2 on the "small" box connect to the 5 micro F capacitor that is our unknown C. All of the ones I have checked fall within +/- 0.1 of the nominal 5.0 muF value.

Connectors 3-4 on the "small" box connect to the unknown R you are to insert in parallel with the multimeter. All have values of about 5 M ohm, ranging from 4.7 to 5.6 M ohm. You will need to turn the box over and read the color code on the resistor to determine the nominal value of the one you used. All of the ones I have checked fall within 1 or 2 % of the nominal value, so you can use the color-coded value as "known".


This photo shows the equipment laid out the way you will find it when you arrive. Please be sure it looks like this when you leave so the next group will find it in the same condition.

new items are stopwatch and box with C and R

Most of this should look familiar. The new items are the stopwatch (which has a "lap time" feature so you can stop it and take an intermediate reading while it continues keeping time) and the "small" box containing the capacitor and resistor for this experiment (and also the inductor we will use in later experiments). The back of this box is transparent so you can see what is inside. Take a look, and read the color code on the resistor you will use.

A separate page gives some additional details on the RC measurement, showing this equipment in use in both part 1 and part 2 of the lab.


The set of cables shown in the picture above contains the ones you will need to use for this lab. You will be supplied with the full set shown below 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.


 ?? Contact me if you have any questions.