chapter 20: electricity section 20.1 electric charge and static electricity section 20.2 electric...

Chapter 20: Chapter 20: ElectricityElectricity

Section 20.1 Electric Charge and Static Section 20.1 Electric Charge and Static ElectricityElectricity

Section 20.2 Electric Current and Section 20.2 Electric Current and Ohm’s LawOhm’s Law

Section 20.3 Electric CurrentsSection 20.3 Electric Currents

Section 20.1 Electric Section 20.1 Electric Charge and Static Charge and Static

ElectricityElectricity Lightning and static cling result from Lightning and static cling result from

the movement of electric charges.the movement of electric charges. Electric ChargeElectric Charge

Def.-a property that causes subatomic Def.-a property that causes subatomic particles such as protons and electrons particles such as protons and electrons to attract or repel each otherto attract or repel each other

Two types of electric charge: Two types of electric charge: positive positive and negativeand negative

Protons (+ charge) and electrons (- Protons (+ charge) and electrons (- charge)charge)


ElectricityElectricity Electric ChargeElectric Charge

Examples of movement: lightening Examples of movement: lightening boltsbolts

Examples of attraction: clothes taken Examples of attraction: clothes taken out of the dryerout of the dryer

Almost everything in our day to day Almost everything in our day to day lives is affected some way by charges.lives is affected some way by charges.


ElectricityElectricity Electric ChargeElectric Charge

Cloud of (–) electrons surround (+) nucleusCloud of (–) electrons surround (+) nucleus Atom is neutral b/c Atom is neutral b/c #protons=#electrons#protons=#electrons Atom gains 1 or more electrons becomes a Atom gains 1 or more electrons becomes a

negatively charged ionnegatively charged ion Atom loses electrons become a positively Atom loses electrons become a positively

charged ioncharged ion Key Concept: An excess or shortage or Key Concept: An excess or shortage or

electrons produce a net electric charge.electrons produce a net electric charge.


ElectricityElectricity Electric ChargesElectric Charges

SI Unit of electric charge= C or SI Unit of electric charge= C or coulombcoulomb

Takes 6.24 x 10Takes 6.24 x 1018 18 electrons to produce electrons to produce a single coulomba single coulomb


ElectricityElectricity Electric ForcesElectric Forces

Key Concept: Like charges repel, and Key Concept: Like charges repel, and opposite charges attract.opposite charges attract.

Electric force-the force of attraction or Electric force-the force of attraction or repulsion between electrically charged repulsion between electrically charged objectsobjects

Charles-Augustin de Coulomb (1736-Charles-Augustin de Coulomb (1736-1806)-discovered that electric forces 1806)-discovered that electric forces obey laws similar to the law of obey laws similar to the law of universal gravitationuniversal gravitation


ElectricityElectricity Electric ForcesElectric Forces

Electric force b/t 2 objects directly Electric force b/t 2 objects directly proportional to net charge on each object proportional to net charge on each object and inversely proportional to the square of and inversely proportional to the square of the distance between them.the distance between them.

Doubling net charge on one object doubles Doubling net charge on one object doubles the electric forcethe electric force

Doubling distance between objects: Doubling distance between objects: electric electric force is one fourth as strongforce is one fourth as strong

Electric forces inside atoms are much Electric forces inside atoms are much stronger than gravitational forcesstronger than gravitational forces


ElectricityElectricity Electric FieldsElectric Fields

Def.-the effect an electric charge has Def.-the effect an electric charge has on other charges in the space around on other charges in the space around itit

Key Concept: The strength of an Key Concept: The strength of an electric field depends on the amount of electric field depends on the amount of charge that produces the field and on charge that produces the field and on the distance from the charge.the distance from the charge.

Figure 4


ElectricityElectricity Electric FieldsElectric Fields

Exert forces on any charged object Exert forces on any charged object placed in the fieldplaced in the field

Force depends on: Force depends on: net charge in the net charge in the object, strength and direction of the object, strength and direction of the field at the object’s positionfield at the object’s position

The greater the net charge an object The greater the net charge an object has, the greater the force on it.has, the greater the force on it.


ElectricityElectricity Static Electricity and ChargingStatic Electricity and Charging

Static electricity-the study of the behavior Static electricity-the study of the behavior of electric charges, including how charge is of electric charges, including how charge is transferred between objectstransferred between objects

Key Concept: Charge can be transferred Key Concept: Charge can be transferred by friction, by contact, and by induction.by friction, by contact, and by induction.

