Physics
ys cs 132:
3 Lecture
ectu e 17
Elements of Physics II
A
Agenda
d for
f T
Today
d


Resistance
 Resistors
 Series
 Parallel
 Ohm’s law
Electric Circuits
 Current
 Kirchoff
Kirchoff’ss laws
Physics 202: Lecture 8, Pg 1
Kirchhoff’s Junction Law
For a junction, the law
of conservation of
current requires that:
where the  symbol
means summation
summation.
This basic
conservation
statement is called
Kirchhoff’s junction
law
law.
Physics 202: Lecture 8, Pg 2
Kirchhoff’s Loop Law

For any path that starts and
ends at the same point:

The sum of all the potential
differences encountered
while moving around a loop
or closed path is zero.

This statement is known as
Kirchhoff’s loop law.
Physics 202: Lecture 8, Pg 3
Steps for Using Kirchhoffs Rules





Label all currents
Label +/+/ for all elements
 Emf (regular +/-)
 Resistors
 current goes from + to Choose a loop and direction (your choice)
W it down
Write
d
voltage
lt
d
drops
across resistors
i t
 Loop rule (use first sign you get to)
 List voltages
g
Write down current equation
 Junction rule
Physics 202: Lecture 8, Pg 4
Simplest Circuit in the World
Physics 202: Lecture 8, Pg 5
Clicker Question 1:
The current through
g the 3 
resistor is
A.
B.
C.
D.
E.
9 A.
6 A.
5 A.
3 A.
1 A.
Physics 202: Lecture 8, Pg 6
Example
Physics 202: Lecture 8, Pg 7
Physics 202: Lecture 8, Pg 8
Physics 202: Lecture 8, Pg 9
Physics 202: Lecture 8, Pg 10