Electric CurrentElectric Current

Chapter 34Chapter 34

Flow of ChargeFlow of Charge

When the ends of an electric conductor When the ends of an electric conductor are at different electric potentials, charge are at different electric potentials, charge flows from one end to the otherflows from one end to the other

Potential Difference –Potential Difference – the difference in the difference in voltage between the ends of a conductorvoltage between the ends of a conductor

The flow of charge will continue until both The flow of charge will continue until both ends reach a common potentialends reach a common potential

To attain a sustained flow of charge in a To attain a sustained flow of charge in a conductor, some arrangement must be conductor, some arrangement must be provided to maintain a difference in provided to maintain a difference in potential while charge flows from one end potential while charge flows from one end to the otherto the other

Flow of ChargeFlow of Charge

Electric CurrentElectric Current Electric Current –Electric Current – the flow of electric the flow of electric

chargecharge In solid conductors the electrons carry the In solid conductors the electrons carry the

charge through the circuit because they are charge through the circuit because they are free to move throughout the atomic free to move throughout the atomic network (network (conduction electronsconduction electrons))

Protons are bound inside atomic nuclei that Protons are bound inside atomic nuclei that are more or less locked in fixed positionsare more or less locked in fixed positions

Ampere –Ampere – the unit of electric current (A); 1 the unit of electric current (A); 1 coulomb of charge per secondcoulomb of charge per second

A current-carrying wire does not have a A current-carrying wire does not have a net net electric charge, because the number of electric charge, because the number of electrons electrons still still equals the number of protonsequals the number of protons

Electric CurrentElectric Current

Voltage SourcesVoltage Sources

Charges will not flow unless there is a Charges will not flow unless there is a potential differencepotential difference

Voltage Source –Voltage Source – something that something that provides a potential differenceprovides a potential difference

Dry cells, wet cells, and generators are Dry cells, wet cells, and generators are capable of maintaining a steady flow capable of maintaining a steady flow (batteries are two or more cells connected (batteries are two or more cells connected together), by supplying energy that allows together), by supplying energy that allows charges to movecharges to move

The voltage provides the “electric The voltage provides the “electric pressure” to move electrons between the pressure” to move electrons between the terminals in a circuitterminals in a circuit

Voltage SourcesVoltage Sources

Electric ResistanceElectric Resistance Electric Resistance –Electric Resistance – the resistance the resistance

that the conductor offers to the flow of that the conductor offers to the flow of chargecharge

The resistance of a wire depends on the The resistance of a wire depends on the conductivity of the material used in the conductivity of the material used in the wire and also on the thickness of the wire and also on the thickness of the wirewire

Thick wires have less resistance than Thick wires have less resistance than thin wires; longer wires have more thin wires; longer wires have more resistance than short wiresresistance than short wires

Electric resistance is also dependent on Electric resistance is also dependent on temperature; the more temperature, the temperature; the more temperature, the more resistancemore resistance

Ohms –Ohms – unit of electric resistance ( unit of electric resistance (ΩΩ))

Electric ResistanceElectric ResistanceMaterial

Resistivity(ohm•meter)

Silver 1.59 x 10-8

Copper 1.7 x 10-8

Gold 2.4 x 10-8

Aluminum 2.8 x 10-8

Tungsten 5.6 x 10-8

Iron 10 x 10-8

Platinum 11 x 10-8

Lead 22 x 10-8

Nichrome 150 x 10-8

Carbon 3.5 x 105

Polystyrene 107 - 1011

Polyethylene 108 - 109

Glass 1010 - 1014

Hard Rubber 1013

Ohm’s LawOhm’s Law

Ohm’s Law –Ohm’s Law – the current in a circuit is the current in a circuit is directly proportional to the voltage directly proportional to the voltage impressed across the circuit and impressed across the circuit and inversely proportional to the resistance inversely proportional to the resistance of the circuitof the circuit

Current = voltage/resistanceCurrent = voltage/resistance1 ampere = 1 volt/ohm1 ampere = 1 volt/ohm

Inside electrical devices, such as radio Inside electrical devices, such as radio and television receivers, the current is and television receivers, the current is regulated by circuit elements called regulated by circuit elements called resistors, whose resistance may range resistors, whose resistance may range from a few ohms to millions of ohmsfrom a few ohms to millions of ohms

Ohm’s LawOhm’s LawCircuit

Diagram

BatteryVoltage(     V)

TotalResistance

(     )Current(Amps)

1.              1.5 V 3    0.50 Amp

2.                  3.0 V 3    1 Amp

3.                     4.5 V 3    1.5 Amp

4.                  1.5 V 6    0.25 Amp

5.                      3.0 V 6    0.5 Amp

6.                         4.5 V 6    0.75 Amp

7.                          4.5 V 9    0.50 Amp

Ohm’s Law and Electric Ohm’s Law and Electric ShockShock The damaging effects of shock are the The damaging effects of shock are the

result of current passing through the result of current passing through the bodybody

This current depends on the voltage This current depends on the voltage supplied, and also on the electric supplied, and also on the electric resistance of the human bodyresistance of the human body

The resistance of your body depends on The resistance of your body depends on its condition and ranges from about 100 its condition and ranges from about 100 ohms if you’re soaked with salt water to ohms if you’re soaked with salt water to about 500,000 ohms if your skin is very about 500,000 ohms if your skin is very drydry

