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ELECTRICITY
Chapter 17
17.1 ELECTRIC CHARGE & FORCE
Essential Questions:What are the different kinds of electric charge?
How do materials become charged when rubbed together?
What force is responsible for most everyday forces?
Electric Charge Video Clip
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If your car gets struck by lightning in a thunderstorm, will you be safe. Why?
Answer: You will be safe because your cars metal chassis acts like a Faraday Cage. The charged particles travel around the outside of the car and into the ground.
Video
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Copyright © 2010 Ryan P. Murphy
What would life be like without it electricity?
Copyright © 2010 Ryan P. Murphy
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Much different than it is for most of us.
ELECTRIC CHARGE
Electricity is a variety of independent science concepts all with one single name.
Copyright © 2010 Ryan P. Murphy
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Electricity is related to charges, and both electrons (-) and protons (+) carry a charge.
Electric charge is an electrical property of matter.
All matter is made of atoms that contain electrons, neutrons, and protons
An object can have a negative charge, positive charge or no charge.
Copyright © 2010 Ryan P. Murphy
ELECTRIC CHARGE
Charge of proton Positive
Charge of electron Negative
Charge of neutron NONE
Atoms have no charge because the charges
of the protons and electrons cancel each
other out.
Atoms become charged by gaining or
losing electrons
ELECTRIC CHARGE
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Lightning is a big spark that occurs when electrons move from one place to another very quickly because of the unequal distribution of electrons.
Copyright © 2010 Ryan P. Murphy
Charges in objects can produce a force between the objects
Objects are forced together or attracted when their charges are different
“Opposite charges attract”
Same electric charges they push apart
“Like charges repel”
Like charges REPEL Opposites attract!
ELECTRIC CHARGE
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Like Charges Repel
Opposite Charges Attract
Which one is right and which is wrong?
Answer: They are both wrong.
Copyright © 2010 Ryan P. Murphy
Answer: Now they are both right.
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ELECTRIC CHARGE
Electrons can be moved around
When different materials are rubbed together, electrons can be transferred from one material to the other.
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The SI Unit of electric charge is the coulomb (C).
ELECTRIC CHARGE
STATIC ELECTRICITY
Static electricity- buildup of excess negative charge on an object Excess electrons on an object
Very short electric discharge
Static electricity is electricity at rest
Friction can cause it
Law of conservation of charge- charge may be transferred from object to object, but it cannot be created or destroyed
Opposite Charges attract, and like charge repel
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STATIC ELECTRICITYWhy you end up with a static shock after
walking across the carpet: Atoms in the carpet hold their electrons more
loosely than atoms in your shoes
Shoes gain electrons from the carpet,
becoming negatively charged
Carpet loses electrons & becomes positively
charged
Shock occurs when electrons are suddenly
transferred from one object to another- this
appears as a spark
TRANSFER OF ELECTRIC CHARGE
Different materials are rubbed together
Electrons can be transferred from one material
to the other
The direction depends on the material
Conduction:
Electrons move more easily through
conductors, like metals
Metals conduct well because: atoms in
metals have electrons that move easily
through the material
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Electrical conductors, such as copper wiring, allow electrons to move freely.
TRANSFER OF ELECTRIC CHARGE
Insulation:
Insulator- a material that doesn’t allow electrons to move
through it easily
Occurs because electrons are held tightly to the atoms in
insulating materials – like wood, plastic, glass
Charging by contact- the process of transferring charge by
touching or rubbing two objects together
Charging by friction- rearrangement of electrons on a neutral
object by a nearby charged object
TRANSFER OF ELECTRIC CHARGE
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We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid.
The water in the air helps electrons move off you more quickly, so you can not build up as big a static charge.
ELECTROSCOPE
Used to detect static
electricity
Electrons are
transferred to the
metal ball and down
to the foil
Foil becomes negative
and repels
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ELECTROSCOPE
No Charge-
leaves hang
straight down
INDUCTION
Rod with negative
charge
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INDUCTION
Rod with negative
charge
Pushes electrons in
electroscope down
INDUCTION
Rod with negative
charge
Pushes electrons in
electroscope down
Extra negative charge
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INDUCTION
Rod with negative
charge
Pushes electrons in
electroscope down
Extra negative charge
Leaves move apart
Induction
Remove rod
everything returns
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Conduction
Rod with negative
charge
Conduction
Rod with negative
charge
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Conduction
Rod with negative
charge
Transfers electrons
Conduction
Rod with negative
charge
Transfers electrons
Extra negative charge
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Conduction
Rod with negative
charge
Transfers electrons
Extra negative charge
Moves leaves apart.
Conduction
Remove rod leaves
stay apart.
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Electric force: the force of attraction or repulsion on a charged particle that is due to an electric field.
Electric Force
Electric Force
• Electric force depends on charge and distance.
• Electric force acts through a field
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• Electric field: the space around a charged object in which another charged object experiences an electric force.• Electric fields surround charged objects.• Any charged object that enters a region with an electric field experiences an electric force.
