Download Document

D.C.
Circuits
Unit 20
x x
Take Note
• Ex 19.1 and holiday homework due
Friday; performance Task due
yesterday
• Don’t touch / dismantle / destroy
/ eat circuits on table yet
• Correction: Activity Worksheet,
page 1 para 2: “140 rows of 5000
electroplaques”
Outline
• Electric Animals
• Lab
• Kirchhoff’s Laws
» Current Law
» Voltage Law
• Series and parallel circuits,
revisited
Electric Animals
Quick Revision
Equivalent resistance of:
Resistors in series?
Resistors in parallel?
Eel Circuit
Rw
Gustav Kirchhoff
• German physicist
• 1824-1887
• Two sets of
“Kirchhoff’s Laws”
• Formulated circuit
laws as a student
• Co-discovered
caesium and
rubidium
Lab 20.1.2
A4
A5
C
Q
P
A6
R
A7
E
D
Kirchhoff’s Current
Law (or First Law, or Junction Theorem)
The current flowing into
any junction in an
electrical circuit is
equal to the current
flowing out of it.
KCL Example
Copy down and solve for currents a through e
a
6.5 mA
b
d
2.1 mA
c
7.9 mA
15.3 mA
e
Lab 20.1.3
V1
F
G
T
S
V2
V3
Kirchhoff’s Voltage
Law (or Second Law, or Loop Theorem)
In any closed loop of a
circuit, the sum of the
voltage drops must equal the
sum of e.m.f.s in the loop.
(or: the sum of potential
differences must be zero.)
KVL Example
Solve for the unknown voltages.
A
B
+
6V
-
C
D
+
5V
-
I
+
4V
-
+ 2V E
G
F
H
In series:
R = R1 + R2 + …
In parallel:
1/R = 1/R1 + 1/R2 + …
Why??
Resistors Revisited
Torture your friends
Conclusion
• Kirchhoff’s Laws
» Current Law
» Voltage Law
• Prove combined resistance
• Make torturous resistance
problem
• Due FRIDAY with Ex 19.1 and
holiday homework