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Chapter 21

PHYS 241 Chapter 21 Notes.docx

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Department
Physics
Course
PHYS 24100
Professor
Oanas Malis
Semester
Fall

Description
Chapter 21 Section 1—Charge • Every object has a normal amount of electricity that can be transferred from one object to another when two objects are in close contact (i.e. rubbing) o One object would have an excess charge and other would have a deficiency of charge (+/-) o Electrically neutral = neither positively or negatively charged • Charge of a proton is e, electron charge is –e o e = fundamental unit of charge o changes in e mean charge is quantized • law of conservation of charge • SI unit of charge is the coulomb o Amount of charge flowing through a cross-section of wire in one second when the current is one ampiere o e=1.602177x10^-19 • Example 21-1 How Many in a Penny? P696 o Penny mass = 3.1 grams, what is total charge? o Number of electrons in copper = 29 o 1. Total charge Q is the number of electrons multiplied by the charge Q=Ne(-e) o 2. Number of electrons is Z multiplied by the number of copper atoms Nat.  Ne=ZNat o 3. Compute number of copper atoms in 3.1 g  Nat=(3.1g)(6.02x10^23/63.5(g/mol))=2.93x10^22 atoms o 4. Compute Ne  Ne=ZNat=(29 electrons/atom)(2.91x10^22 atoms)=8.53x10^23 electrons o 5. Find Q  Q=Ne(-e)=(8.53e23)(-1.6e-19)=-1.37e5 C Section 2—Conductors and Insulators • Conductors—electrons are free to move about the entire material • Insulators—electrons are bound to nearby atoms • When a large number of copper atoms are combined in a piece of metallic copper, strength of attractions of electrons to nucleus of atom is reduced • Copper ions are arranged in a lattice • Charging by induction o Two uncharged spheres touch each other o When a positively charged rod is brought near one of the spheres, conduction electrons flow from one sphere to another, toward the rod o Sphere closest to rod acquires electrons from other sphere; net negative charge • A conductor that has separated equal and opposite charges is said to be polarized Section 3—Coulomb’s Law • The force exerted by one point charge on another acts along the line between the charges. It varies inversely as the square of the distance separating the charges and is proportional to the product of the charges. The force is repulsive if the charges have the same sign and attractive if the charges have opposite signs. o F=(k|q1q2|)/r^2 by q1 on q2 where k is an experimentally determined positive constant (9e9Nm^2/C^2) Section 4—The Electric Field • One charge produces an electric field E everywhere in space, an
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