Chapter 7 Electricity

Syllabus Posted on Web Site due 1/13/2014

Page 192

Section 7:1Electrical Charge

Page 192-199

Electrical Charge• We can’t see electrical charge.• We can see the effect of electrical charge.• Walk across a carpet on a dry day, then touch

a metal door knob.• A spark jumps.• Pull a sweater off over your head and your

hair stands up.• Take clothes out of the dryer and they stick

together.

Van der Graaf Generator

Explaining the Unseen• To explain these and other phenomenon,

scientist have developed a theory.• Theory of Charge

– Matter is made up of + and – charged particles– - charged are called electrons– + charged called protons– (another uncharged particle, the neutron)– Like charges repel each other (-,- or +,+)– Unlike charges attract each other (-,+ or +,-)

– Conservation of Charge– Charge can be transferred but it cannot be created

nor destroyed

• All solids, liquids, and gases are made of tiny particles called atoms.

• Atoms are made of even smaller particles called protons, neutrons, and electrons.

Electricity

• Protons and neutrons are held together tightly in the nucleus at the center of an atom, but electrons swarm around the nucleus in all directions.

• Protons and electrons have electric charge, but neutrons have no electric charge.

Electricity

Electric Charge

Electric Fields (P194)

• Electrical forces work through space• Surrounding each charge is a “field” of influence• A charge placed in an electric field will be either

attracted or repelled by the electric field.• Fields are represented by arrows pointing in the

direction a positive charge in the field would move.

Electric Field

Electrical Forces (P194)

• Force of gravity is much weaker than electrical forces.

• Force of electrical attraction between proton (+) and electron (-) is a thousand trillion trillion trillion times larger than the gravitational attraction between these particles. (1039)

• Atoms held together by electrical forces.• Chemical bonds between atoms in molecules are

due to electrical forces.• Most objects are electrically neutral so no force.

Flow of Charge (P195)• Spark that jumps from you to the door knob

represents the flow of charge from one object to another.

• The flow is electrons. Protons do not flow!• Conductors – materials through which

electrons move easily.– Metals

• Insulators – materials through which electrons are not able to move easily.– Plastic, wood, rubber, glass

Charging

• Charging by contact– Rubbing two materials together can transfer

electrons from one material to the other

• Charging by induction– A charged object near a neutral object can cause

the movement of electrons.

Lightning

• Moving air masses cause the build up of negative charges in one area and positive charges in another area.

• A static discharge takes place as electrons move to the positive charged areas

• Moving charges strike molecules and cause them to emit light

• Moving charges also generate heat which causes the air to expand rapidly producing sound waves.

Grounding (P198)

• The sudden discharge of built up charge can damage electronic devices and harm people.

• Connect devices to the ground so that excess charge flows to the earth and does not build up.

• Grounding – connecting an object to the earth to prevent the build up of electrical charge.

Detecting Charge (P199)

• Electroscope – a device for detecting electrical charge.

• Thin metal leaf electroscope.• When charge flows to the leaves they become

charge with the same charge and are therefore repelled.

• Uncharged the leaves hang close together.

Chapter 7 Schedule• 1/6 Monday- Electrical Charge• 1/7 Tuesday – No School• 1/8 Wednesday- Separating Charge, Charge Flow• 1/9 Thursday- Detecting Charge, Static and Current Electricity• 1/10 Friday- Electric current, Electric Circuits• 1/13 Monday- – Types of Circuits• 1 /14 Tuesday- Electric Power and Energy• 1/15 Wednesday - Circuit Lab• 1/16 Thursday Review & Practice Problems• 1/17 Friday NTWS• 1/21 Tuesday- Chapter 7 Review Page 218:1-15, 21-25• 1/22 Wednesday- – Test Chapter 7 • 1/23 Thursday

Chapter 7 Section 2Electrical Currents

Page 200-205

Two Types of Charge Flow

• Static electricity– Gradual separation of + and - charges– Sudden recombining of + and – charges

• Current electricity– Continual separation of charges– Continual recombining of charges

Static Electricity

• Charges are first separated– Charging by contact – materials rub together– Charging by induction- objects don’t touch

• Unlike charges are attracted to each other– Separated charges “want” to get back together– When the charges build up enough they move from

one place to another (electrons=negative, move)• This happens suddenly but only once until charge

is equalized (net 0 + = -)• Start the process over again

Electrical Current

• Net movement of electrical charge in one direction. Electrons and protons are in constant motion but in all directions.

