Current And ResistanceCurrent And Resistance

•Chapter 17

Electric CurrentElectric Current

•Current exists whenever charges of like signs move

17.1

•Current is the rate of flow of charge

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I =ΔQ

Δt

•The unit for current is the Ampere (A).

1 Ampere = 1 Coulomb/Second

•Conventional (positive) current flow is used.–Charge carriers may be positive or negative

75

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

Current Produces HeatCurrent Produces Heat

•Internal collisions between charge carriers produce heat.

17.4A

Current Flow Analogies

•Water flow in a pipe•Marbles in a long pipe•Links in a bicycle chain

20-21

ResistanceResistance

• Resistance (R) is opposition to the flow of current– Measured in units called ohms

•Symbol ()

– Occurs because of collisions between electrons

Ohm’s Law

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R =ΔV

I

Current And Voltage Measurements

• Multimeters can measure voltage,current and resistance in an electrical circuit.

It is very important to choose the correct range and to connect the multimeter leads properly.

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

Ohmic Resistance

Non-Ohmic ResistanceNon-Ohmic Resistance

ResistorsResistors

•A resistor is a device which has a specific amount of resistance–Circuit diagram symbol

Examples of ResistorsExamples of Resistors

Resistor Color CodeResistor Color Code

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)

Resistance and Resistivity

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R = ρl

A

Applications Involving Applications Involving ResistanceResistance

•The carbon microphone

•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.

Temperature Variation Temperature Variation Of ResistanceOf Resistance

•Usually, the resistivity of a metal increases with an increase in temperature.

Temperature Variation Temperature Variation Of ResistanceOf Resistance

•Temperature coefficient of resistivity ()–Values are in Table 17.1

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=o 1+α T −To( )[ ]

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R = Ro 1+α T −To( )[ ]

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)

•Currents in a superconductor, could flow forever without an applied voltage.

The Meissner Effect

•The goal is to produce high temperature superconductors–Mercury-oxide-based

•The highest temperature so far is 150 K (-123 oC)

•What is the ultimate goal?

Superconductivity at room temperature!

Applications of Applications of SuperconductorsSuperconductors

–Microwave devices–Superconducting power lines

–Mag-Lev trains–Motors

Cells and BatteriesCells and Batteries

•A dry cell transforms chemical energy into electrical energy•Some of this energy becomes thermal energy

178, 179

Resistive CircuitsResistive Circuits

•A simple resistive circuit•Assume no loss of energy in the wires.

•Joule heating occurs in the resistor.

17.11

Electrical PowerElectrical Power

•Electrical power is measured in Watts (W)

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P = IV = I2R =V2

R

Electrical EnergyElectrical Energy

•Electrical energy is measured in kilowatt hours (kWh)•Consumers Energy bill?

1 kWh = 3.60 x 106 J

A Kilowatt-Hour MeterA Kilowatt-Hour Meter

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

• If the atrial and ventricular contractions become irregular, we have fibrillation.– Defibrillators can be used to restore the normal heart rhythm

17.13

A Bad EKGA Bad EKG

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.

Pacemaker LocationPacemaker Location

An Actual PacemakerAn Actual Pacemaker

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)

Electrical Activity In Electrical Activity In The BrainThe Brain

•The brain produces electrical waves–Monitored by electroencephalographs•EEG

An EEGAn EEG

Questions

1, 3 - 5, 7, 8, 10 - 12

Pg. 587