1
Quiz 2 solutions are in order of the questions for version 1.
(1). Correct answer is E
As
R
decreases, the resistance of the rightmost branch decreases. This branch is in
parallel to
R
1
and the combination is then in series with
r
, so the eﬀective resistance of
the whole circuit goes down. This of course means that the current out of the battery
goes up. Since we can use the leftmost loop to obtain
V
batt

rI
=
R
1
I
1
this immediately gives that
I
1
must decrease. But since the total current went up,
I
2
must clearly increase.
(2). Correct answer is C
Based on the electrical ratings, we can determine the resistances of the three lamps
from either
P
=
I
2
R
or
V
2
/R
; this gives
R
A
= 24Ω,
R
B
= 160Ω,
R
C
= 240Ω. If we
add bulbs B and C in parallel, we get an eﬀective resistance of 96Ω, giving a total
circuit resistance of 112Ω and a current through bulb A of 1.34A. This leads to a power
consumption of 38W.
(3). Correct answer is A
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 Winter '07
 BRIANKEATING
 Resistance, Magnetic Field, Electrical resistance, 10.6 degrees, 38W

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