LAB# 32
CAPACITORS
INTRODUCTION:
The charge
q
on a capacitor’s plate is proportional to the potential difference
V
across the capacitor.
We express this with
V
q
C
,
where
C
is a proportionality constant known as the
capacitance
.
C
is measured in the unit of the
farad, F, (1 farad = 1 coulomb/volt).
If a capacitor of capacitance
C
(in farads), initially charged to a potential
V
0
(volts) is connected
across a resistor
R
(in ohms), a timedependent current will flow according to Ohm’s law. This
situation is shown by the RC (resistorcapacitor) circuit below when the switch is closed.
Figure 1
As the current flows, the charge
q
is depleted, reducing the potential across the capacitor, which in
turn reduces the current. This process creates an exponentially decreasing current, modeled by
V t
V e
t
RC
( )
0
.
The rate of the decrease is determined by the product
RC
, known as the
time constant
of the circuit. A
large time constant
means that the capacitor will discharge slowly.
When the capacitor is charged, the potential across it approaches the final value exponentially,
modeled by
V t
V
e
t
RC
( )
0
1
.
The same time constant
RC
describes the rate of charging as well as the rate of discharging.
To better understand the principles of charging and discharging answer the following questions.
Voltage
probe
SPDT switch
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View Full Document1. Consider a candy jar, initially with 1000 candies. You walk past it once each hour. Since you
don’t want anyone to notice that you’re taking candy, each time you take 10% of the candies
remaining in the jar. Sketch a graph of the number of candies for a few hours.
2. How would the graph change if instead of removing 10% of the candies, you removed 20%?
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 Fall '08
 RABE
 Physics, Exponential Function, Capacitance, Charge, Resistor, RC time constant

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