20.1 electric charge and static electricity

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20.1 Electric Charge and Static Electricity

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Think About This…Have you ever taken the laundry

out of the cloths dryer and had some of them stick together?

Have you taken these things apart and seen the small sparks?

When you do this, you are creating lightening on a small scale!

Both lightening and static cling are caused by the same thing - electric charges.

http://thatcostumegirl.com/gallery/d/2578-2/scling.jpg

We are going to learn about…

What is electric charge? How do I get something

to have an electric charge?

Why do some static charges cause things to stick together?

Why do I get shocked when I touch certain objects?

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Electric ChargeElectric charge is a property that causes subatomic

particles such as protons and electrons to attract or repel each other.

There are two types of electrical charge:Positive and Negative

Protons have positive charge. Electrons have negative charge.

http://ael.gsfc.nasa.gov/images/saturn/atom.gif

Electric ChargeThe atom is neutral (no electric charge) if it has the

same number of positive and negative charges.If an atom gains one or more electrons, it becomes a

negatively charged ion (since it gains a negative charge).

If an atoms loses electrons, it becomes a positively charged ion. QuickTime™ and a

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Electric ChargeAn excess or shortage of electrons produces a

net electric charge.The SI unit of electric charge is the coulomb (C).

1 coulomb = 6.24 X 1018 electrons

A lightening bolt is about 10 to 20 coulombs of charge.

http://superstruny.aspweb.cz/images/fyzika/electrons_acrylic.jpg

Electric ForcesIf you rub an inflated rubber balloon on your clean, dry

hair, you can use that balloon to attract your hair!If you do this with two balloons and then put the balloons

together, the balloons with repel.

They are both negatively charged.



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Electric ForcesMajor Concept:

Like charges repel, and opposite charges attract.

The attraction or repulsion between electrically charged objects is electric force.


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Electric ForcesThe amount of electric force depends on the amount of

charge that is involved and how close the charged objects are to each other.

If the objects are close to each other, then they can “sense” each others’ charges easier.

So the closer they are, the more force there is between them.

It is the same with the amount of charge. The more charge, the more the force.



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Coulomb’s LawThe French scientist Charles-Augustin de Coulomb

(1736-1806) found that electric forces act much like gravity, and formulated a mathematical expression to express the amount of electric force.F = kc q1 q2 / r2

kc is Coulomb’s constant = 8.99 X 109 N m2 / C2

q1 and q2 are the charges of the objects.r is the distance between the charged objects.

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Electric ForcesInside an atom, electric forces are much stronger

than gravitational forces.But on a large scale, matter is mostly neutral,

also the distance between charges makes the electric force between objects mostly nonexistent.

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Electric FieldsAlong with the idea of electric force is the idea of

electric fields.An electric field is like a “force field” around a

charged object; it exerts forces on other charged object placed in the field.

The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.QuickTime™ and a


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Electric FieldsJust like electric forces, the more charge a particle has,

the stronger the electric field it will produce.The lines representing the field are closer together near

the charge, where the field is stronger.However, if the object that is coming into an electric field

has no charge, then the field does not act upon the object.



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Creating Electric ChargeSo far, we have learned about electric forces and

electric fields.Both help us understand how charged particles interact.But how can an object become “charged?”



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Creating Electric ChargeThere are several ways that a net charge can build up

on an object or move from one object to another.The study of electric charges, including how they are

transferred between objects, is called static electricity (“statics”).

In statics, charge can be transferred by friction, by contact, and by induction.

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Creating Electric ChargeBefore we learn more about these three methods, it is

important to know that only the electrons are transferred. The protons are never transferred.

This is because the protons are in the center of the atom and the electrons are on the outer parts of the atom.

The electrons are free to move about, and thus they are able to be transferred.



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Creating Electric ChargeSince the transfer of electrons is what causes a

charge transfer, we must have the overall charge between two objects remain the same.

This is called the law of conservation of charge.

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The Law of Conservation of ChargeLet’s say you start off with two neutral objects and then

one of them transferred two electrons to the other object. What are their individual and collective charges before and after the transfer?

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Back to Creating ChargesAs stated before, there are three ways to transfer

electrons: friction, induction and contact.Let’s start with friction…Imagine, once again, rubbing a balloon in your hair.This results in hair attracting to the balloon because

opposite charges attract.This is an example of

charging by friction.QuickTime™ and a


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Charging by FrictionHow does this situation work?Electrons move from your hair to the balloon because

the atoms in rubber have a greater attraction for electrons than atoms in hair.

So the balloon picks up a net negative charge.Since your hair loses

electrons, it becomes positively charged.

Then, once again, opposites attract.

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Charging by InductionRemember that like charges

repel each other and unlike charges attract, and they do not have to be touching to do this.

Induction uses this fact to create a transfer of charge without any contact between materials.

http://www.school-for-champions.com/science/images/static_induction-electroscope_charged.gif



Charging by InductionIf your hand has a net negative

charge, as you reach for the metal doorknob, the other negative charges in the doorknob will want to get away from your hand, and the positive charges will want to get to your hand.

So the doorknob will be positively charged at the part that is closest to your hand.

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Charging by ContactBesides friction, charges can also be transferred by

simply making contact with a charged object.A Van de Graaff generator has a metal sphere that can

be charged.If somebody is making contact with this sphere, they will

receive some of the charge as well, no friction is necessary.

This machine is responsible for making people’s hair stick straight up.



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Static DischargeLet’s go back to the idea of charging by induction and

the charged hand and doorknob from earlier.Sometimes in this situation, you will get shocked.This shock is a static discharge - when charges “jump”

from one object to another.Lightening operates on the

same idea as static discharge!



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Static DischargeAn important thing to note is that you do not always get

a shock every time you try to open a door, nor do you always have lightening reaching the ground.

In order for any charge to “jump,” the air between the objects must also get charged.



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Static DischargeIf the air between the objects does not have a charge,

then there will be no shock.However, if the air is charged, then the air can act like a

connector between the two objects and the charge is able to be transferred rapidly.

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20.1 electric charge and static electricity

Documents

net electric charge

electric forcesinside

idea of electric

coulombs of charge

positive charge

thing electric charges

si unit of electric

chargeelectric charge