Capacitor Lab

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Capacitor Lab

• Oh what a charge it is..

2
You will be building capacitors and then
determining the capacitance based on the ?. In
order to increase the time constant such that a
digital multimeter (DMM) can used to record the
data a very large resistance is required. This
large resistance is accomplished using an
operational amplifier. However, we do not know
that value so you will have to experimentally
determine the resistance. To do this you will
calibrate the circuit using a known capacitance.
The process you will follow is discussed
later. Now the capacitors you will build are
simple parallel plate designs. You will build
two capacitors. The first one will have a wax
paper dielectric and the second will use acetate
sheets as a dielectric. Capacitance is determine
using the following formula C ??0A/d You
will need to measure the area of the plates as
well as the thickness of the dielectric (use
calipers). Use two sheets of aluminum foil
(8x10) and alternate each sheet of foil with a
piece of dielectric. Offset the two sheets of
foil so that when you roll the sheets onto the
wooden dowel you will have two terminals.
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It should look something like this You
can use tape to keep everything aligned. When
you are done you should have a capacitor that
looks like this-
Wood dowel
Aluminum foil
Dielectric
Dielectric facing out
4
terminal
TL082 Op-amp
20 ohms
V
200 ohms
220 ohms

-
-V
Capacitor under test
3G?
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The lab uses an active load. This load must be
calibrated before testing your capacitors. Attach
the voltmeter and record the open circuit
voltage. This voltage is due to leakage in the
op-amp circuit and will have to be subtracted
later to normalize your data. With the test
board still open-circuited depress the pushbutton
and record the starting voltage. This is the
maximum voltage that will be applied to the
capacitor. Now connect the capacitor between the
capacitor terminal (C) on the board and the
ground terminal ( ) with jumper
leads. Depress the push button and charge the
capacitor (2 seconds). Let go of the button at
t 0 and record the voltage on the DMM at 5
second intervals (practice until you can get
reliable results). The voltage should decrease.
Record the voltage until it drops to
approximately the open circuit voltage. Repeat
this process for each capacitor.
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• You are done when you have all the data recorded.
  You will need the leakage voltage, the charging
  voltage, voltage readings for the calibration
  capacitor, and voltage readings for your two
  capacitors.
• You will use the data from the calibration
  capacitor to determine the resistance R. Use
  the voltage value at the first time constant.
• Then you will use the experimentally calculated
  R to determine the capacitance of the wax paper
  and acetate capacitors.
• Questions
• How does the experimentally determined value of
  capacitance compare to the theoretically
  determined value ( error)?
• What is the experimentally derived ? for wax
  paper and acetate? How does this compare to the
  published values for wax paper and plastic (
  error)?
• What was the amount of charge on each capacitor?
• What was the energy stored in each capacitor?
• What was the greatest single source of error?

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To evaluate you data it is best if you use a
spreadsheet. List your data points in one
column. Subtract the leakage and store the
result in the next adjacent column. Plot the
adjusted data versus time. Determine the time
for ? 1 and use this to calculate the circuit
resistance and the unknown capacitances. Include
the spreadsheet, formulas, and plots in the lab
report.