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© Houghton Mifflin Harcourt Publishing Company
Section 3 Current and ResistanceChapter 17
Current and Charge Movement
• Electric current is the rate at which electric charges pass through a given area.
charge passing through a given area
electric current = time interval
QI
t
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 17Section 3 Current and Resistance
Conventional Current
© Houghton Mifflin Harcourt Publishing Company
Section 3 Current and ResistanceChapter 17
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.
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 17Section 3 Current and Resistance
Drift Velocity
© Houghton Mifflin Harcourt Publishing Company
Section 3 Current and ResistanceChapter 17
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
potential difference
resistancecurrent
VR
I
© Houghton Mifflin Harcourt Publishing Company
Section 3 Current and ResistanceChapter 17
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.
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 17Section 3 Current and Resistance
Factors that Affect Resistance
© Houghton Mifflin Harcourt Publishing Company
Section 3 Current and ResistanceChapter 17
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.
© Houghton Mifflin Harcourt Publishing Company
Section 4 Electric PowerChapter 17
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.
© Houghton Mifflin Harcourt Publishing Company
Section 4 Electric PowerChapter 17
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.
© Houghton Mifflin Harcourt Publishing Company
Section 4 Electric PowerChapter 17
Energy Transfer
• Electric power is the rate of conversion of electrical energy.
• Electric power
P = I∆V
Electric power = current potential difference
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 17 Section 4 Electric Power
Energy Transfer
© Houghton Mifflin Harcourt Publishing Company
Section 4 Electric PowerChapter 17
Energy Transfer, continued
• Power dissipated by a resistor
• Electric companies measure energy consumed in kilowatt-hours.
• Electrical energy is transferred at high potential differences to minimize energy loss.
22 ( )V
P I V I RR
© Houghton Mifflin Harcourt Publishing Company
Section 1 Schematic Diagrams and CircuitsChapter 18
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.
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 18
Schematic Diagram and Common Symbols
Section 1 Schematic Diagrams and Circuits
© Houghton Mifflin Harcourt Publishing Company
Section 2 Resistors in Series or in ParallelChapter 18
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…
© Houghton Mifflin Harcourt Publishing Company
Chapter 18Section 2 Resistors in Series or in Parallel
Resistors in Series
© Houghton Mifflin Harcourt Publishing Company
Section 2 Resistors in Series or in ParallelChapter 18
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.
eq
VI
R
© Houghton Mifflin Harcourt Publishing Company
Chapter 18
Sample Problem
Resistors in SeriesA 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.
Section 2 Resistors in Series or in Parallel
© Houghton Mifflin Harcourt Publishing Company
Chapter 18
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
Section 2 Resistors in Series or in Parallel
Unknown:
Req = ?
I = ?
Diagram:
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 18
Comparing Resistors in Series and in Parallel
Section 2 Resistors in Series or in Parallel
© Houghton Mifflin Harcourt Publishing Company
Section 2 Resistors in Series or in ParallelChapter 18
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
Req
1
R1
1
R2
1
R3
...
© Houghton Mifflin Harcourt Publishing Company
Chapter 18Section 2 Resistors in Series or in Parallel
Resistors in Parallel
© Houghton Mifflin Harcourt Publishing Company
Chapter 18
Sample Problem
Resistors in ParallelA 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.
Section 2 Resistors in Series or in Parallel
© Houghton Mifflin Harcourt Publishing Company
Chapter 18
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
Section 2 Resistors in Series or in Parallel
Unknown:
Req = ?
I = ?
Diagram:
© Houghton Mifflin Harcourt Publishing Company
Chapter 18Section 2 Resistors in Series or in Parallel
Resistors in Series or in Parallel
© Houghton Mifflin Harcourt Publishing Company
Section 3 Complex Resistor CombinationsChapter 18
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.
© Houghton Mifflin Harcourt Publishing Company
Click below to watch the Visual Concept.
Visual Concept
Chapter 18
Analysis of Complex Circuits
Section 3 Complex Resistor Combinations
© Houghton Mifflin Harcourt Publishing Company
Section 3 Complex Resistor CombinationsChapter 18
Sample Problem
Equivalent ResistanceDetermine the equivalent resistance of the complex circuit shown below.