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C.1
hermannWellenstein
Spring 08
CAPACITORS
THE FUNCTION OF CAPACITORS IN ELECTRIC CIRCUITS, PULSER.
A. INTRODUCTION:
The purpose of this experiment is to learn about the function of capacitors in
electric circuits. The capacitor, next to the resistor, is the most important electric circuit
element. It has the capacity of storing electric charge, and its function is sometimes
compared to containers filled with liquid or gases. Although charging and discharging of
containers through tubes follow similar mathematical laws, there is a limit of this
analogy. Electric circuits are governed by the interaction of freely moving electrons with
"stationary" positive ions (and neutrals). This requires that electric circuits are "closed
loop", whereas a reservoir discharging through a tube is "open loop".
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View Full Document CAPACITORS
C.2
Hermannn Wellenstein
Spring 08
B. THEORY:
The simplest capacitor consists of a pair of parallel plates. If connected to a
battery, electrons flow away
from
one
plate,
making
it
positive, to the other, which
becomes negative. The resulting
electric field gradually brings the
flow to a stop. Here, we only
collect the formulae we need for
the experiment; refer to the
lecture notes and the book for
more detail. The equations
below describe the behavior of a
capacitor qualitatively.
The charge on a capacitor
is
Q = C V,
(1)
where Q is the charge in Coulombs, C is the capacitance in Farads, and V is the Voltage
in Volts.
Discharging of capacitor
: (See Figure 1.) If Q = Q
o
at t = 0, then
Q
=
Q
o
e
!
t
/
RC
(2)
but Q = C V and Q
o
= CV
o
, hence,
V
=
V
o
e
!
t
RC
=
V
o
e
!
t
"
(3)
Charging of capacitor
: (See Figure 2.) If V = 0 at t = 0, then
V
=
V
o
1
!
e
!
t
RC
"
#
$
%
’
(4)
where V
o
= V
,
at t =
!
.
τ
= RC is the characteristic time of the RCcircuit. In time
τ
the voltage (or the charge) decreases by the factor e = 2.72, i.e. to approximately one
Figure 1.
R
C
V
t
V
0
0
V
0
Figure 2.
CAPACITORS
C.3
Hermannn Wellenstein
Spring 08
third.
Before coming to lab, show, by using the definitions of R and C, that the units of
τ
are sec.
C. THE EXPERIMENTS
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This note was uploaded on 02/02/2009 for the course PHYS 10a taught by Professor Martens during the Spring '07 term at Brandeis.
 Spring '07
 Martens

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