A. Manalaysay – PHY2054, Fall 2004
CHAPTER 18
Chapter 18 talks about how resistors act in circuits.
I can combine resistors to get an equivalent resistance
.
The rules for combining resistors are exactly
opposite the rules for combining capacitors.
Here they are:
Adding resistors in series:
R
eq
= R
1
+ R
2
+
. . .
Adding resistors in parallel:
1/R
eq
= 1/R
1
+ 1/R
2
+
. . .
Knowing that, you do the same thing you would with capacitor problems: break the circuit up into sections
of series and parallel (if you can).
The Kirchhoff Rules
: these rules are simple and helpful.
There are two of them.
Junction rule
:
the sum of the currents going into
a junction must equal the sum of the currents leaving
that
junction.
This make sense if you imagine water flowing through pipes; suppose one pipe split into two (as
in figure 18.12 on page 564), the amount of water flowing in the original pipe must be the same as the
water flowing in the two pipes, otherwise that means you’re either leaking water or adding water
somewhere.
Loop rule
:
the sum of the voltage differences around a closed loop must equal zero.
Here’s a helpful way
of thinking about it: imagine the circuit is a fence (wide enough that you can walk on it).
Capacitors,
resistors, batteries… these correspond to places where the fence has a step up or down.
Voltage would be
the elevation of the fence.
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 Spring '08
 Avery
 Physics, Resistance, Electric charge, Electrical resistance, Series and parallel circuits

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