PASCO Physics Experiments

Set Up

MAIN PROCEDURE

To observe the discharge of a capacitor by measuring the time constant (T=RC). To experience the effect of different meters as discharge loads.

Multi-meter
Digital Electronic Multi-meter (SE-9786A)
Xplorer GLX (PS-2002)
Stopwatch (ME-1234)
10 mF 100V Epoxy Capacitor
9V Battery
Pair of Alligator to Banana Test Leads

Microsoft Word version of this lab:
- RC_TimeConstant.zip (.doc, 5.6 MB)

This lab will involve connecting a capacitor to three measuring devices in turn; a standard multi-meter, a digital electronic multi-meter and an Xplorer GLX datalogger. It is important that the meters be set as pictured and that the capacitor is connected to the meter before it is charged with the 9V battery. After each part of the experiment, discharge the capacitor by shorting the leads. Please be careful to avoid bending the capacitor leads any more than is absolutely necessary.

Two primary issues are related to this lab. The first idea of capacitor charge and discharge is fully explained in your text and has been discussed in class. You should summarize these ideas and attach your interpretation to this lab upon completion. The second issue is the internal resistance of the three devices we will use to discharge the capacitor. If we measure a duration equal to five time constants with a stopwatch we can work backwards and calculate the internal resistance of the three devices. Care should be taken in measuring the times.

A. The first test with a standard multi-meter

Set up the circuit pictured below with the knobs in the indicated positions. Do not connect the meter until the knob positions are set and do not move them for any reason as damage to the meter can easily occur. Also do not charge the capacitor at this point.

Note the red lead, the red mark on the capacitor, and the red mark on the 9V battery are in line. As a brief test momentarily touch the leads of the capacitor to the battery. The meter should read 9V on the 10V scale as long as you keep the battery connected. The instant you disconnect the battery the meter will start to return to zero. With the stopwatch, time this event 5 times; that is, measure the time the meter takes to return to zero after the battery is disconnected. Enter the times below:

TIME 1	TIME 2	TIME 3	TIME 4	TIME 5

B. The second test with the digital electronic multi-meter

Note the red lead, the red mark on the capacitor, and the red mark on the 9V battery are in line. As a brief test, momentarily touch the leads of the capacitor to the battery. The meter should read 9V on the 20V scale as long as you keep the battery connected. The instant you disconnect the battery, the meter will start to return to zero. With the stopwatch, time this event 5 times; that is, measure the time the meter takes to return close to zero after the battery is disconnected. Enter the times below:

TIME 1	TIME 2	TIME 3	TIME 4	TIME 5

STOP AT THIS POINT FOR AN INTRODUCTION TO THE XPLORER GLX

C. The third test with the Xplorer GLX

To the right is a picture of the side of the Xplorer GLX showing the correct connection point for the voltage probe cable provided.

Turn on the Xplorer GLX by holding the power button for a couple seconds.
Wait for the Xplorer GLX to boot-up to the "home screen" seen on the left.
Plug in the Voltage probe and note that the screen changes to the view on the picture to the right.

We want a graph so press the home key and again you will see the home screen. Using the arrow keys move the highlighted screen to the lower left Graph screen, select it with the check key (surrounded by the arrow keys). You should now see a Voltage vs. Time graph as shown to the right.
The "play" key will start and stop the recording of voltage data. Once you have finished this step go to the procedure that follows.

PROCEDURE
Pictured to the right is the setup we will use with the Xplorer GLX. Make the connections as they appear in the picture. Be careful to connect the red alligator clip to the red dot side of the capacitor and the black alligator clip to the other lead from the capacitor! Align the battery with the red dot (+) side up.

RC Time Constant Press the "play" button on the Xplorer GLX, then momentarily touch the leads of the capacitor to the battery. The Xplorer GLX graph should read 9V on the graph as long as you keep the battery connected. The instant you disconnect the battery the capacitor will discharge through the internal resistance of the Xplorer GLX. The screen of the Xplorer GLX will draw a graph similar to the one pictured above. Once you see the graph indicate 0 volts press the "play" button to stop taking data. Function key 1 will auto-scale the graph.

Record the time from the instant the battery is disconnected until the curve reaches zero (very close to zero is best as we are looking for 99% discharge). This interval can be found on the X time axis. You may also want to use the stop watch as in the previous parts of this lab. Enter the times measured in the table below. Repeat the charge and discharge cycle 5 times. Each time a new graph will be displayed.

TIME 1	TIME 2	TIME 3	TIME 4	TIME 5

CALCULATE THE AVERAGE TIME OR THE TIME YOU BELIEVE IS MOST ACCURATE: _______ SECONDS.
Recall that this time represents 5 time constants for a 99% complete discharge, hence you will need to divide the time by 5 to get T. You will use the formula T=RC to calculate the resistance of the meter (discharge resistance). Using T from your collected data and C= 10 μF calculate R _____________ Ω

We now have the internal resistance of the Xplorer GLX. Note that since it is high the Xplorer GLX can be used to measure voltages from feeble sources whereas the multi-meter places a much greater "load" on any source it is connected to. For this very reason multi-meters are not used in electronics.

STOP AT THIS POINT FOR FURTHER INSTRUCTIONS

Extracting Data from the Xplorer GLX

FIRST: Press the home key and using the arrow keys highlight the upper left data files icon, then press the check mark key. You should see a display similar to the one shown to the right. You will see a list of files or perhaps just the current "untitled" file. If not, push the up arrow key and the left or right arrow key to highlight RAM. (This is where all active files will be stored). The "flash" memory icon is where future activities will be permanently stored. Use the arrow keys to return to the lower part of the screen and highlight the untitled file (your file).

SECOND: Now we need to title and save your currently untitled file. Press function key 4 (Files) and a pop-up menu will appear. Curser to "Save As" and press the check key. You should now see a screen like the one pictured on the left. Use the back space to delete the untitled name and enter your name by pressing the alphanumeric keys in rapid sequence to enter the desired letters. (Like a cell phone). If you make an error, use the backspace key and re-enter the correct letter. Your results should look similar to the screen pictured below. Now press function key F2 to Save.

THIRD: Insert a USB flash drive as shown in the picture at left. Note that a new icon appears in the upper part of the screen. Your teacher will supply the USB flash drive.

FOURTH: Press the F4 File key and you should see a pop-up menu as pictured to the right. Curser to "Copy File" and you will see a small icon appear next to the RAM memory icon (see photo below). This represents your file and you can use the left-right arrow keys to move it to USB flash drive icon. Then press F1 "OK". File transfer will be indicated. Once you are done, wait a moment then remove the USB flash drive. I will take the USB drive and print out the graphs.