Transcript Electricity and Magnetism Pt 1

12/5 Bell Work
How can it be that a man was once married to the
sister of his widow?
• Hint: What is a widow? Who did he marry first?
Widow’s walk
12/5 Schedule
• Notes Ch 17.1 “Static Electricity”
• Work
o Coulomb’s Law Worksheet due WEDNESDAY
o Finish test
Barbies, Bratz, GI Joe? 5
pts each, can bring in 3
Assignments:
1. Coulomb’s Law Worksheet due WEDNESDAY
Electricity and
Magnetism
Ch 17-18 Objective:
Describe how electricity and magnets do
work.
Ch 17.1 “Charge & Force”
Objectives:
• Identify particles causing electric charge.
• Describe and diagram how objects transfer
charge.
Static Electricity Intro
1. Fabric softeners are commonly used because
they eliminate static cling. Why do clothes in the
dryer get static cling?
2. Why can walking across a carpeted room be a
shocking experience?
Electric Charge
Kinds of electric charge
• Negative
• Positive
• Neutral
• Electric charge: an electrical property of matter
that creates electric and magnetic forces and
interactions
Electric Charge
Electric charge depends on the imbalance of protons
(+) and electrons (-).
Features of charge
• Neither created or destroyed.
• Same charges repel, and opposites attract.
• Equal + and – charges = 0
Electric Charge
• The SI unit of electric charge is the coulomb, C.
o A proton has a charge of +1.6  10–19 C.
o An electron has a charge of –1.6  10–19 C.
o The amount of electric charge on an object depends on
the number of protons and electrons.
• The net electric charge is always a multiple of 1.6  10–19
C.
Transfer of Charge
Charges can move within uncharged objects.
o Charges in a neutral conductor can move without
changing the overall charge of the object.
o Areas missing those charges now have an induced charge.
Induced charge
Transfer of Charge
Charge by contact.
o Contact charge: touching a charged object to a
neutral object
Charged by friction
o One material gains electrons (-), and the other loses
electrons (now +).
o Ex: static cling, walk in socks on carpet
Girl touching a van de
graaff generator
Electric Force
Electric force is caused by…
〉attraction/repulsion of charged particles
Electric force is responsible for…
〉friction
〉lightning
〉shocks
〉atomic bonds
Electric Force
Electric force depends on charge and distance.
o The force is proportional to the product of the charges.
o Force is inversely proportional to the square of the distance.
Coulomb’s Law
Q in Coulombs
d in meters
12/6 Bell Work
If there are nine players on a baseball team, and
three strikes make an out and four balls make a walk,
how many outs are there in an inning?
12/6 Schedule
• Notes Ch 17.1 “Static Electricity”
• Static Electricity Lab, finish procedure today
• Work
o Coulomb’s Law Worksheet due WEDNESDAY
o Finish test
Barbies, Bratz, GI Joe? 5
pts each, can bring in 3
Assignments:
1. Coulomb’s Law Worksheet due WEDNESDAY
Electric Force
Electric force acts through a field.
o electric field: the space around a charged object in which another
charged object experiences an electric force
Show electric fields by drawing field lines.
o Electric field lines point in the direction of the electric force on a positive
charge.
The electric field lines
around a positive charge
point outward.
The electric field lines
around a negative charge
point inward.
Electric Force
• Electric field lines never cross.
• Lines near two like charges point away from each other, and show that
the charges repel.
• Lines show direction and relative strength of a given charge.
– More lines are drawn for greater charges to indicate greater force.
Two positive charges repel each other.
The positive charge is twice as large as the
negative charge.
12/7 Bell Work
What common substance is more dense in its liquid
form than in its solid?
• HINT: What FLOATS when it freezes?
12/7 Schedule
• Notes Ch 17.2 “Current”
• Static Electricity Lab due TODAY
• Work
o Finish waves test
o Coulomb’s Law Worksheet due WEDNESDAY
Barbies, Bratz, GI Joe? 5
pts each, can bring in 3
Assignments:
1. Coulomb’s Law Worksheet due WEDNESDAY
Ch 17.2 Current
Objectives:
1. Compare/contrast electrical and
gravitational potential energy.