• Measured in amperes = the flow of 6,250 million, billion electrons flowing past a point. (6.250 X 1018)

• The net movement of charge is caused by electric forces acting on the charge.

Electrical Current

• Charges are first separated- continuously– Charging by contact – materials rub together– Charging by induction- objects don’t touch– Chemical reactions separate charges- battery– Magnetic field will separate charges

– Must have a path (material) through which the charge can flow.– Conductor– Closed loop to prevent the build up of charge

Water in a Pipe analogyFigure 11 Page 200

-- ------ ++++

High Pressure

Low Pressure

Low Voltage

High Voltage

Water Flow and Current FlowPage 201 Figure 12

+ -

Water Pump

Electron Pump

Water Wheel doing work

Light bulb or motor doing work

Voltage and work

• Voltage is related to the force that causes electrons to flow.

• Falling water can do work- GPE can be changed into motion of a water wheel.

• “Falling” electrons ie falling from a high voltage to a lower voltage can do work.

• Figure 12 the work is done lighting the light bulb.

• Voltage is also called voltage drop.

Two Types of Charge Flow

• Static electricity– Gradual separation of + and - charges– Sudden recombining of + and – charges

• Current electricity– Continual separation of charges– Continual recombining of charges

Battery (cell)

• A chemical reaction in a battery provides constant energy to separate charges and keep them flowing constantly.

• A battery is “like” an electron pump.• Chemical which separates charges

– Dry cell- the chemicals are a paste– Wet cell – the chemical are a liquid

• Electrodes to which the charges are attracted.

Chemical reaction which separates electrons from the cathode (carbon rod)into the electrolyte (+ electrode)

Chemical reaction which separates electrons from the electrolyte into the anode (zinc) (- electrode)

Dry Cell• Carbon Rod in the middle• Zinc container on the outside• Chemical paste in between• Carbon rod and the chemicals react causing

electrons to be transferred from the carbon rod to the paste.

• Another chemical reaction transfers excess electrons from the chemical paste to the zinc outside.

• Middle carbon rod becomes positive charged and the zinc outside becomes negatively charged.

Dry Cell battery

Chemical

Carbon RodZinc

Dry Cell Battery

Electric circuit (P201)• If a wire is connected outside the battery from

the negative charged zinc to the carbon rod in the middle then electrons can and will flow through this wire.

• These moving electrons can then do work as in Figure 12.

• Circuit – the closed path that electrons follow.• Electrons will flow through the wire as long as

the path is closed and as long as the chemical reaction continues.

Water Flow and Current FlowPage 201 Figure 12

+ -

Water Pump

Electron Pump

Water Wheel doing work

Light bulb or motor doing work

Circuit Components

• Source of Voltage Difference• Conductors (wires)• Useful Device• Closed Loop

Resistance (P203)• Resistance is the tendency of a material to

oppose the flow of electrons.• This resistance produces heat• Resistance depends on

– Temperature– Length– Thickness– Type of material

• The unit for resistance is the ohm (Ω)

Electrical VariablesVariable Definition Symbol Unit

Current Number of charges flowing per second

I Ampere (A)

Voltage The force moving the charges

V Volt (V)

Resistance The resistance to the flow of charge

R Ohms (Ω)

Ohm’s Law (P205)I = current (amperes)R = resistance (ohms)

V = voltage difference (volts)

I = VR

V = I x R

R =

VI

Applying Math page 205 #6, #7

Electrical EnergySection 3

Page 207

Electrical Circuits

• To use electrical energy we must use circuits.• The components of a circuit

– Source of voltage (something to separate charge)• Batteries, electrical outlets

