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Ohm’s Law
Electronic Resistance:
Material’s resistance to the flow of electric
current
Thin wire
120V
Thick wire
Ohm’s Equation
voltage
Current =
resistance
directly proportional to voltage
 Current is ________
 Current is inversely
________ proportional to resistance
Equation: symbol/units
V
I =
R
Volts (V)
Amperes (A) =
Ohms (Ω)*
* Ω = OMG
Practice #1
 How much current will flow through a
lamp that has a resistance of 60 Ω
when 12V are impressed across it?
voltage
Current =
resistance
12 V
Current =
60 Ω
= 0.200 amps
Practice #2
 What is the resistance of an electric
frying pan that draws 12 A when
connected to a 120 V circuit?
voltage
Current =
resistance
120 V
12 A
=
R
=
10.0 Ω
Resistance in series
2Ω
I =
V
R
6V
I
6V
=
2Ω
Current = 3.00 amps
Multiple Resistance in
series: ADD!
2Ω
I =
V
R
6V
I
6V
=
2Ω + 2Ω
2Ω
Current = 1.50 amps
Resistance in parallel:
R equivalent = (R1R2) / (R1+R2)
2Ω
R = (2x2)/(2+2) =
R = 4/4 = 1 Ω
2Ω
1Ω
=
I
=
V
R
I
6V
6V
=
1Ω
Current = 6.00 A
Series and Parallel:
R equivalent = R1R2 / R1+R2
4Ω
2Ω
= 4+2= 6Ω
parallel
(6x6) /(6+6) =
6Ω
=
3Ω
36/12 =
V
I =
R
I
=
6V
3Ω
6V
Add series Ω first
Current = 2.00 A
Series and Parallel:
R equivalent = R1R2 / R1+R2
4Ω
6Ω
4 + 6 = 10 Ω
= top series
10 Ω
5Ω
10 Ω
inside parallel
(10x10) / (10+10) =
100/20 = 5 Ω
= inside series 5 + 5 = 10 Ω
final parallel
(10x10) / (10+10) =
100/20 = 5 Ω
V
I =
6V
Work from the inside out
R
I
=
6V
5Ω
Current = 1.20 A
More than two Parallel:
Solve two at a time!
4Ω
8Ω
4 + 8 = 12 Ω
= top series
top two parallel
(4x12) / (4+12) =
48/16 = 3 Ω
4Ω
final parallel
(10x3) / (10+3) =
10 Ω
30/13 = 2.31 Ω
V
I =
R
6V
I
6V
=
2.31 Ω
Current = 2.60 A
Parallel Then Series:
2Ω
Parallel:
R = (2x2)/(2+2) =
R = 4/4 =
2Ω
1Ω
Series:
1+2=3Ω
2Ω
I
6V
Solve parallel first
6V
=
3Ω
Current = 2.00 A
1/R
equivalent
= 1/R1 +1/R2 + …
 Alternate equation to calculate
resistance.