2. Explain what causes electric resistance.
Batteries and Appliances
1. Name five devices that use batteries.
2. Give reasons why copper is normally used to
wire a home for electricity.
Voltage and Current
Relating electric and gravitational potential
energy
〉Depend on position
〉Move from high to low energy
• electrical potential energy: the ability to move an
electric charge from one point to another
Electrical Potential
Energy
The electrical potential energy between two negative charges
decreases as the distance between them increases.
Voltage and Current
• Potential/voltage difference in circuit
o Force increases as similar charges move closer to each other.
o SI unit is the volt,
o V, = one joule per coulomb
(1 J/C).
Voltage and Current
Voltage moves across the terminals of a battery.
o cell: a device that produces an current by converting chemical or
radiant energy into electrical energy
o One terminal is positive, and the other is negative.
o
Voltage and Current
• Voltage sets charges in motion.
• Current is the rate of charge movement.
o electric current: the rate at which charges pass through a given point
o The SI unit of current is the ampere, A.
• 1 amp = 1 C/s
Voltage and Current
• Direct current: the charges always move from one
terminal to the other in the same direction.
o EX: battery
• The direction of current is opposite to the direction
that electrons move.
Electrical Resistance
Resistance causes
〉internal friction
〉slows the movement of charges through
a material.
Electrical Resistance
• Resistance can be calculated if current and
voltage are known.
o Resistance indicates how much the motion of charges
within it is resisted because of collisions of electrons with
atoms.
o Ohms’ law:
voltage
resistance =
current
o The SI unit of resistance is the ohm (Ω)
• 1 Ω = 1 V/A
V
R=
I
Math Skills
Resistance
Typical car headlights are powered by a 12 V
battery. What is the resistance of the headlights if
they draw 3.0 A of current when turned on?
1. List the given and unknown values.
Given: current, I = 3.0 A
voltage, V = 12 V
Unknown: resistance, R = ? Ω
Math Skills
2. Write the equation for resistance.
voltage
resistance =
current
3.
V
R=
I
Insert the known values into the equation, and solve.
V
12 V
R=
=
I
3.0 A
R = 4.0 
Electrical Resistance
• Ranking Resistance
• Superconductors = 0 when below a critical temp
• Conductors = low
• Semiconductors = certain conditions.
• Insulators = high
o Once a current is established in a superconductor, the
current continues even if the applied voltage is removed.
Superconducting magnet
Stop Notes
Ch 17.3 “Circuits”
Objectives:
〉Describe the two ways that devices can be
connected in a circuit.
〉ID what happens to the energy that
charges have in a circuit.
Electricity Brainstorm
1. Inexpensive electrical power is essential. List at least six
electrical devices that you have used today.
2. In some strings of Christmas lights, none of the lights work
if one light is burned out. What is a possible explanation for
this?
What Are Circuits?
Electric Circuits
• The voltage source (battery, outlet, etc.) is always
part of the conducting path of a closed circuit.
• Without a complete path and voltage source, there
is no charge flow and no current.
o This is an open circuit.
What Are Circuits?,
continued
Switches
0 open and close a circuit
0 interrupt the flow of charges
0 allow control
What Are Circuits?,
continued
Schematic diagrams are used to represent circuits.
0 schematic diagram: a diagram of a circuit that uses lines to represent
wires and standard symbols to represent components
o Can be used for all electronic devices.
What Are Circuits?
Diagram two light bulbs connected in parallel to 2
batteries…
• Draw as picture first.
• Change to diagram.
• + terminal of battery is shown with
a longer line.
What Are Circuits?
Diagram the pickle battery…
Series and Parallel
Circuits
2 ways to Connect Circuits
• series circuit: parts are joined one after another so
that the current in each part is the same
• parallel circuit: parts are joined in branches so the
potential difference across each part is the same
Series and Parallel
Circuits
Series circuits have a single path.
0 Current is the same.
0 Resistances and voltages may be different.
0 If one element along the path is removed, the
circuit will not work.
Ibattery = I1 = I2 = I3 = ...
Write these in notes!
ΔVbattery = ΔV1 + ΔV2 + ΔV3 + ...
