Download Honors Physics Unit 11 Notes

Chapter 17
Section 3 Current and
Resistance
Current and Charge Movement
• Electric current is the rate at which electric charges
pass through a given area.
I
electric current =
Q
t
charge passing through a given area
time interval
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Chapter 17
Section 3 Current and
Resistance
Conventional Current
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Visual Concept
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Chapter 17
Section 3 Current and
Resistance
Drift Velocity
• Drift velocity is the the
net velocity of a charge
carrier moving in an
electric field.
• Drift speeds are
relatively small because
of the many collisions
that occur when an
electron moves through
a conductor.
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Chapter 17
Section 3 Current and
Resistance
Drift Velocity
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Visual Concept
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Chapter 17
Section 3 Current and
Resistance
Resistance to Current
• Resistance is the opposition presented to electric
current by a material or device.
• The SI units for resistance is the ohm (Ω) and is
equal to one volt per ampere.
• Resistance
V
I
potential difference
resistance 
current
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R
Chapter 17
Section 3 Current and
Resistance
Resistance to Current, continued
• For many materials resistance is constant over a
range of potential differences. These materials obey
Ohm’s Law and are called ohmic materials.
• Ohm’s low does not hold for all materials. Such
materials are called non-ohmic.
• Resistance depends on length, cross-sectional area,
temperature, and material.
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Chapter 17
Section 3 Current and
Resistance
Factors that Affect Resistance
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Visual Concept
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Chapter 17
Section 3 Current and
Resistance
Resistance to Current, continued
• Resistors can be used to control the amount of
current in a conductor.
• Salt water and perspiration lower the body's
resistance.
• Potentiometers have variable resistance.
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Chapter 17
Section 4 Electric Power
Objectives
• Differentiate between direct current and alternating
current.
• Relate electric power to the rate at which electrical
energy is converted to other forms of energy.
• Calculate electric power and the cost of running
electrical appliances.
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Chapter 17
Section 4 Electric Power
Sources and Types of Current
• Batteries and generators supply energy to charge
carriers.
• Current can be direct or alternating.
– In direct current, charges move in a single
direction.
– In alternating current, the direction of charge
movement continually alternates.
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Chapter 17
Section 4 Electric Power
Energy Transfer
• Electric power is the rate of conversion of electrical
energy.
• Electric power
P = I∆V
Electric power = current  potential difference
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Section 4 Electric Power
Chapter 17
Energy Transfer
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Visual Concept
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Chapter 17
Section 4 Electric Power
Energy Transfer, continued
• Power dissipated by a resistor
2
(

V
)
P  I V  I 2R 
R
• Electric companies measure energy consumed in
kilowatt-hours.
• Electrical energy is transferred at high potential
differences to minimize energy loss.
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Chapter 18
Section 1 Schematic Diagrams
and Circuits
Schematic Diagrams
• A schematic diagram is
a representation of a
circuit that uses lines to
represent wires and
different symbols to
represent components.
• Some symbols used in
schematic diagrams are
shown at right.
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Chapter 18
Section 1 Schematic Diagrams
and Circuits
Schematic Diagram and Common Symbols
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Visual Concept
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Series
• A series circuit describes two or more components of
a circuit that provide a single path for current.
• Resistors in series carry the same current.
• The equivalent resistance can be used to find the
current in a circuit.
• The equivalent resistance in a series circuit is the sum
of the circuit’s resistances.
Req = R1 + R2 + R3…
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Series
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Series, continued
• Two or more resistors in
the actual circuit have the
same effect on the current
as one equivalent resistor.
• The total current in a series
circuit equals the potential
difference divided by the
equivalent resistance.
V
I
Req
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Sample Problem
Resistors in Series
A 9.0 V battery is
connected to four light
bulbs, as shown at
right. Find the
equivalent resistance
for the circuit and the
current in the circuit.
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Sample Problem, continued
Resistors in Series
1. Define
Given:
∆V = 9.0 V
R1 = 2.0 Ω
R2 = 4.0 Ω
R3 = 5.0 Ω
R4 = 7.0 Ω
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Unknown:
Req = ?
I=?
Diagram:
Chapter 18
Section 2 Resistors in Series or
in Parallel
Comparing Resistors in Series and in Parallel
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Visual Concept
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Parallel, continued
• Resistors in parallel have the same potential
differences across them.
• The sum of currents in parallel resistors equals the
total current.
• The equivalent resistance of resistors in parallel can
be calculated using a reciprocal relationship
1
1
1
1



...
Req R1 R2 R3
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Parallel
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Sample Problem
Resistors in Parallel
A 9.0 V battery is
connected to four
resistors, as shown at
right. Find the
equivalent resistance
for the circuit and the
total current in the
circuit.
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Chapter 18
Section 2 Resistors in Series or
in Parallel
Sample Problem, continued
Resistors in Parallel
1. Define
Given:
∆V = 9.0 V
R1 = 2.0 Ω
R2 = 4.0 Ω
R3 = 5.0 Ω
R4 = 7.0 Ω
© Houghton Mifflin Harcourt Publishing Company
Unknown:
Req = ?
I=?
Diagram:
Chapter 18
Section 2 Resistors in Series or
in Parallel
Resistors in Series or in Parallel
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Chapter 18
Section 3 Complex Resistor
Combinations
Resistors Combined Both in Parallel and in
Series
• Many complex circuits can be understood by isolating
segments that are in series or in parallel and
simplifying them to their equivalent resistances.
• Work backward to find the current in and potential
difference across a part of a circuit.
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Chapter 18
Section 3 Complex Resistor
Combinations
Analysis of Complex Circuits
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Visual Concept
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Chapter 18
Section 3 Complex Resistor
Combinations
Sample Problem
Equivalent Resistance
Determine the equivalent resistance of the complex
circuit shown below.
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