Eric Chong
PHYS C1493
Partner: Albert Lee
Lab Date: November 16, 2006
Experiment 8: Capacitance and the Oscilloscope
Introduction
In this lab we use different components of an electrical circuit to investigate the behavior of a
capacitor as it charges by some current
I
provided by a battery source of some voltage
V
, for
varying levels of the capacitance
C
given in units of
μF
.
We also observe its discharging and
relate the values of the time constant
τ
of both procedures, using the equations
RC
τ
=
and
0
log
log
log
e
I
I
t
RC
=

where by plotting the base10 log of the current values recorded at certain time intervals
t
, and
plotting these against each other, the linear estimation will give us a slope that allows us to
calculate
RC
with errors.
The time constant values for varying levels of the capacitance are
found and compared, for the processes of discharging and charging.
The second part of this
experiment requires the use of an oscilloscope to gauge the value of the time constant
τ = RC
of
a short RC, and compare the mathematically calculate value of this time constant and the value
given by the oscilloscope.
Sources of error are discussed at the end of this lab.
Procedure
Large RC Charging
For this part of the experiment, we set up circuit elements that comprise the large RC, and for
different values of the capacitance in increments of
10
μF
, we observe the current
I
in the system
as the capacitor bank charges at regular time intervals.
The circuit was connected using a
capacitor bank, a power supply, a
10 kΩ
resistor, and a switch used to complete the elements in
the circuit and to dictate when current should flow to charge the capacitor.
We expect the
current to be a decreasing function of time as
I
in the system should decrease as the capacitor
charges to completion.
Before starting each new trial (multiple trials were enacted for each
capacitance level) the switch should be momentarily
flipped so that any charge in the capacitor is released
across the resistor so as to be able to observe the
capacitor charging from its initial state.
Values of
I
were recorded in time intervals of
2 seconds
, for
capacitances of
10, 20 and 30 μF
.
Large RC Discharging
This procedure was similar to the charging procedure above, accept this time we observed the
discharging of a capacitor for the same values of capacitance,
10, 20 and 30 μF
.
The switch is
flipped such that the capacitor is allowed to charge to completion, and once charged, the switch
is flipped again to allow us to view the current of the system as it discharges through the
ammeter.
Values of
I
are again recorded in intervals of
2 seconds
through the ammeter, and
these values are taken in multiple trials for each level of capacitance.
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 Spring '08
 LAB
 Physics, Capacitance, Charge, Electric charge, RC circuit, Oscilloscope, RC time constant

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