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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 RepelOpposite 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 togetherElectrons can be transferred from one material to the other

The direction depends on the material

Conduction:

Electrons move more easily through conductors, like metalsMetals 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.

ELECTROSCOPEUsed to detect static electricityElectrons are transferred to the metal ball and down to the foilFoil 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 CURRENTElectric 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

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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.