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Introduction to Electrical & Computer Engineering
Ohm’s Law
Dr. Cynthia Furse
University of Utah
Dr. Cynthia Furse
University of Utah
1
Ohm’s Law
• Voltage = Current x Resistance (V=IR)
• What is Resistance ?
– Conductivity
– Resistivity
– Resistance
• Resistance is used for:
– Control of Voltage & Current
– Sensing of physical properties
• I-V Curves
2
Ideal Voltage and Current Sources
is
vs
is
+
-
3
Voltage = Potential
a
Vab
b
+
+++
+
Think of Voltage
as a ‘stack of
positive charges’
at the top of a hill.
It has Potential
Energy!
Current = Flow
is
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Resistance to Flow of Current
R (Ω)
R (Ω)
Water-Circuit Analogy
Resistance
(Rocks/Pipes)
Voltage
(Potential)
Current
(Flow)
7
Ohm’s Law: v = i R
i
R (Ω)
v
+
-
8
Ohm’s Law: v = i R
i=10A
R= 1Ω or 2 Ω
v
+
-
How much voltage required to drive 10 A through 1 Ω resistor?
V = (10A) (1 Ω) = 10V
How about for a 2 Ω resistor?
V = (10A)(2 Ω) = 20V
For a constant current, more resistance requires more voltage
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Ohm’s Law: v = i R
i
V=10V
+
-
R= 1Ω or 2 Ω
How much current will a 10V source drive thru a 1Ω resistor?
I = V/R = (10V) / (1 Ω) = 10A
How about for a 2 Ω resistor?
I = (10V)/(2 Ω) = 5A
For a constant voltage, higher resistance reduces the current.
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Ohm’s Law: v = i R
I = 10A
V=10V
+
-
R
For a 10V, 10A source, what is the resistance?
R = V/ I= (10V) / (10A) = 1 Ω
How about for a 5A source?
R = (10V)/(5A) = 2 Ω
Measuring V & I can tell us R. R is often used as a sensor.
11
Resistor Color Codes
Circuits, Second Edition by Fawwaz T. Ulaby
and Michel M. Maharbiz, © NTS Press, Used
with Permission by the Publisher
All rights reserved. Do not reproduce
or distribute. © 2013 National
Technology and Science Press
I-V Curves used for diagnosis,
I
design, etc.
Current (I)
R=0
R=1kΩ
1mA
V
0.5mA
R
R=2kΩ
R= ∞
1V
Voltage (V)
V=IR
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I-V Curves for LED
(Vf is where light turns on)
http://www.electronics-tutorials.ws/diode/diode_8.html
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Resistance (R) and Resistivity (ρ)
Resistance: ability to resist flow of electric current
Resistance (R ) Ω (ohms)
Resistivity (r) Ω/m (ohm-meters)
-1
Conductivity (σ ) m/Ω
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
All rights reserved. Do not reproduce
or distribute. © 2013 National
Technology and Science Press
Resistance can be used as a sensor
Measuring Resistance of Sea Ice
Other Resistive Sensors
• Thermistor = R changes with Temperature
• Piezoresistor = R changes with Strain or
Pressure
• Light-dependent
Resistor (LDR) =
R changes with light
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
17
Conductivity
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
Superconductivity
• Critical temperature TC is the
temperature at which a
material becomes
superconducting (zero
resistance & no power
dissipation)
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
Resistance can be
used to control
Voltage and
Current
R (Ω)
Variable Resistors
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
Conclusion -- Ohm’s Law
• Voltage = Current x Resistance (V=IR)
• What is Resistance ?
– Conductivity
– Resistivity
– Resistance
• Resistance is used for:
– Control of Voltage & Current
– Sensing of physical properties
• I-V Curves
22
Introduction to Electrical & Computer Engineering
Antelope Island, Great Salt Lake, Utah
Dr. Cynthia Furse
University of Utah
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