When charge transfer occurs, the total When charge transfer occurs, the total charge is the same before and after the charge is the same before and after the transfer (transfer (law of conservation of chargelaw of conservation of charge))

Section 20.1 Electric Section 20.1 Electric Charge and Static Charge and Static ElectricityElectricity

Charging by FrictionCharging by Friction Ex. Balloon attracts hairEx. Balloon attracts hair Electrons move from hair to balloon Electrons move from hair to balloon

making balloon negative and hair making balloon negative and hair becomes positivebecomes positive

Ex. Walking across a carpetEx. Walking across a carpet


ElectricityElectricity Charging by ContactCharging by Contact

Hair standing on endHair standing on end Person acquires a charge large Person acquires a charge large

enough where hairs have like chargesenough where hairs have like charges Like charges repelLike charges repel


ElectricityElectricity Charging by InductionCharging by Induction

Ex. Walk across carpet and reach for doorknobEx. Walk across carpet and reach for doorknob You pick up electrons from the carpet: hand is You pick up electrons from the carpet: hand is

negatively chargednegatively charged Net negative charge of hand repels the Net negative charge of hand repels the

electrons in the metal doorknob electrons in the metal doorknob Electrons move to base of doorknob=doorknob Electrons move to base of doorknob=doorknob

is positively charged in partis positively charged in part Doorknob is still neutral but charge moved into Doorknob is still neutral but charge moved into

itit Induction-a transfer of charge without contact Induction-a transfer of charge without contact

between materialsbetween materials


ElectricityElectricity Static DischargeStatic Discharge

Shock from doorknobs or other objects=Shock from doorknobs or other objects=result result of static dischargeof static discharge

Key Concept: Static discharge occurs when a Key Concept: Static discharge occurs when a pathway through which charges move forms pathway through which charges move forms suddenly.suddenly.

Air becomes charged suddenly when the gap Air becomes charged suddenly when the gap b/t your finger and the doorknob is small. b/t your finger and the doorknob is small. Air Air provide path for electrons to flow from your provide path for electrons to flow from your hand to the doorknob. hand to the doorknob. Spark can be seen in Spark can be seen in the dark.the dark.


ElectricityElectricity Static DischargeStatic Discharge

Lightning-more dramatic dischargeLightning-more dramatic discharge Friction between moving air masses causes Friction between moving air masses causes

charge to build up in storm cloudscharge to build up in storm clouds Negative charge in lower part of cloud induces Negative charge in lower part of cloud induces

a positive charge in the ground below ita positive charge in the ground below it Amt. of charge in cloud Amt. of charge in cloud ↑, ↑, attraction b/t attraction b/t

charges in cloud and ground charges in cloud and ground ↑↑ Air becomes charged (forms pathway for Air becomes charged (forms pathway for

electrons to flow from cloud to the ground)electrons to flow from cloud to the ground)

Section 20.2 Electric Section 20.2 Electric Current and Ohm’s Current and Ohm’s

LawLaw Electric CurrentElectric Current

Def.-the continuous flow of electric chargeDef.-the continuous flow of electric charge SI Unit of electric current-ampere (A); SI Unit of electric current-ampere (A);

equals 1 coulomb/secondequals 1 coulomb/second Key Concept: The two types of current are Key Concept: The two types of current are

direct current and alternating current.direct current and alternating current. Direct current (DC)-charge flows in one Direct current (DC)-charge flows in one

directiondirection Flashlight; most battery-operated devicesFlashlight; most battery-operated devices

Section 20.2 Electric Section 20.2 Electric Current and Ohm’s Current and Ohm’s

LawLaw Electric CurrentElectric Current

Alternating current (AC)-a flow of Alternating current (AC)-a flow of electric charge that regularly reverses electric charge that regularly reverses its directionits direction

Houses and schoolsHouses and schools

A A FlashlightFlashlight

Figure 8

Section 20.2 Electric Section 20.2 Electric Current and Ohm’s LawCurrent and Ohm’s Law

Conductors and InsulatorsConductors and Insulators Electrical conductor-a material through which charge Electrical conductor-a material through which charge

can flow easilycan flow easily Electrical insulators-a material through which charge Electrical insulators-a material through which charge

cannot flow easilycannot flow easily Metals-Metals-have ions in a lattice that don’t move, but have ions in a lattice that don’t move, but have have

some electrons that a not bound to the lattice some electrons that a not bound to the lattice =they can =they can conduct chargeconduct charge

**Most materials don’t conduct charge b/c they have no **Most materials don’t conduct charge b/c they have no free electronsfree electrons

Metal such as copper and silver are good electrical Metal such as copper and silver are good electrical conductors. Wood, plastics, rubber and air are good conductors. Wood, plastics, rubber and air are good electrical insulators.electrical insulators.

