Capacitors

(Using Charge sensors) 

 

Introduction 

 

In this lab, we use a ìcharge-sharingî technique to determine the capacitance of various conductor – 

dielectric geometries. 

 

Procedure 

 

You  will  use  a  power  supply  to  charge  a  capacitor; then  carefully  touch  that  capacitorís  leads  to  the 

leads of a second capacitor (uncharged in most cases) so that the charge on the first capacitor is shared 

with the second. 

Before beginning the formal experiment, you must learn how to measure the voltage across a charged 

capacitor. Digital voltmeters, due to finite internal resistance, always draw some current when 

connected across a charged capacitor, thereby leaking charge. To overcome this problem, we will use a 

charge sensor. 

 

Mount a 0.1 µF capacitor on the Styrofoam block. Use the power supply to charge this capacitor to a 

voltage of 5.0 V; then disconnect the power supply. Attempt to measure the capacitor voltage directly 

with a digital voltmeter (DVM).  You should observe the voltage on the DVM dropping rather quickly 

to zero. 

 

Now mount the 0.1 µF capacitor leads to the input of the charge sensor.  Again use the power supply to 

charge this capacitor to a voltage of 5.0 V; this time the reading should show a constant 5.0 V on the 

computer display. Discharge the capacitor by touching both capacitor leads simultaneously with an 

alligator clip. Once again the voltage should drop rather quickly to zero. You may observe some short-lived ìreboundî in the voltage due to a lingering polarization in the dielectric of the capacitor. 

 

For the formal experiment, begin with a pair of capacitors in parallel.  Leave the 0.1 µF capacitor 

mounted on the charge sensor and mount a 1.0 µF capacitor on the Styrofoam block; connect the two 

capacitors (the connectors with small hooks are useful here) to create 1.1 µF capacitor. Make sure the 

1.1 µF is completely discharged. Charge a second 1.0 µF capacitor to 5.0 V (you can do this by 

holding the second capacitor by its plastic cover and inserting its leads into the power-supply outlets) 

and touch it to the uncharged 1.1 µF capacitor (be sure to connect positive to positive). For safety, 

discharge the ìcharge-transferî capacitor before putting it down. Compare the measured voltage to 

what is expected from a theoretical calculation. 

 

Now discharge the 1.1 µF capacitor. Repeat the above steps using a 0.1 µF capacitor as the charge-transfer capacitor (instead of a 1.0 µF capacitor).  Once again compare the reading to what is expected 

from a theoretical calculation. 

 

This time, do not discharge the 1.1 µF capacitor.  Recharge the 0.1 µF  charge – transfer capacitor to 

5.0 V and connect it again across the 1.1 µF capacitor, taking care to maintain the proper polarity. You 

should observe that the increase in the voltage is a little less this time. Compare the voltage reading to 

what is expected from a theoretical calculation and explain why the increase in voltage is a little less 

for the second charge transfer. What do you expect for a third charge transfer without discharging the 

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1.1 µF capacitor? Do the experiment and report your results. 

 

Now charge the 1.1 µF capacitor to 5.0 V. Share the charge on this capacitor with an uncharged 

capacitor of unknown capacitance (the capacitor covered with black electrical tape), and from the final 

reading of the voltage determine the unknown capacitance. You can share the charge by holding the 

uncharged capacitor and touching its leads to the leads of the charged 1.1 µF capacitor. Report this 

measurement of capacitance and compare with a second measurement taken directly with the 

capacitance capability of the DVM. 

 

Finally, we wish to measure the capacitances of a length of coaxial cable and of a parallel-plate 

capacitor.  Construct the parallel-plate capacitor by inserting the waxed paper between the aluminum 

plates; press the combination together by putting a 1.0 kg weight on top of the top plate. Both of these 

capacitances are quite small, so we will need to alter our standard capacitor. Remove the 0.1 µF 

capacitor and mount a 0.01 µF capacitor (the smaller brown capacitor: check with capacitance 

capability of the DVM) on the charge sensor; disconnect the capacitor mounted on the Styrofoam. We 

must take into account the small capacitance of the charge sensor, which we have thus far ignored. Use 

the capacitance capability of the DVM to measure the combined capacitance of the 0.01 µF capacitor 

and the charge sensor (it should be approximately twice the capacitance of the 0.01 µF capacitor 

alone); this is our standard capacitor. Charge the standard capacitor to 5.0 V using the power supply. 

Use the charge-sharing method (as summarized below) to determine the two unknown capacitances: 

capacitances of a length of coaxial cable and of a parallel- plate capacitor. 

1.  Charge the standard capacitor to V

1

 = 5.0 V. (Fig. 1).   

2.  Disconnect one wire between power supply and the charge sensor. 

 

 

 

3.  With a quick touch, share the charge of the standard capacitor and a parallel-plate capacitor (Fig. 2). 

4.  Record the resultant voltage V

2

 across capacitors in parallel: the standard capacitor and parallel- 

plate capacitor. 

5.  Calculate the capacitance of the parallel-plate capacitor. 

 

How would pressing the two parallel plates together more tightly affect their capacitance? Try it and 

report your results. 

Follow the same procedure to measure the capacitance of a length of coaxial cable. Compare your 

results to direct measurements of those two capacitances with the capacitance capability of the DVM 

(the DVM internally performs a similar experiment). 

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