 # Chapter 20 Static Electricity. Objectives  Charged Objects  Conductors and Insulators  Forces on Charged Bodies  Coulomb’s Law  The unit of charge.

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Chapter 20 Static Electricity

Objectives  Charged Objects  Conductors and Insulators  Forces on Charged Bodies  Coulomb’s Law  The unit of charge “The Coulomb” (coo – lome)

Electrical Charges  Electrical comes from the Greek word for Amber: “ Elektron ”  Amber, when rubbed, attracts things. Balloons, when rubbed, stick to walls.  Benjamin Franklin called the two states of charge “Positive” and “Negative”.  Static electricity is produced when one object takes negative charges from another object  Like charges repel, opposites attract.

Charging Amber Amber before rubbing, neutral chargeWool before rubbing, neutral charge Amber after rubbing, more negatives than positives = negatively charged. Wool after rubbing, more positives than negatives = positively charged.

Charge; Microscopic Level  Electrons have a charge of “-e”.  Protons have a charge of “+e”  Neutron has no charge.  e = 1.60 x 10 – 19 C  The total electric charge in the universe is Constant

Charge: Microscopic  Mass of an electron: m e = 9.11 x 10 -31 kg  Mass of a proton: m p =1.673 x 10 – 27 kg  Mass of a neutron: m n = 1.675 x 10 -27 kg  The atom that loses an electron is a positive “ion”  The atom that gains an electron is a negative “ion”

Conductors and Insulators  Charges in insulators are not free to move about.  Charges on conductors are free to move.  Conductors give up an electron easily.  Most insulators are non-metallic.  Most metals are good conductors.  Some materials exhibit qualities of both and are called “Semiconductors”.

Charged Bodies  There are two charges: + and –  Like charges repel, opposites attract.  When a charged object touches a neutral object charges are transferred. This is termed “Charging by Conduction”.  When a charged object is “near” a neutral object, the charges on the neutral object will separate as opposite charges gather near one another and like charges repel one another.  This is termed “Charging by Induction”.

Charging by Conduction  If the charged object is negative, electrons can flow into the neutral object resulting the object acquiring negative charge.  Although the originally negatively charged object loses some of its negative charge, it still is negative.  The originally charged object and the newly charged object will repel one another because they have the same charge.

Charging by Induction  first induce a charge separation.  “-” charged object is brought close to a neutral conductor  “-” electrons on the neutral object will be repelled by the “-” charged object.  This results in a charge separation or “polarization”  In order to charge the object, we need to provide a passage out for “-” charges.  We ground the object. Since the electrons repel one another, they will freely flow into the earth where they can spread out.  As the electrons flow out of the neutral object, the object becomes positively charged.

Electrical Force: F e  F e is measured in Newtons.  Unlike gravity, F e can attract OR repel.  Gravity is a weak force compared to F e  F e will vary inversely with the square of the distance between two charges; the farther apart, the less F e  Coulomb put forth his ideas as “Coulomb’s Law” Formula to use to determine Force of gravity on two charges Coulomb’s Law

The Unit of Charge  In Coulomb’s Law k = 9 x 10 9 N(m 2 /C 2 )  One Coulomb “C” is the charge of 6.25x10 18 electrons.  When talking of F e, if two like charges (q 1 and q 2 ) repulse one another, the sign of the force is positive.  If the two charges are attracted, then the force has a negative sign.

Recap  There are two types of charges  Positive +  Negative-  Insulators prohibit electron movement.  Conductors allow electron movement.  The unit of charge is the Coulomb.  Electrical Force is in Newtons (N).

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