Experiment 4
The RC Time Constant
Mark Sawh
Date Preformed: 09/22/09
Date Due: 09/29/09
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View Full DocumentPurpose
•
To investigate how the voltage across a capacitor varies as it changes ( or
discharges) and to find the time constant (
τ
) of a RC circuit.
Theory
•
When a DC voltage source is connect across an uncharged capacitor, the rate at
which the capacitor charges up decreases as time passes. As a result, the capacitor
charges exponentially, quickly at the beginning and more slowly as the capacitor
becomes fully charged. The charge on the plates at any time is given by:
o
=
q qo1

e

=
tRC qo1

e

t
τ
o
Where
qo
is the maximum charge on the plates and
=
RC
is the capacitive
time constant (R is resistance and c is capacitance).
•
In the experiment the charge on the capacitor is measured indirectly by measuring
the voltage across the capacitor since these two values are proportional to each
other:
=
q CV
•
At time t =
τ
= RC, the voltage across the capacitor has grown to the value of

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 Spring '11
 Chen
 Physics, Charge, Electric charge, RC circuit, RC time constant, capacitor change

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