– Device to use convert electrical energy to another form of energy

• Lights, motors, heaters, speakers

– Conductors to carry the electrons• Wires

Electric Circuits

ω

ω

V= 6volts

I= 1 ampere

R=

Series Circuit

ω

ω ω

ω V= 6volts

I= .5 amperes

R= 6 ohms

R= 6 ohms

V= 3volts

V= 3volts

Parallel Circuit

ω ω

ω ω

V= 6volts V= 6volts

R= 6 ohms

I= 2 amperes

I= 1 amperesI= 1 amperes

ω

ω ω

ω

ω ω

ω

ω

Series Circuit Parallel Circuit

Types of Circuits

Series vs ParallelSeries Parallel

*Current is the same in all elements of the circuit.*Resistance is the sum of the individual element resistance.*One element burned out, the whole circuit is off.*Each light bulb gives less light than would be given if only one bulb in the circuit.*Used in some cheap Christmas lights

*Voltage drop is the same across all the elements*Resistance is less than any of the element resistance.*If one element burns out, the others are still on.*Each light bulb gives the same light as would be given by 1 bulb in the circuit.*Used in household circuits.*The more elements added in parallel the higher the current.

Fuses and Circuit Breakers P210• Fuses – a protective element in a circuit with a

small piece of metal that melts when it gets too hot and when it melts the circuit is broken. Must replace the fuse when it breaks.

• Circuit Breaker – a protective element in a circuit with a small strip of metal that bends when it gets too hot, the bending “trips” the switch and opens the circuit. The breaker can be reset.

• If a fuse “blows” or a breaker breaks more than once you need to find the cause before you reactivate the circuit.

Electrical Power

• Power is the rate of doing work or the rate of using energy.

• Electrical power = Current x voltage• P= IV• Remember that V= IR so P= I2R• Also remember that I= V/R so P=V2/R• Current in Amperes, Voltage in volts• The unit for Power is Watt usually

1000W or kilowatt or kW

Electrical Power Equation (P211)

• The current in a clothes dryer is 15A when it is plugged into a 240-volt outlet. How much electrical power does the clothes dryer use?

• Given: I=15A, V=240 volts• Asked: P= ? Units watts or kilowatts• Formula: P= IV• P= (15A)(240v) = 3600 watts = 3.6 kW• Do practice problems 1-3

Electrical Energy

• Power x Time = Energy• Power in kiloWatts x time in Hours = Energy• Energy in kiloWatt hours• kWh • Remember kW= 1000 W and Hours = 60 min

Power = EnergyTime

P= I V

Electrical Energy Equation P212

• A microwave oven with a power rating of 1200 W is used for .25 hours. How much electrical energy is used by the microwave?

• Given: P=1200W= 1.2kW, t= .25 hours• Asked: E=? kWh• Formula: E=Pt• E= (1.2kW)(.25h) = 0.30kWh• Do practice problems 1-3

Chapter 7 Schedule• 1/6 Monday- Electrical Charge• 1/7 Tuesday – No School• 1/8 Wednesday- Separating Charge, Charge Flow• 1/9 Thursday- Detecting Charge, Static and Current Electricity• 1/10 Friday- Electric current, Electric Circuits• 1/13 Monday- resistance, voltage, work• 1 /14 Tuesday – Types of Circuits• 1/15 Wednesday - Circuit Lab• 1/16 Thursday Electric Power and Energy• 1/17 Friday Review & Practice Problems• 1/21 Tuesday- NTWS• 1/22 Wednesday- Chapter 7 Review Page 218:1-15, 21-25• 1/23 Thursday– Test Chapter 7

Cost of Electrical EnergyAppliance Hair Dryer Stereo Color TV Light BulbPower 1000W 100W 200W 60WTime Daily 15min 2 hrs 4 hrs 24 hrsDays/mo 30 30 30 30kWh per mo 7.5 6.0 24.0 43.2Cost per kWh $.08 $.08 $.08 $.08Monthly Cost $0.60 $0.48 $1.92 $3.46

Friday 1/7

• Open your book to page 218• Get out chapter review• Look first at question 14• Be prepared to work a problem on the board.

Problem 14

Circuit Current ResistanceA 2.3B 0.6C 0.2D 1.8

Chapter 7 ElectricityAssignments

• NTWS• Chapter Review Page 218: 1-15, 21-25• Page 205 Applying Math: 6,7• Page 211 Practice Problems 1-3• Page 213 Practice Problems 1-3• Practice Worksheet• Circuit Lab

Problem 14

Resistance

Curr

ent