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Electricity

Chapter 21

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Charged objects

NeutralEqual positive and negative charges

PositiveFewer negative charges (lost electrons)

NegativeMore negative charges (gained electrons)

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Static electricity

The accumulation of electric charges on an object.

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Charges

Like charges repelOpposite charges attract

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Charging by induction

A charged object causes the electrons on a neutral object to rearrange themselves.The charged object and the neutral object are then attracted to each other.See page 547

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Electric field

How an electron exerts a force on other particles.The field is strongest closest to the electron.

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Conductor

Material that allows electrons to move easily through it

metals

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Insulator

A material that doesn’t allow electrons to move through it easily

PlasticWoodRubberGlass

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Grounding

Providing a path for electrons to reach the ground.A grounded object cannot accumulate excess charge.

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Electroscopes

Detect the presence of chargesContain two thin metal leaves that separate when a charged object is brought near

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Lightning

A large discharge of static electricity.Electrons accumulate on the bottom of a cloud. When there are too many, they are transferred to the ground.They can also go to another cloud.Causes air to expand rapidly

thunder

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Lightning rod

Pointed metal rod on the highest point of a structure.Connected to the ground with a cable.Allow the electrons from lightning to travel safely to the ground.

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discuss

What is static electricity?What is the difference between a conductor and an insulator?What is an electric field?What is lightning?What is thunder?

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Moving electrons

Electrons move from areas of high potential energy to areas of low potential energyWhen the potential energies are equal, the electrons stop

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Electric potential

Potential energy difference divided by chargeMeasured in volts (V)Often called voltageMeasured by a voltmeter

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Circuit

A closed path for electronsElectrons continue to flow as long as there is a potential difference

Common source is a battery

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Current

The flow of electrons through a conductorAmount of current depends on number of electrons passing a point in a given time.Measured in amperes or amps (A)1 ampere is 1 coulomb of charge per second

1 coulomb is the charge of 6.24 x 1018 electrons

Measured with an ammeter

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Dry cells

AAA, AA, C, D, 9V, etc.Contain a carbon rod, a zinc plate, and a moist conducting paste.Have a potential difference between the positive and negative terminalsElectrons are released in a chemical reactionAs long as the reaction occurs, the battery works

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Wet cells

Car batteriesContain two conducting plates in an electrolyte solutionA chemical reaction between a plate and the solution causes a potential difference

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Resistance

The tendency of a material to resist electron flowMeasured in ohms ()Long wires have more resistance than short wiresThin wires have more resistance than thick wires

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Ohm’s Law

The current in a circuit depends on both voltage and resistance

R

VI

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Example

Find the current flowing through a wire if its resistance is 20 and it is connected to a 12-V battery.

R

VI

20

V 12I A 6.0I

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You try

Find the current flowing through a wire if its resistance is 20 and it is connected to a 6-V battery.

R

VI

20

V 6I A 3.0I

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Discuss

What is current?Describe dry cell batteries.Describe wet cell batteries.What is resistance?What three quantities are related by Ohm’s law?

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Series circuits

The current only has one possible path.The current is the same everywhere.If one part goes out, the circuit is broken and it won’t work.

Christmas lights

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Parallel CircuitsContain separate branches for current to move through.The current is different in each branch.

More current goes through branches with less resistance.

The potential difference is the same in each branch.When one branch is opened, the current continues to flow through the other branches

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Household circuitsMostly parallel circuits.Overheating protection is needed.

FusesContain small pieces of metal that melt when the current gets too highMust be replaced when blown

Circuit breakersContain metal that bends when the current gets too highCan be reset by flipping a switch

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Discuss

The current splits up to move through separate branches in a(n) _____________ circuit.What kind of circuit is most of your home wiring connected in?What are two devices that can keep a circuit from overheating?

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Power

The rate at which work is done – ch 7Electrical power – the rate at which electrical energy is converted to another form of energy

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Power

Expressed in watts (W) or kilowatts (kW)Found by multiplying current times voltage.

IVP

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Example

A lamp operates with a current of 0.625 A and a potential difference of 120 V. How much power does it use?

IVP V 120A 625.0P

W75P

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You try

A microwave oven uses 1000 W of power. The voltage source is 120 V. What is the current flowing through the microwave?

IVP V 120 W1000 I

A 8.3I IV 120

W1000

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Electrical energy

What you pay forDepends on power rating and time of useMultiply power times time

tPE

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Kilowatt-hour

Unit of electrical energyEqual to 1000 watts (1 kW) of power used for 1 hourUsually charged 5 - 10 cents per kWhSee table 21-2 on page 567

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Example

A 100-W light bulb is left on for 5.5 hours. How many kilowatt-hours of energy is used?

tPE h 5.5kW 100.0E

kWh 55.0E

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Example

A 1500 W hair dryer is used for 6 minutes. How many kilowatt-hours of energy is used?

tPE h 1.0kW 500.1E

kWh 15.0E

h 1.0min 60

h 1min 6

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Discuss

What is electrical power?What is the product of electrical power and time?What is a kilowatt-hour?SB on page 568