Rtotal = R1 + R2 + R3 + ...
Series and Parallel
Circuits
Parallel circuits have multiple paths.
o The voltage across each device is the same.
o The current does not have to be the same.
• The sum of the currents in all of the devices equals the total current.
o A break in any one path does not interrupt the other paths.
Vbattery = V1 = V2 = V3 = ...
Write these in notes!
Itotal = I1 + I2 + I3 + ...
1/Rtotal = (1/R1) + (1/R2) + (1/R3) + ...
Series and Parallel
When light bulbs are
connected in series, charges
must pass through both
light bulbs to complete the
circuit.
When light bulbs are connected
in parallel, charges have more
than one path to follow. The
circuit can be complete even if
one light bulb burns out.
12/7 Bell Work
What common substance is more dense in its liquid
form than in its solid?
• HINT: What FLOATS when it freezes?
12/7 Schedule
• Finish Ch 17.3 “Circuits”
• Ohm’s Law and Power
o V = IR
o P= IV
• Work Time
o Coulomb’s Law Worksheet LATE
o Ohm’s Law worksheet due TODAY
o Ohm’s Law and Power due WED
Assignments:
1. Coulomb’s Law Worksheet LATE
2. Ohm’s Law Practice due TODAY
3. Ohm’s and Power due WED
Electric Power and
Energy
Energy and Charges
〉Some energy is transformed into useful
work, (turn motor, play music, etc.)
〉 Some lost heat.
The SI unit for power is the watt (W).
• 1W=1A1V
power = current ´ voltage
P = IV
Electrical Energy,
continued
• Combine P=IV with V = IR, you can calculate the
power lost by a resistor.
2
V
P = I 2R =
R
Electric companies measure energy in kilowatt-hours.
o One kilowatt-hour is the energy delivered in 1 h at the rate of 1 kW.
• 1 kW•h = 3.6  106 J
Math Skills
Electric Power
When a hair dryer is plugged into a 120 V outlet, the
hair dryer has a 9.1 A current in it. What is the hair
dryer’s power rating?
1.List the given and unknown values.
Given: voltage, V = 120 V
current, I = 9.1 A
Unknown: electric power, P = ? W
Math Skills, continued
2. Write the equation for electric power.
power = current × voltage
P = IV
3. Insert the known values into the equation, and
solve.
P = (9.1 A)(120 V)
P = 1.1 × 103 W
Fuses and Circuit
Breakers
Overloaded (current) circuits
〉Dangerous…shocks or fire
〉Might be caused by missing insulation
creating a short circuit.
Fuses and Circuit Breakers
Overloaded (current) circuits
2 safety measures
〉 fuse: an electrical device that contains a metal strip that melts when
current in the circuit becomes too great
〉 circuit breaker: a switch that opens a circuit automatically when the
current exceeds a certain value
circuit breaker
car fuse
12/8 Bell Work
Two farmers were discussing their farms when Clem
said to John, “My property is a mile square.” John
looked at Clem and said, “As a matter of fact, mine is
a square mile.”
“Then there is no difference,” replied Clem.
Is Clem’s last statement correct? Why?
John
Clem
12/7 Schedule
• Review Circuits Equations
• Ohm’s Law and Power
o V = IR
o P= IV
• Work Time
o Coulomb’s Law Worksheet LATE
o Ohm’s Law worksheet LATE
o Ohm’s Law and Power due WED
Assignments:
1. Coulomb’s Law Worksheet LATE
2. Ohm’s Law Practice due TODAY
3. Ohm’s and Power due WED
Review Circuits and
Diagrams
• Series
Ibattery = I1 = I2 = I3 = ...
ΔVbattery = ΔV1 + ΔV2 + ΔV3 + ...
Rtotal = R1 + R2 + R3 + ...
• Parallel
Vbattery = V1 = V2 = V3 = ...
Itotal = I1 + I2 + I3 + ...
1/Rtotal = (1/R1) + (1/R2) + (1/R3) + ...
• Calculate the total resistance for 650, 350, and 1000
ohm resistors connected in series.
• A 10, 20, and 100 ohm resistor are connected in
parallel. Calculate the total resistance.