ID: A
3
UCSD Physics 2B
Unit Exam 3A
Current, Resistance, Circuits
Answer Section
MULTIPLE CHOICE
1. ANS: B
This is a compound circuit, so we must find the current around the entire loop, and
therefore the
equivalent resistance of the
entire
circuit. First, reduce the parallel leg to its equivalent
1
R
Parallel
=
1
R
1
+
1
R
2
=
1
30.0
Ω
+
1
30.0
Ω
=
1
15.0
Ω
so that
R
Parallel
=
15.0
Ω
This is in series with R
3,
so
R
Series
=
R
1
+
2
+
R
3
=
15.0
Ω +
30.0
Ω =
45.0
Ω
The total circuit current is thus
I
=
V
R
Series
=
75.0
V
45.0
Ω
=
1.67
A
Since the entire circuit current must pass through R
3
,
P
3
=
I
2
R
3
=
1.67
A
(
)
2
30
Ω
(
)
=
83.3
W
TOP: NET + POWER
2. ANS: A
R
=
V
I
−
220
V
8.75
A
=
25.1
Ω
TOP: OHM'S LAW
3. ANS: B
The resistors are in parallel, so they’re at the
same
voltage and
independent
of each other.
P
2
=
V
2
R
2
=
50.0
V
(
)
2
17.2
Ω
=
145
W
TOP: POWER

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