Electric Currents & Circuits
AP – Ch 20 & 21
Intro Stuff
• Circuit?
–Power Supply
–Resistance
Battery?
•
A thing that produces a potential difference
through a chemical reaction.
– battery, electric, device that converts chemical
energy into electrical energy.
•
•
(How Stuff Works . Com)
The act of beating or pounding.
– Law. The unlawful and unwanted touching or striking
of one person by another, with the intention of
bringing about a harmful or offensive contact.
•
(Wiki-Idiot-Pedia.comedy)
Intro Stuff
• Electromotive Force
–emf
• Maximum V of battery
• Current
–Flow of charge
q
I
t
Intro Stuff
q C
I
 A
t
s
(Ampere )
Intro Stuff
• Ohm’s Law
–States linear relationship
between V & R
V  IR
Volt  ( Amp )(Ohm)
Intro Stuff
• Resistance
–Says what it means
• e- has to ‘fight’ thru a wire or
‘resistor’
L
R
A
L
R
A
CONVENTIONAL Current
ELECTRON Current
CONVENTIONAL Current
ELECTRON Current
Intro Stuff
W
• Power
P
–Same as regular P,t but different
qV
P
t
P  IV
P
Stuff
IV
Intro
• Power…
P  I ( IR )
I R
2
2
V
V
P V 
R
R
Watts
Intro Stuff
• Wiring Circuits
–Series / Parallel
• Parallel
I T  I1  I 2  ...
VT
V1 V2


 ...
RT
R1 R2
1
1
1


 ...
RT
R1 R2
1
RT 
1
1

 ...
R1 R2
WHY? C//
Q  Q1  Q2  Q3  ...
CV  C1VQ1 Q C
 Q2V
 ...2  C3V3  ...
1
2
3
C  C1  C2  C3  ...
CV
1
WHY? CSeries
V  V  V  V  ...
1
2
3
Q Q Q Q
     ...
C C C C
1
2
3
1 1 1 1
     ...
C C C C
1
2
3
Quantitative Summary
kq1q2
F 
2
r
Coulomb’s Law:
Force between any 2 charges
F
E
q
Electric Field Strength:
Force on test Q
UE
kq1q2
 qV 
r
Electric Potential Energy:
Fd
V
E
d
Electric Field (av):
Potential at a distance
kq
V 
r
 q1 q2

 k 

 ... 
r2
 r1

Electric POTENTIAL:
NOT a vector…
Q
C
V
Capacitance:
Property of Capacitors
C
o A
d
Capacitance:
Property of Capacitors
UC
1
1
2
 QV  CV
2
2
Energy stored in Capacitor:
Q
I 
t
Definition of Electrical Current:
SI Unit - Amps
Q
I 
t
1C
1A 
1sec
18 
6.25 x10 e
1A 
1sec
Definition of Electrical Current:
SI Unit – Amps (A)
L
R
A
Resistance in a WIRE:
SI Unit – Ohms ()
V  IR
OHM’S LAW:
All Circuits Must Obey…
V  IR
12V
6
V 12V
I 
 2A
R 6
OHM’S LAW:
All Circuits Must Obey…
P  IV
I R
2
2
V

R
Electrical Power:
From P=W/t
RS  R  R1  R2  ...
I S  I1  I 2  ...
VS  V1  V2  ...
Series Circuits
1
1
RP 

1
1
1


 ...
R R1 R2
I P  I  I1  I 2  ...
VP  V1  V2  ...
Parallel Circuits
1
1
CS 

1
1
1


 ...
C C1 C2
V  V  V  V  ...
1
2
3
Q Q Q Q
     ...
C C C C
1
2
3
1 1 1 1
     ...
C C C C
1
2
3
Circuits w/Capacitors
C P  C  C1  C2  ...
Q  Q1  Q2  Q3  ...
CV  C1V1  C2V2  C3V3  ...
C  C1  C2  C3  ...
Circuits w/Capacitors
EXAMPLE #1
• Three 12 resistors are used in two circuits; one
in series and one in parallel. What is the Requivalent
of each circuit?
Series
12
12
12
REq  R  R1  R2  R3
 12  12  12  36
Parallel
12
12
12
1
1
REq 

1
1 1 1

 
R 12 12 12
1 12


 4
3
3
12
Series + Parallel
12
12
12
12
6
18