Rensselaer Polytechnic Institute
Activity 3
Introductory Physics II
PHYS 1200 Fall 2008
Capacitance
In this experiment, you will investigate some of the properties of capacitors. Below is a list of the
equipment provided, and a brief description of its operation. This laboratory consists of two
parts. In part A, you will determine capacitance by measuring voltage and charge on various
combinations of capacitors.
Equipment for Part A:
Capacitance box -- contains three capacitors
Voltage source & Coulomb meter
6 Banana plug connector wires
A) Determination of capacitance by measurement of voltage and charge
Equipment Operation:
The voltage source & meter measures voltage across the capacitor in the normal configuration
(no buttons pushed). If left in this configuration for too long, it will cause the capacitor to
discharge, so measurements should be made with as little delay as possible. The red button is
used to charge the capacitor with a voltage that is variable from zero to about 10 volts. When the
red button is held down, the meter reads the voltage that is being applied to the capacitor. The
black button, when held down, measures the charge that was on the capacitor with total charge
shown on the display. The range that can be measured is from zero to about 1 microcoulomb. (It
does this by discharging the capacitor and constantly monitoring the current flowing from it, or
"integrating" the current for the time it takes to discharge.)
Procedure:
Connect capacitor A to the instrument with the leads provided. Press the red button and set the
voltage to some convenient value (~5V). Release the button and notice that the voltage decreases
very slowly. Then short the capacitor with your fingers. Notice that the briefest touch discharges
the capacitor significantly. This demonstrates that you must use care when dealing with the
capacitors so you do not inadvertently discharge them. Recharge the capacitor and press the
coulomb meter button. You should obtain a reading which represents the charge that was on the
capacitor, in microcoulombs. When you release the button, the reading will fall to zero. Thus if
you wish to measure voltage and charge, measure voltage first and then read charge while the
button is pressed.


Find the capacitance of capacitor A by charging it to a known voltage, ~5V is good. Then
measure the resultant charge on the capacitor. Record the voltage and the charge and
calculate the capacitance using C=Q/V. (Repeat about five times to get an estimate of
uncertainty.)
Repeat for capacitors B and C.
Series Capacitors

Connect capacitors A and B in series* and compute capacitance of the combination using
the Q/V approach.

Repeat the series measurement with capacitors A and C in series.
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(C) 2009 Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008

Compare your measurement to the result computed using the individual capacitor
measurements. (How should you compute capacitance of a system of capacitors in
series?)
Parallel Capacitors

Wire capacitors A and B in parallel** and use the Q/V approach to measure the
combined capacitance.

Repeat with capacitors A and C in parallel.

Compare your measurements to the results computed using the individual capacitor
measurements. (How should you compute capacitance of a system of capacitors in
parallel?)
B) Oscilloscope measurement of voltage on series capacitors
Equipment for Part B:
Oscilloscope + 2 coax to banana connectors + 6 banana leads
Function generator
Capacitance Box
By applying a sinusoidal voltage to series capacitors, one can use an oscilloscope to measure the
voltage on just one or both.
1) Setting up the oscilloscope
a) Set up the function generator to produce a sinusoidal voltage at a frequency of about 100 Hz.
Set the amplitude knob to about ¼ scale. This will produce voltage of a few volts peak to
peak.
b) Wire the function generator directly to the Channel 1 input of the oscilloscope. (Black jack to
black jack and red jack to red jack. The black jack on the banana plug is the low or ground
side of the device. The red side is the high side. Plugging the ground of the oscilloscope into
the high side of the function generator is not recommended.
c) Press Channel 1 on the oscilloscope and set up the buttons for DC input, no filtering, x1
attenuation. Press the Trigger set up on the oscilloscope and set it to Channel 1, Auto. If the
signal is too large, dial it down on the function generator. You should now have a beautiful
signal.
d) Press the measure button on the top row on the oscilloscope. You should now be able to use
the screen related buttons to set the oscilloscope to give you the peak to peak voltage of the
signal.
2) Capacitor measurement
a) Wire the function generator to the A and B capacitors set up in series.
b) Measure the peak to peak voltage across both capacitors together.
c) Measure the peak to peak voltage across capacitor A only.
d) Is the ratio of the voltages what you would expect from your knowledge of the two
capacitances from part A above?
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(C) 2009 Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008
Notes:
*To wire capacitors A and B in series you can use the following diagram.
**To wire capacitors A and B in parallel, you can use the following diagram.
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(C) 2009 Rensselaer Polytechnic Institute