Simple Circuit Analysis
Ohm’s Law
ET 150
Ohm’s Law
Learning Objectives
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In this lesson you will see:
the mathematical relationship between voltage, current and resistance
resistance
call Ohm’
Ohm’s Law.
the Ohm’
Ohm’s Law circle and use it to find the three forms of the formula
examples using Ohm’
Ohm’s Law to find voltage, current and resistance
the linear mathematical relationship between voltage and current
the relationship between current and resistance is nonnon-linear
low resistance causes current to increase quickly
Ohm’s Law
Ohm’s Law – a mathematical formula that relates voltage, current and
resistance
E
2.) R =
1.) E = IR
I
Three forms of Relationship
Ohm’s Law Circle
E
E
Voltage
=
E
I
Current
R
I
R
3.)
I=
Resistance
=
I
R
E
R
E
=
I
R
Ohm’s Law
Example Calculations
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An electric heater draws 7 A from a 120 V dc source. What is the
the
resistance of the heater coil?
I=7 A
E
I
120 V
R=
= 17 .14 Ω (Ohms)
7A
R=
E=120 V
R=?
Ohm’s Law
Example Calculations
‰ A water heater has a resistance of 10 Ω. What current will it draw
from a 120 V dc source?
I= ?
E=120 V
R=10 Ω
E
R
120 V
I=
= 12 A
10 Ω
I=
Ohm’s Law
Example Calculations
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An iron draws 15 A and has a 5 ohm heating element resistance.
What is the supply voltage to the iron?
I=15 A
E=?
R=5 Ω
E = IR
E = (15 A)(5 Ω) = 75 V
Ohm’s Law
Voltage-Current Relationship
Voltage is proportional to current (Linear relationship)
x
Oh m's Law V-I Relat ionship
140 150
Voltage (Volts)
E = RI
100
V( I)
y
R=slope
50
0
0
0
10
0
20
30
40
I
Current (Amps)
As current, I increase,
voltage, V increases
proportionally.
40
Ohm’s Law
Current-Resistance Relationship
Current, I, is inversely proportional to, R, Resistance
20
⎛1⎞
I = V⎜ ⎟
⎝R⎠
Oh m's Law I-R Relationship
20
20
Current (amps)
15
x
I( R)
10
y
I( 20)
As resistance decreases
current increases quickly
5
1.25
0
0
5
20
40
R
Resistance (ohms)
60
80
80
Note: R=0 is short circuit
ET 150
Coming Next: Basic Electric Circuits- Series
Connections
SIMPLE CIRCUIT ANALYSIS
OHM’S LAW