Transcript Chapter 7 Electricity

Chapter 7 Electricity
Syllabus Posted on Web Site due 1/13/2014
Page 192
Section 7:1
Electrical 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
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–
–
–
–
–
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
Electricity
• 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.
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.
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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 2
Electrical 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 analogy
Figure 11 Page 200
High
Pressure
High
Voltage
Low
Pressure
----
++
++
-
Low
Voltage
Water Flow and Current Flow
Page 201 Figure 12
Light bulb or motor
doing work
Water Wheel
doing work
+
-
Electron Pump
Water Pump
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)
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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
Zinc
Carbon Rod
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 Flow
Page 201 Figure 12
Light bulb or motor
doing work
Water Wheel
doing work
+
-
Electron Pump
Water Pump
Circuit Components
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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 Variables
Variable
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= V
R
V=IxR
R=V
I
Applying Math page 205 #6, #7
Electrical Energy
Section 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
I= .5 amperes
ω
ω
V= 6volts
ω
ω
R= 6 ohms
V= 3volts
R= 6 ohms
V= 3volts
Parallel Circuit
I= 2 amperes
ω
ω
I= 1 amperes
R= 6 ohms
ω
V= 6volts
ω
V= 6volts
I= 1 amperes
Types of Circuits
Series Circuit
Parallel Circuit
ω
ω
ω
ω
ω
ω
ω
ω
Series vs Parallel
Series
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 = Energy
Time
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P= I V
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
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
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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 Energy
Appliance
Power
Time Daily
Days/mo
kWh per mo
Cost per kWh
Monthly Cost
Hair Dryer
1000W
15min
30
7.5
$.08
$0.60
Stereo
100W
2 hrs
30
6.0
$.08
$0.48
Color TV
200W
4 hrs
30
24.0
$.08
$1.92
Light Bulb
60W
24 hrs
30
43.2
$.08
$3.46
Friday 1/7
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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
A
B
C
D
Current Resistance
2.3
0.6
0.2
1.8
Chapter 7 Electricity
Assignments
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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
Current
Problem 14
Resistance