• Electric field lines never cross each other
Electric Force
Electric fields
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SECTION 2: CURRENT
VOLTAGE & CURRENT
An electric charge has electrical potential energy that depends on its position in an electric field.
A negative charge that is close to another negative charge has the potential to move away.
The 2 negative charges repel each other as a result of their electric fields being squished together.
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ELECTRICAL POTENTIAL ENERGY
The electrical potential energy between two negative charges
decreases as the distance between them increases.
Voltage: difference in energy per unit charge as the charge moves between two points in the path of a circuit
Higher voltage, the more work the electrons can do.
Voltage can vary
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VOLTAGE AND CURRENT
It is more practical to consider potential difference than electrical potential energy.
Potential difference is the voltage difference in potential between two points in a circuit.Also called voltage
This change occurs as a charge moves from one place to another in an electric field.
SI Unit for Potential difference is Volts (V)
Example: a AA battery has a potential difference of 1.5 V between the two ends (the positive and negative terminals).
There is a Voltage across the terminals of a
battery.
The potential difference or voltage across the two ends, or terminals, of a battery ranges from about 1.5 V for a small battery to about 12 V for a car battery.
VOLTAGE AND CURRENT
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Most batteries are Electrochemical Cells
(or groups of connected cells) that convert
chemical energy into electrical energy.
VOLTAGE AND CURRENT
Electrochemical cells contain
• Electrolyte- a solution that conducts electricity,
• two electrodes- each a different conducting
material.
VOLTAGE AND CURRENT
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THE CELL
WHAT IS A CURRENT?
Current: the rate at which charge passes a given point
A flow of electrons, or individual negative charges.
Made by electrons moving in a wire
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ELECTRIC CURRENT
Electric current- The flow of electrons through a wire or any conductor.
Measured in units of Amperes (A)
Different from static electricity because it lasts longer
Charges flow from High voltage to Low voltage
Voltage difference- push that causes charges to move
For charges to flow, the wire must always be connected in a closed path, or circuit
Ampere: A measure of how much current moves through a wire in one second.
The larger the size of wire, the greater the ampere capacity.
Copyright © 2010 Ryan P. Murphy
ELECTRIC CURRENT
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Where do your see these plugs?Why are they larger?
Copyright © 2010 Ryan P. Murphy
Answer: The Plug to a dryer or stove is much thicker than a standard outlet to account for extra amps.
There are two main kinds of electric current, Direct current (DC)
Alternating current (AC).
Explains how current gets moved
ELECTRIC CURRENT
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ELECTRIC CURRENT
DIRECT CURRENT
(DC) Direct current is a flow of charge always in one direction.
From batteries
ALTERNATING CURRENT(AC) -Alternating current is a flow of charge back and forth, changing its direction many times in one second. (Plugs and outlets / household)
Advantages of AC
Voltage can be raised or lowered
More efficient over long distances
From Generators
Used in your home
Transformers change AC to DC
ELECTRIC CURRENT
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Is this (AC) Alternating Current, or (DC) Direct Current?
AC
DC
AC
AC
DC
AC
?
ELECTRICAL RESISTANCE
Resistance- the tendency for a material to oppose the flow of electrons, changing electrical energy into Thermal energy and light
Opposition to the flow of charge
All materials have some electrical resistance.
Making wires thinner, longer, or hotter increases the resistance
As resistance increases… current decreases!!!!
Measured in ohms: omega Ω
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OHM’S LAW
Ohm’s law states that the current in a circuit is equal to the voltage divided by the resistance
I = VR
The relationship among current, voltage, and resistance.
V
I R
I = Current
V = Voltage
R = Resistance
Amperes (A)
Volts ( V)
Ohm (Ω)
Measured as
PRACTICE PROBLEMS: RESISTANCE
1. A car has a 12 volt system. The headlights are on a 10 amps circuit. How much resistance do they have?
2. Your house uses 120 volts. What amount of current would flow through a 20 ohm resistor?
V= 12 volts
I= 10 amps
R= ?
R= V/I R= 12 volt/10 amp R= 1.2 Ohm (Ω)
V= 120 volts
I= ?
R= 20 ohm
I= V/R I= 120 volts/20 ohmI= 6 amps
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3. A refrigerator’s circuit has a current equal to 0.647 A in it when the voltage across the circuit equals 116 V. What is the resistance of the circuit?
4. The resistance of a wire in a hair dryer is 7.7 Ω . If the current through the wire equals 15.6 A, what is the voltage across the terminals of the hair dryer?
PRACTICE PROBLEMS: RESISTANCE
V= 116 volts
I= 0.647 amps
R= ?
R= V/I R= 116 volts/0.647 amps R= 179 ohms
V= ?
I= 15.6 amps
R= 7.7 Ω
V= I x R V= 15.6 amps x 7.7 Ω V= 120 Volts
Conductors have low resistances.
