Lecture S5
Muddiest Points
General
Comments
Today,
we finished up
one PRS
question
(the first
one on
light
bulbs), and
did
another similar
one.
The point of
these questions
is
to
get
you
to
think
about
circuits in
terms of potentials.
Many
students
found
these questions
challenging, which
is good
— I
need
to
challenge you
preconceptions
in
order
to
change them.
Responses
to MuddiestPartoftheLecture
Cards
(45
cards)
1.
How
do we know
if a battery is
absorbing or
supplying power?
(1)
How
does
the 12 V
battery in
the PRS
question
absorb power?
(1)
For any circuit
element
with current
i
and
voltage
v
,
the element
is dissipating
or absorbing
power if
iv
>
0; the
element is
supplying
power
if
iv
<
0. But
to
make this work, you
must
use the passive sign
convention,
which
requires
that you
label
i
as going
into
the + terminal (or out
the
−
terminal).
2.
I
can’t do the KVL equation
for
complex loops. (1)
There is plenty
of practice on
the homework.
Please see me at oﬃce hours if there is a
problem.
3.
In
the
PRS
question, you sad that
the battery
forces
the node voltage to
be 12
V.
What
do you mena by “force”?
(1)
The constitutive law of the battery
(a
voltage
source,
really) is
that the voltage across its terminals must
be exactly
the source strength,
in this
case 12
V.
Since the
−
terminal is at
ground, the + terminal is at
exactly
12
V.
When the switch
is
closed,
the middle node is at
the same potential as the + terminal, so it
must
be at 12
V.
Using
the word
“force”
is a
bit
of colorful language to
make this point.
4.
Muddy point: How
to deal with
split
currents
in
KVL
in
the last
concept
question. (1)
Not sure I understand
the question
—
please ask
again, or see me at
oﬃce
hours or
recitation.
5.
In
the first PRS
question, how
do
you
apply KCL
at
the junction
between
the
two bulbs? (1)
The sum of
currents out
of the node sum
to
zero. Since the current
into
the node through
bulb
A
is
the same as the current
out
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 Fall '05
 MarkDrela
 Incandescent light bulb, Thévenin's theorem, Norton's theorem, Bulb

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