This preview shows page 1. Sign up to view the full content.
154
Direct Current Circuits
P28.66
Start at the point when the voltage has just reached
2
3
∆
V
and the switch has just closed. The voltage is
2
3
∆
V
and is
decaying towards 0 V with a time constant
RC
2
∆∆
Vt
Ve
C
tRC
af
=
L
N
M
O
Q
P
−
2
3
2
.
We want to know when
∆
C
will reach
1
3
∆
V
.
Therefore,
1
3
2
3
2
VV
e
=
L
N
M
O
Q
P
−
or
e
−
=
2
1
2
or
tR
C
12
2
=
ln .
V
R
1
R
2
∆
V
+
C
∆
V
c
Voltage
controlled
switch
FIG. P28.66
After the switch opens, the voltage is
1
3
∆
V
, increasing toward
∆
V
with time constant
RRC
+
bg
:
∆
V
C
tR RC
=−
L
N
M
O
Q
P
−+
2
3
.
When
V
C
=
2
3
2
3
2
3
∆
V
e
or
e
=
1
2
.
So
R
C
21
2
2
=+
ln
and
Tt t
R RC
=+=
+
1
2
22
ln
.
P28.67
(a)
First determine the resistance of each light bulb:
P
=
∆
V
R
2
R
V
==
=
∆
Ω
af a
f
120
60 0
240
V
W
.
.
We obtain the equivalent resistance
R
eq
of the network of light
bulbs by identifying series and parallel equivalent resistances:
FIG. P28.67
RR
eq
+
1
23
1
11
240
120
360
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 12/14/2011 for the course PHY 203 taught by Professor Staff during the Fall '11 term at Indiana State University .
 Fall '11
 Staff
 Physics, Current

Click to edit the document details