Section 20. 2 Electrical Section 20. 2 Electrical current and Ohm’s Lawcurrent and Ohm’s Law

ResistanceResistance Def.-opposition to the flow of charges in a Def.-opposition to the flow of charges in a

material (SI unit=ohm)material (SI unit=ohm) Key Concept: A material’s thickness, Key Concept: A material’s thickness,

length, and temperature affect its length, and temperature affect its resistance.resistance.

Resistance > in long wire b/c charges Resistance > in long wire b/c charges have to travel fartherhave to travel farther; ; Temp. >=more Temp. >=more resistance b/c electrons collide more oftenresistance b/c electrons collide more often

Section 20.2 Electric Section 20.2 Electric Current and Ohm’s Current and Ohm’s LawLaw

ResistanceResistance Can resistance be 0?Can resistance be 0? Superconductor-material that has Superconductor-material that has

almost zero resistance when it is almost zero resistance when it is cooled to low temperature.cooled to low temperature.


VoltageVoltage Key Concept: In order for charge to flow in a Key Concept: In order for charge to flow in a

conducting wire, the wire must be connected conducting wire, the wire must be connected in a complete loop that includes a source of in a complete loop that includes a source of electrical energy.electrical energy.

Potential DifferencePotential Difference Objects at greater height have more Objects at greater height have more

potential than those at a lower height. So potential than those at a lower height. So those objects fall from higher to a lower those objects fall from higher to a lower potential energy. potential energy. This is true of charges. This is true of charges. Charges flow spontaneously from a higher to Charges flow spontaneously from a higher to lower potential energy.lower potential energy.


Voltage (Potential Difference)Voltage (Potential Difference) Potential difference of a charge depends on its Potential difference of a charge depends on its

position in an electric field.position in an electric field. Def.-the difference in electrical potential energy Def.-the difference in electrical potential energy

between two places in an electric fieldbetween two places in an electric field Measured in Joules/Coulombs (Volts) aka VoltageMeasured in Joules/Coulombs (Volts) aka Voltage

Voltage SourcesVoltage Sources Batteries, solar cells, and generators.Batteries, solar cells, and generators. Have terminals (positive and negative) that connect to Have terminals (positive and negative) that connect to

wires in a circuitwires in a circuit Battery-a device that converts chemical energy to Battery-a device that converts chemical energy to

electrical energy.electrical energy.


Ohm’s LawOhm’s Law Ohm unit-German scientist-Georg Ohm Ohm unit-German scientist-Georg Ohm

(1789-1854)(1789-1854) 11stst determined how resistance and current determined how resistance and current

affect voltage.affect voltage. Discovered voltage is not the same Discovered voltage is not the same

everywhere in a circuit; everywhere in a circuit; Hypothesized that Hypothesized that resistance reduces voltage.resistance reduces voltage.

Found a relationship b/t voltage, current, and Found a relationship b/t voltage, current, and resistanceresistance


Ohm’s LawOhm’s Law Voltage (V) in a circuit equals the product of Voltage (V) in a circuit equals the product of

the current (I) and the resistance (R).the current (I) and the resistance (R). V=I x R or I=V/RV=I x R or I=V/R If current in amps and resistance in ohms, If current in amps and resistance in ohms,

voltage in voltsvoltage in volts Key Concept: Increasing the voltage Key Concept: Increasing the voltage

increases the current. Keeping the same increases the current. Keeping the same voltage and increasing the resistance voltage and increasing the resistance decreases the current.decreases the current.

Section 20.3 Electric Section 20.3 Electric CurrentsCurrents

Circuit DiagramsCircuit Diagrams Electric current-a complete path through Electric current-a complete path through

which charge can flow; which charge can flow; wires in houses, etc. wires in houses, etc. have complex networks of circuitshave complex networks of circuits

Electricians use Electricians use circuit diagrams circuit diagrams to monitor to monitor how elements in a circuit are connected.how elements in a circuit are connected.