Insulators have high resistances.
Semiconductors conduct under certain conditions. materials that have electrical properties between those of insulators and conductors
Some materials can become superconductors.
Some metals and compounds have zero resistance when their temperature falls below the critical temperature.
Once a current is established in a superconductor, the current continues even if the applied voltage is removed.
ELECTRICAL RESISTANCE
Superconductor clip
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SECTION 3: CIRCUITS
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CIRCUITS
For current to flow there must be a complete loop
Electric circuit: complete, a closed path through which electrons travel.
Electrons flow from negative to positive terminal
Work is done if there is a resistance in the wire.
Closed Open
Open Circuit- if any part of the circuit is disconnected, no current flows
Example- old style Christmas lights
Closed Circuit- A closed-loop path for electrons to flow through, creating a current.
CIRCUITS
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Resistance is supplied by a resistor.
A resistor is a device that uses electric energy to do work.
A wire connected from the resistor to the positive terminal completes the circuit.
An open switch breaks the circuit.
CIRCUITS
DRAWING A SCHEMATIC DIAGRAM
A schematic diagram is a model of an electric circuit with standard symbols for the electrical devices.
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SCHEMATIC DIAGRAM SYMBOLS
TWO TYPES OF CIRCUITS
Series circuits: A circuit with only one path.
All the resistors in a series circuit lie along a single path.
The amount of current in a series circuit is the same at all parts of the circuit.
Resistance in the circuit changes if resistors are added or taken away.
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Series Circuits
Series Circuits
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Series Circuits
Series Circuits
Break in the wire turns off all the lights
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Parallel circuits: The electrons in a parallel circuit can travel through more than one path, each path is separate.
If there’s a break in one path in the circuit, electrons can still flow through the other paths and maintain a complete circuit.
Parallel circuits in your home allow each light or appliance to use the amount of current it needs to work.
A parallel circuit prevents all the lights or appliances from shutting off when one of them stops working.
TWO TYPES OF CIRCUITS
Parallel Circuit
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Parallel Circuit
Series vs. Parellel Circuit Video
ELECTRIC POWER
Electric Power is the rate at which electrical energy is used in a circuit.When a charge moves in a circuit, it ______ energy.
This ________ is transformed into useful work (like turning a motor) and is lost as heat in the circuit.
losesenergy
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Power: The rate at which electricity does work or provides energy The amount of electric power a device uses to do work is determined by its resistance.
P = V x I
(P) power = (V) voltage x (I) current in the circuit.
Electrical power is expressed in watts (W)
P (watts) = V (volts) x I (amperes)
ELECTRIC POWER
P
V I
PRACTICE PROBLEMS: ELECTRICAL POWER
1. An electric mixer draws 200.0 W of power. If the mixer is plugged into an outlet across a voltage of 115 V, what current is in the mixer’s circuit?
2. A nightlight uses 4.00 W of power when plugged into an outlet. Assume that the only resistance in the circuit is provided by the light bulb’s filament. The current in the circuit is 3.40 x 10-2 A. What is the voltage across the filament?
P= 200.0 Watts
V=115 Volts
I=?
I=P/V I=200.0 Watts/115 Volts I= 1.7 amps
P= 4.00 Watts
V=?I= 3.40 x 10-2 A
V=P/I V=4.00 Watts/ 3.40 x 10-2 A
V=117 volts
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3. A current of 5.83 A is used to produce the microwave radiation in a microwave oven. If the voltage across the oven is 120 V, how much power does the use?
4. A refrigerators uses a current of 0.62 A and a voltage of 116 V. How much power does the refrigerator use?
PRACTICE PROBLEMS: ELECTRICAL POWER
P= ?
V=120 Volts
I= 5.83 A
P= V x I P= 120 V x 5.83 A P= 699.6 watts
P= ?
V=116 Volts
I= 0.62 A
P= V x I P= 116 V x 0.62 A P= 71.92 watts
Fuses and circuit breakers protect against overloaded circuits.Fuses- contain a small piece of metal that melts if the current becomes too high, opening the circuit and stopping the flow of current
ELECTRIC SAFETY
Circuit breakers- contain a small piece of metal that bends when it gets hot, opening the circuit and stopping the currentCircuit breakers are often used in place of fuses.
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Broken wires or water can cause electric appliances to short-circuit.
A short circuit occurs when electricity takes a short path and bypasses the resistors in the circuit.
Because of this the resistance of the circuit is less and the circuit wire increases.
The increased current can produce enough heat to melt wires and start a fire, or cause serious electric shock.
Short Circuits
ELECTRIC SAFETY
Don’t over connect outlets because they could short circuit.
Copyright © 2010 Ryan P. Murphy
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CLASSWORK1. Identify the components, and the number of
each in this diagram.
2. Draw a schematic diagram with 4 lights in parallel.
3. Draw a schematic diagram of 2 lights in series.
4. Draw a schematic diagram with 2 lights in parallel, and one in series.