physical science chapter 7 (new book)

7/29/2019 Physical Science Chapter 7 (New Book)

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Physical ScienceCh. 7: Electricity
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Yes, we all know what electricity is, but

exactly what is it?

-where does it come from

-can you see it

-how is it created


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Electricity

Electricity is a force created by a difference incharges (+ & -) due to gained or lostelectrons. (an electron is a negatively

charged particle.)

When electricity is flowing between twopoints, this is actually electrons moving frompoint A to point B. This is called an electricalcurrent.


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In order for these electrons to flow,

however, there must be a difference in

charges (# of electrons built up) betweenthe 2 points. Just like heat flow needs a

difference in temperatures.

Electricity always flows from a locationwith a negative charge to a location with a

positive charge. (like charges repel, opposites attract)

- +


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Think of a battery, the

top has a + charge andthe bottom has a

negative charge. So

when they areconnected, electrons

flow from the bottom to

the top.


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Static Electricity

You may have noticed that if youwalk across the carpet in socks,and then touch your cats nose, it

will shock him/her (it). This is because, you picked up

some free electrons walking over

the carpet (kind of like dust), andtherefore gained a negativecharge.


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Touching kittys nose allows you to dissipate that

charge (send the extra electrons to an area with

fewer electrons, negative to positive) The shock will continue until both surfaces are at

the same charge (neutralized)


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Static electricity is a temporary 1 time

charge produced by an excess of

electrons.

Electrons in the air will attach to moisture

in the air. Thats why very dry air (like in

the winter, or in the dryer) may hold alarge number of free electrons, because

theres not much moisture for them to

attach to.

i.e. Theres more static in the air in the winter

because there are more free electrons just

floating around


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Van der Graff generators basically induce a strong

electrical field (can be either positive or negative depending on

the setup) into a hollow metal sphere. This field can

the release electrons into the surrounding air when

the voltage becomes great enough.


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Voltage

The difference in charges between 2points is called the voltage.

2 points can both have a tremendouscharge, but if the charge is the same then

there will be no flow of electrons between

them.

- - - - - -

- - - - - -

- - - - - -


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The number of electrons actually flowing

through a conductor is called the current,

and is measured in amperes (amps).

1 amp = 6.25 x 1018 electrons per sec.

(6,250,000,000,000,000,000!)


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So lets say that using a

voltmeter you determinethat there are no electrons

flowing from point A to point

B through an extensioncord connecting the two.

Does that mean it is safe

for you to touch either ofthe 2 points? Hmmm


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No! There may be no electrons flowing

because the charges are the same.

However, your charge may be much

different, which would allow the electricity

to flow into you.


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Conductors and Insulators

Although an electric current can

occasionally jump from 1 point to another,

it normally travels through a conductor. A

conductor is any material which allowselectrons (electricity) to flow easily through

it.

Metals in general are good conductors of

electricity.


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Insulators are materials which prevent the flow of

electrons (electricity).

Some good insulators include rubber, glass,

wood, and plastic.

Any type of electric cord usually consists of ametal conducting wire surrounded by a layer of

rubber or plastic insulation.


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Resistors

Materials called resistors can slow the flow ofelectricity, without stopping it all together.

Almost all materials, even good conductors,provide at least a small amount of resistance toan electric current.

Resistance is measured in ohms.

W


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Direct Current (DC)

In most electricalcircuits, the current will

flow in only 1 direction.

This is called directcurrent or DC.

Examples of direct

current includebatteries, lightning, and

static electricity.
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Lightning

Lightning is a form of direct current (DC)

produced by static electricity in clouds.

The static is formed when air molecules

move past each other (just like clothes in a

dryer).


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The negative charges group at thebottom of the cloud and transfer

electrons to the ground, which has takenon a positive charge.

Why do you think the ground has taken on apositive charge?


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Does lightning always travel from cloud to

ground? Hmmm


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Alternating Current (AC)

An alternating current will send a

flow of electrons in 1 direction

through a circuit, and then it will

reverse the flow in the otherdirection.

Household outlets are an example

of AC current. They reverse thedirection of the current about 120

times per second.


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What would

happen if you stuck

one finger in oneside of an outlet?

One finger in eachside?


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Transformers

The AC which comes from your household

outlets usually has a voltage of about 120V.

The voltage of the power lines outside,

however, is much higher. Before entering

your house, the electricity from the power

lines must pass through a transformer. This

is a device which can increase or decreasethe voltage which exists between 2 points.


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Transformers outside your house make the voltage

in your outlets safe for household appliances.

Occasionally a lightning strike make take out atransformer, and send a power surge into your

house.
http://en.wikipedia.org/wiki/Image:PoleMountTransformer02.jpghttp://en.wikipedia.org/wiki/Image:Polemount-singlephase-closeup.jpg


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Step-up & step-down transformers

Transformers can also increase the

voltage between 2 points.

An amplifier and a stun gun are examples

of step-up transformers.
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Electric Generators

The electricity supplied toyour home is produced by

an electric generator. This

is a device which changesmechanical (kinetic) energy

into electricity.

Most electric power which isgenerated in Missouri is

either from hydroelectric or

fossil fuel power plants.
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Electric Motors

Electric motors are devices which convertelectricity to mechanical energy.