Key Concept: Circuit diagrams use symbols Key Concept: Circuit diagrams use symbols to represent parts of a circuit, including a to represent parts of a circuit, including a source of electrical energy and devices that source of electrical energy and devices that run by the electrical energyrun by the electrical energy

Section 20.3 Electric Section 20.3 Electric CircuitsCircuits

Circuit DiagramsCircuit Diagrams Show one or more complete paths where Show one or more complete paths where

charge can flowcharge can flow Switches show where circuit can open; Switches show where circuit can open; if if

open circuit not in a complete loop so open circuit not in a complete loop so current current stops=open circuitstops=open circuit

If switch closed, the circuit is complete and If switch closed, the circuit is complete and charge can flow=closed circuitcharge can flow=closed circuit

Section 20.3 Electric Section 20.3 Electric CircuitsCircuits Electrons in a wire flow in the opposite direction.Electrons in a wire flow in the opposite direction.

Series CircuitsSeries Circuits ****Charge has only one path through which it can Charge has only one path through which it can

flowflow Key Concept: If one element stops functioning Key Concept: If one element stops functioning

in a series circuit, none of the elements can in a series circuit, none of the elements can operate.operate.

One light bulb blows it becomes an open circuit.One light bulb blows it becomes an open circuit. Bulbs are a source of resistance; the more Bulbs are a source of resistance; the more

present the more resistance there is.present the more resistance there is. >resistance=<current;decrease brightness of >resistance=<current;decrease brightness of

bulbsbulbs


Parallel CircuitsParallel Circuits Circuits in homes are mostly parallel.Circuits in homes are mostly parallel. Def.-an electric circuit with two or more Def.-an electric circuit with two or more

paths through which charges can flowpaths through which charges can flow Key Concept: If one element stops Key Concept: If one element stops

functioning in a parallel circuit, the rest of functioning in a parallel circuit, the rest of the elements still can operate.the elements still can operate.

Allows independent operation of devices.Allows independent operation of devices.

Series and Parallel CircuitsSeries and Parallel Circuits

Figure 12

Section 20.3 Electric Section 20.3 Electric CircuitsCircuits

Power and Energy CalculationsPower and Energy Calculations Electric power-the rate at which electrical Electric power-the rate at which electrical

energy is converted to another form of energyenergy is converted to another form of energy Units of joules per second (watt, W). Units of joules per second (watt, W). Power Power

measured in watts or kilowatts (kW)measured in watts or kilowatts (kW) Key Concept: Electric power can be Key Concept: Electric power can be

calculated by multiplying voltage by current.calculated by multiplying voltage by current. P (watts)=I (amps) x V (volts)P (watts)=I (amps) x V (volts) ****Appliances vary in the amount of power Appliances vary in the amount of power

they use.they use.

Section 20.3


Electrical SafetyElectrical Safety Inspectors check all new homes to make sure Inspectors check all new homes to make sure

electrical wiring is installed safely.electrical wiring is installed safely. Key Concept: Correct wiring, fuses, circuit Key Concept: Correct wiring, fuses, circuit

breakers, insulation, and grounded plugs breakers, insulation, and grounded plugs help make electrical energy safe to use.help make electrical energy safe to use.

The amount of current in a circuit depends The amount of current in a circuit depends on how many devices in the circuit. > # of on how many devices in the circuit. > # of devices turned on the > the currentdevices turned on the > the current

If current exceeds the circuits safety limit, If current exceeds the circuits safety limit, wire may overheat and start a fire.wire may overheat and start a fire.


Home SafetyHome Safety Fuses prevent current overload in a circuit; Fuses prevent current overload in a circuit;

wire in the center of a fuse melts if there is wire in the center of a fuse melts if there is too much current passing through it “blowing too much current passing through it “blowing a fuse”. a fuse”. Has to be replaced with a new one Has to be replaced with a new one to use circuitto use circuit

Most homes use circuit breakers (a switch Most homes use circuit breakers (a switch that opens when current in a circuit is too that opens when current in a circuit is too high); has to be reset for circuit to be used high); has to be reset for circuit to be used againagain


Personal SafetyPersonal Safety Electrical wiring is insulated to protect from Electrical wiring is insulated to protect from

shockshock Don’t touch electrical devices with wet handsDon’t touch electrical devices with wet hands Insulation prevents short circuits; three pronged Insulation prevents short circuits; three pronged

plugs help prevent shocks from short circuitsplugs help prevent shocks from short circuits Circular prong connects to ground; current Circular prong connects to ground; current

takes easier path to ground instead of entering takes easier path to ground instead of entering your bodyyour body

Grounding-the transfer of excess charge Grounding-the transfer of excess charge through a conductor to Earththrough a conductor to Earth

chapter 20: electricity section 20.1 electric charge and static electricity section 20.2 electric...

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