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Current And ResistanceCurrent And Resistance
•Chapter 17
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Electric CurrentElectric Current
•Current exists whenever charges of like signs move
17.1
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•Current is the rate of flow of charge
€
I =ΔQ
Δt
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•The unit for current is the Ampere (A).
1 Ampere = 1 Coulomb/Second
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•Conventional (positive) current flow is used.–Charge carriers may be positive or negative
75
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Drift SpeedDrift Speed
•The speed of the charge carriers is an average speed called the drift speed (d).
n is the number of charge carriers per unit volume
17.2€
I =ΔQ
Δt= nqvdA
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Current Produces HeatCurrent Produces Heat
•Internal collisions between charge carriers produce heat.
17.4A
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Current Flow Analogies
•Water flow in a pipe•Marbles in a long pipe•Links in a bicycle chain
20-21
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ResistanceResistance
• Resistance (R) is opposition to the flow of current– Measured in units called ohms
•Symbol ()
– Occurs because of collisions between electrons
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Ohm’s Law
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R =ΔV
I
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Current And Voltage Measurements
• Multimeters can measure voltage,current and resistance in an electrical circuit.
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It is very important to choose the correct range and to connect the multimeter leads properly.
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Ohmic and Non-Ohmic Ohmic and Non-Ohmic ResistanceResistance
•Ohm’s Law–Ohmic resistance is constant over a wide range of voltages.
–Non ohmic resistance varies with voltage•Semiconductor diodes
–One way valves for electricity
17.5, 226, 175
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Ohmic Resistance
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Non-Ohmic ResistanceNon-Ohmic Resistance
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ResistorsResistors
•A resistor is a device which has a specific amount of resistance–Circuit diagram symbol
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Examples of ResistorsExamples of Resistors
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Resistor Color CodeResistor Color Code
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ResistivityResistivity
• Resistivity is a constant of proportionality which affects the resistance of a specific material.– Units: (m)
•Values are in Table 17.1 (Pg. 577)
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Resistance and Resistivity
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R = ρl
A
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Applications Involving Applications Involving ResistanceResistance
•The carbon microphone
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•The carbon microphone–Sound waves cause a change in resistance.
–Frequency response is very poor. (20 Hz to 4,000Hz)•The normal human hearing range is between 20 Hz and 20,000 Hz.
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Temperature Variation Temperature Variation Of ResistanceOf Resistance
•Usually, the resistivity of a metal increases with an increase in temperature.
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Temperature Variation Temperature Variation Of ResistanceOf Resistance
•Temperature coefficient of resistivity ()–Values are in Table 17.1
€
=o 1+α T −To( )[ ]
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R = Ro 1+α T −To( )[ ]
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SuperconductorsSuperconductors
•The resistance of some metals and compounds falls to virtually zero below the critical temperature. (Tc)–Values are in Table 17.2 (Pg. 584)
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•Currents in a superconductor, could flow forever without an applied voltage.
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The Meissner Effect
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•The goal is to produce high temperature superconductors–Mercury-oxide-based
•The highest temperature so far is 150 K (-123 oC)
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•What is the ultimate goal?
Superconductivity at room temperature!
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Applications of Applications of SuperconductorsSuperconductors
–Microwave devices–Superconducting power lines
–Mag-Lev trains–Motors
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Cells and BatteriesCells and Batteries
•A dry cell transforms chemical energy into electrical energy•Some of this energy becomes thermal energy
178, 179
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Resistive CircuitsResistive Circuits
•A simple resistive circuit•Assume no loss of energy in the wires.
•Joule heating occurs in the resistor.
17.11
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Electrical PowerElectrical Power
•Electrical power is measured in Watts (W)
€
P = IV = I2R =V2
R
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Electrical EnergyElectrical Energy
•Electrical energy is measured in kilowatt hours (kWh)•Consumers Energy bill?
1 kWh = 3.60 x 106 J
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A Kilowatt-Hour MeterA Kilowatt-Hour Meter
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Electrical Activity In Electrical Activity In The HeartThe Heart
• Muscle action is initiated by electrical activity
• Voltage pulses cause the heart to beat– These electrical waves travel through the body•Monitored by electrocardiographs
• EKG (electrocardiogram)
177
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• If the atrial and ventricular contractions become irregular, we have fibrillation.– Defibrillators can be used to restore the normal heart rhythm
17.13
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A Bad EKGA Bad EKG
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Cardiac PacemakersCardiac Pacemakers
• Cardiac pacemakers may be implanted under the skin and attached with a wire to the right ventricle.– A lithium battery is used to charge a capacitor. It discharges 60 times per minute to stimulate the heart beat. If the heart is beating normally, the capacitor is not allowed to charge.
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Pacemaker LocationPacemaker Location
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An Actual PacemakerAn Actual Pacemaker
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Cardioverter Cardioverter DefibrillatorsDefibrillators
• Implanted Cardioverter Defibrillators (ICDs) can monitor, record, process information, and send corrective signals to the heart.– The lithium batteries last 4 – 5 years.
– Table 17.3 (Defibrillator Properties)
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Electrical Activity In Electrical Activity In The BrainThe Brain
•The brain produces electrical waves–Monitored by electroencephalographs•EEG
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An EEGAn EEG
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Questions
1, 3 - 5, 7, 8, 10 - 12
Pg. 587