One effect of electric shock is to overheat One effect of electric shock is to overheat tissues in the body or to disrupt normal tissues in the body or to disrupt normal nerve functionsnerve functions

Ohm’s Law and Electric Ohm’s Law and Electric ShockShock

Current in Current in AmperesAmperes

EffectEffect

0.0010.001 Can be feltCan be felt

0.0050.005 PainfulPainful

0.0100.010 Involuntary muscle contractions Involuntary muscle contractions (spasms)(spasms)

0.0150.015 Loss of muscle controlLoss of muscle control

0.0700.070 If through the heart, serious If through the heart, serious disruption; probably fatal if current disruption; probably fatal if current lasts for more than 1 secondlasts for more than 1 second

Effect of Various Electric Currents on the Body

Direct Current and Direct Current and Alternating CurrentAlternating Current Direct Current –Direct Current – a flow of charge which a flow of charge which

always flows in one directionalways flows in one direction A battery produces direct current in a circuit A battery produces direct current in a circuit

because the terminals always have the same because the terminals always have the same sign of charge (electrons always move sign of charge (electrons always move through the circuit in the same direction)through the circuit in the same direction)

Alternating Current – Alternating Current – electric current that electric current that repeatedly switches directionrepeatedly switches direction

Nearly all commercial AC circuits in North Nearly all commercial AC circuits in North America involve voltages and currents that America involve voltages and currents that alternate back and forth at a frequency of 60 alternate back and forth at a frequency of 60 cycles per secondcycles per second

Voltage of AC in North America is normally Voltage of AC in North America is normally 120 volts, whereas in Europe the voltage is 120 volts, whereas in Europe the voltage is standardized at 220 volts (that’s why you standardized at 220 volts (that’s why you need a voltage adapter in other countries)need a voltage adapter in other countries)

Direct Current and Direct Current and Alternating CurrentAlternating Current

Direct CurrentAlternating Current

Converting AC to DCConverting AC to DC

Diode –Diode – a tiny electronic device that a tiny electronic device that acts as a one-way valve to allow acts as a one-way valve to allow electron flow in only one directionelectron flow in only one direction

Since alternating current vibrates in two Since alternating current vibrates in two directions, only half of each cycle will directions, only half of each cycle will pass through a diodepass through a diode

The output is rough DC, off half the timeThe output is rough DC, off half the time To maintain continuous current while To maintain continuous current while

smoothing the bumps, a capacitor is smoothing the bumps, a capacitor is usedused

Converting AC to DCConverting AC to DC

The Speed of Electrons in a The Speed of Electrons in a CircuitCircuit At room temperature, the electrons inside At room temperature, the electrons inside

a metal wire have an average speed of a a metal wire have an average speed of a few million kilometers per hour due to their few million kilometers per hour due to their thermal motionthermal motion

There is no net flow in any one direction; There is no net flow in any one direction; but when a battery is connected, an but when a battery is connected, an electric field is established inside the wireelectric field is established inside the wire

The electrons continue their random The electrons continue their random motions in all directions while motions in all directions while simultaneously being nudged along the simultaneously being nudged along the wire by the electric fieldwire by the electric field

The conducting wire acts as a guide or The conducting wire acts as a guide or “pipe” for the electric field lines“pipe” for the electric field lines

Because the electrons are constantly Because the electrons are constantly bumping into each other as they move, the bumping into each other as they move, the wire becomes hotwire becomes hot

The Speed of Electrons in a The Speed of Electrons in a CircuitCircuit

The Source of Electrons in The Source of Electrons in a Circuita Circuit

The source of electrons in a circuit is The source of electrons in a circuit is the conducting circuit material itselfthe conducting circuit material itself

When you plug a lamp into an AC When you plug a lamp into an AC outlet, outlet, energyenergy flows from the outlet flows from the outlet to the lamp, NOT electronsto the lamp, NOT electrons

Energy is carried by the electric field Energy is carried by the electric field and causes vibratory motion of the and causes vibratory motion of the electrons that already exist in the electrons that already exist in the lamp filamentlamp filament

The Source of Electrons in The Source of Electrons in a Circuita Circuit

Electric PowerElectric Power Electric Power –Electric Power – the rate at which the rate at which

electrical energy is converted into another electrical energy is converted into another form such as mechanical energy, heat, or form such as mechanical energy, heat, or lightlight

Electric power = current x voltageElectric power = current x voltage

1 watt = (1 ampere) x (1 volt)1 watt = (1 ampere) x (1 volt) A A kilowatt kilowatt is 1000 watts, and a is 1000 watts, and a kilowatt-kilowatt-

hourhour represents the amount of energy represents the amount of energy consumed in 1 hour at the rate of 1 consumed in 1 hour at the rate of 1 kilowattkilowatt

This is how the electric company charges This is how the electric company charges for the power you receivefor the power you receive

AssignmentAssignment

Read Chapter 34 (pg. 531-544)Read Chapter 34 (pg. 531-544) Do Chapter 34 #26-52 (pg. 546-547)Do Chapter 34 #26-52 (pg. 546-547) Appendix F, Chapter 34 #1-12 (pg. Appendix F, Chapter 34 #1-12 (pg.

689)689)