Most household appliance are examples of

electric motors.Ex.: washing machine, fan, refrigerator,

VCR, dishwasher, hair dryer, etc.


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Electric cars have engines which run on

electrical energy rather than chemical potential

energy (gasoline). These cars are more efficient, cleaner to

operate, and decrease our dependency on fossil

fuels. However, the engines are not as powerful

and the cars have a limited range.


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Batteries Electric motors are usually specified to run on

either alternating current or direct current

(produced by batteries).

A battery is a device which produces a

continual direct current, through a chemicalreaction.


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Batteries can be composed of a liquid electrolyte

(a liquid which will conduct an electric current)

(wet cell) or a paste electrolyte (dry cell).Whether liquid or paste, the electrolytes are

usually acidic.

Car and boat batteries are examples of wet cells,

flashlight batteries are examples of dry cells.
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A chemical reaction insidethe battery pulls electronsaway from 1 of the

terminals and gives themto the other. This causes1 terminal to be negative(-) and the other to be

positive (+).

When these terminals areconnected through a device,

they produce a flow ofelectrons from negative topositive, which power thedevice.


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Electric Circuits

A circuit is a pathway along which anelectric current can travel, en route to a

device.

Locating a device along that pathway willallow it to be powered by the electric

current.

There are 2 different types of circuits:series and parallel.


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Series Circuit

A series circuit is onewhich has only 1 pathway

for electricity to flow. All

devices are located along

this single pathway, andany break in the circuit will

stop the flow of electricity.

Ex.: old type Christmas

tree lights


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Parallel Circuits

A parallel circuit has

multiple paths which theelectricity can travel down,

with different devices

along the different paths. If one pathway is open or

broken, the others can still

carry electricity.

Ex.: newer Christmas tree

lights, household circuits


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Schematic Diagrams

A schematic diagram

uses symbols to

show electrical

circuits and wiring.


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A short circuitoccurs when the

current stops short

of the intended

device, usually

because of the

circuit being

completed prior toreaching the device.


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A circuit breaker is a device which prevents toomuch electricity from flowing through a circuit.

Breakers can be reset usually with the flip of aswitch.


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Fuses do the same thing as a breaker, but they

use a small wire connector which will burn out if

the flow of electricity is too high. Fuses usuallyhave to be replaced after they burn out. Fuses

are often found in electric appliances also.


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Electromagnets

An electromagnet is a temporarymagnet made by passing an

electric current through a wire

coiled around a metal core.

Electromagnets have 2 advantages

over normal magnets:

1. They can be turned on

and off.2. Their strength can vary

based on the amount of

current flowing.


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Examples of electromagnets include

cranes in scrap yards, telegraphs, and

certain types of doorbells.


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History of Electricity

600 B.C.-Thales discoveredstatic electricity.

1600 - William Gilbert names

the force electricity Mid 1700's - Ben Franklin shows

that lightning is made of

electricity, and does numerousexperiments


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1800 - Volta makesfirst battery

1878 - T. Edison createselectric light bulb


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1943 - First computer

(ENIAC) created

1942 - Fermi produces thefirst fission reaction

Th V D G ff t i l d d


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The Van De Graff generator in class produced

around 15,000 v of electricity, while a typical

household outlet produces only 120 v.Therefore, youre safer sticking your finger in

an outlet.

Right????? Explain.

An automated external defibrillator (AED),


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An automated external defibrillator (AED),

which is similar to the manual defibrillator

often seen used by EMTs, is a portable

device used to diagnose and treat victims ofcardiac arrest.

How do you think this device works?

L i i G l i


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Luigi Galvani was

cooking frog legs in

an iron skillet. Henoticed that when he

picked one up with

copper tongs to flip it

over, sometimes itwould kick as if it

were still alive. Why

do you think thisoccurred?


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Draw a schematic diagram of a circuit witha power source, 1 switch, 2 individual

devices along a series circuit, and 3

devices on separate branches of a parallel

circuit.


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The step-down transformer shown below has 8

coils of wire on the primary side and 4 coils

going out on the secondary side. If thistransformer is hooked up to a typical household

outlet, how many volts will be produced?

Wh d thi k


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Why do you think

most gas pump

handles arecoated in either

rubber or plastic?

What purpose do you think the yellow connectors


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What purpose do you think the yellow connectors

on the electric fence shown below serve?


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The device shown below is an electronic

body fat tester. The subject holds the tester

by the handles, and a weak electric current ispassed through their body. How do you think

this is able to determine their percentage of

body fat?

Although both


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g

dangerous, a lightning

strike near the beach in

Miami may be morepotentially hazardous

than the same strike at

Truman Lake. Why do

you think this is so?

A car battery (wet cell) works similar to the


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A car battery (wet cell) works similar to the

voltaic piles which we built in class.

Do you think it would be possible to build avoltaic pile out of our

lab materials big

enough to havethe same power

as a car battery?

Why or why not.

What is the object shown and how do


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What is the object shown, and how do

you think it works?

Sh b l i d f lt i it i t t


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Shown below is a ground fault circuit interrupter

(GFCI) outlet. Whats special about it, how

does it work, and where might one be found?


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I th i t b l h t i th h f


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In the picture below, what is the charge of

the ground? How do you know?


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Is the man in the car safe or in danger? Explain.


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physical science chapter 7 (new book)

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