Chapter 28
139
(e)
Increasing resistor 3 increases the equivalent resistance of the entire circuit. The current in
the circuit, which is the current through resistor 1, decreases. This decreases the potential
difference across resistor 1, increasing the potential difference across the parallel
combination. With a larger potential difference the current through resistor 4 is increased.
With more current through 4, and less in the circuit to start with, the current through
resistors 2 and 3 must decrease. To summarize,
II
I
I
41
2
3
increases and
and
decrease
,,
.
(f)
If resistor 3 has an infinite resistance it blocks any current from passing through that branch,
and the circuit effectively is just resistor 1 and resistor 4 in series with the battery. The circuit
now has an equivalent resistance of 4
R
. The current in the circuit drops to
3
4
of the original
current because the resistance has increased by
4
3
. All this current passes through resistors 1
and 4, and none passes through 2 or 3. Therefore
I
I
I
I
12
3
4
3
4
0
3
4
==
=
=
.
Section 28.3
Kirchhoff’s Rules
P28.20
+−
−
=
15 0
7 00
2 00 5 00
0
1
..
afaf
af
I
500 700
1
=
I
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 Fall '11
 Staff
 Physics, Current, Resistance, Resistor, Electrical resistance, equivalent resistance, ε

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