0708 Static Electricity

7/30/2019 0708 Static Electricity

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What Is Static

Electricity? A stationary

electrical charge

that is built up on

the surface of a

material


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Two kinds of charges

After being rubbed, aplastic ruler can attract

paper scraps.

Ruler carries electric charge.It exerts electric force on paper.

The interaction between static electric charges is called

electrostatics.

This charging method is called charging by friction.


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Where do charges come from?

Matter is made up ofatoms.

+

+

+

+

Proton (positive charge)

neutron (neutral)

electron (negative charge)

atom nucleus


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If electrons = protons neutral


If electrons > protons gaining electrons, negativechargeIf electrons < protons losing electrons, positive

charge


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Electro-negativity

Relative

electro-negativity

ranking for some

common materials

from electron donating

materials (+, glass) to

electron accepting

materials (-, teflon)

Glass

Human Hair

Nylon

Silk

Fur

Aluminum

Paper

Cotton

Copper

Rubber

PVC

Teflon

+ + + + +

+ + + +

+ + +

+ ++

-

- -

- - -- - - -

- - - - -


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When a balloon rubs a piece of wool...


electrons are pulled from the

wool to the balloon.

The balloon has more electrons than

usual.

+

+

+

+

+

The balloon:charged,

The wool: +charged

wool


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Insulators and conductors

Insulators: materials that do NOT allow

electrons to flow through them easily.

Insulators can be easily charged by friction as the extra

electrons gained CANNOT easily escape.


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4 Insulators and conductors

Conductors: materials that allow electrons to flow

through them easily.

Conductors CANNOT be easily charged by friction as

the extra electrons gained can easily escape.


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Induction: The production of a charge in anuncharged body by bringing a charged

object close to itWhen negatively charged rod is put near a metal can...

electrons of the can are pushed

away from the rod.

top of the can: positive

& attraction > repulsion

++++

+

metalcan

--

--

-

----- --

buttom of the can: negative

induced

charges attraction

repulsion


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Attraction of uncharged objects

Similarly, when charged rod is close to paper

scrap...

----- --

molecules of paper align.

attraction between the

rod and + charge >

repulsion between the rod

and - charge.paper +

+

+

+

+

+

+

+

attraction

repulsion


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Static Discharge

Human body can not feel less than

2,000 volts of static discharge

Static charge built up by scuffing

shoes on a carpet can exceed 20,000volts?


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Gas Station Fires

Carol said a static gas pump fire is blamed forburning her daughter so badly she needed skingrafts on her legs.

Carol had put the gas pump nozzle on automaticand re-entered her car to write a check. When herthen-12-year-old daughter, wearing a sweater and

jacket that may have created static electricity,reached for the nozzle, flames suddenly ignited herclothing.


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1 If the balloon can attract some paper scraps, which of the

following cannot be the charge of paper scraps?

A Neutral B Positive

C Negative

A balloon has a negative chargewhen rubbed by a woollen cloth.


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2 During rubbing, what have been transferred between the

woollen cloth and the balloon?

A Electrons B Protons

C Neutrons

A balloon has a negative chargewhen rubbed by woollen cloth.


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When a + charged rod is put near neutral object,

______________ is induced on the side of the objectnear the rod and _____________ is induced on the side

away from the rod. The rod can attract the netural object

because _________ between rod and induced charge >

the ________ between rod and + induced charge.

How does a positively charged rod attract a

neutral object?

negative charge positive charge

attraction

repulsion


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Grounding

An object is grounded when it is connected

to the earth through a connecting wire.

What is grounding?

If a charged conductor is grounded, it

will become neutral.


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Grounding

b How does grounding occur?

++

++

+

When we touch a metal ball ofpositive charge...

electrons flow from theearth to the metal ball to

neutralize the metal ball.

Metal ball becomes neutral.


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Similarly, if the metal ball is ofnegative charge...

Grounding

How does grounding occur?

extra electrons flowfrom the metal ball to

the earth and the ballbecomes neutral.


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Why do gasoline tankers usually have metal

chains at the back?

When cars run, their tires and bodies are usually

charged by _______. For gasoline tankers, if theaccumulated charge is large enough, _______can be

produced and _________ will occur if gasoline vapor is

ignited. Those metal chains conduct the charge on thebodies of tankers to the _______ and avoid the danger.

friction

sparks

explosion

ground


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Lightning

kills more

than 60

people and injures more

than 400

people a yearin the US

one mile every

five seconds

about 20,000 C

Voltage of up to

1.2x108 volts


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Limit Exposure


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Do Now


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0708 Static Electricity