# Ch. 21: Electric charge Ch. 22: Electric fields Ch. 23: Gauss’s law Ch. 24: Electric potential Ch. 25: Capacitance and dielectrics Ch. 26: Current and.

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Ch. 21: Electric charge Ch. 22: Electric fields Ch. 23: Gauss’s law Ch. 24: Electric potential Ch. 25: Capacitance and dielectrics Ch. 26: Current and resistance Ch. 27: Direct current circuits Ch. 28: Magnetic fields Ch. 29: Sources of the magnetic field Ch. 30: Faraday’s law Ch. 32: Inductance Physics 102 Electricity and Magnetism : Beyond phys-102: AC circuits E&M waves Light and Optics

Chapter 21: Electric Charge 21.1 Electromagnetism Electron - Elecktron - Greek word for Amber Magnetism - Magnesia - where magnetite (Fe 3 O 4 ) was found Since matter is mostly space, why can’t your hands run through one another? What causes pushes and pulls? What is responsible for the intermolecular interactions in solids and liquids? Is magnetism independent of electricity? There is a connection [Maxwell equations]

E&M in Science and technology: Electrostatic precipitator Photocopy machine Electric spray painting The electric range [induction and/ or resistance] Electronics Nanotechnology!! …etc.

21.2 Electric Charge After combing your hair, the comb may acquire the capability to attract pieces of paper. When you rub your shoes on a wool rug, you may feel “zapped” when touching the door knob. What’s happening? Material can become electrified (i.e. electrically charged). A rubber rod rubbed with fur attracts (and is attracted to) a glass rod rubbed with silk! Coulomb’s torsion balance: F ~ 1/r 2

Like charges repel; unlike charges attract We take the convention that the charge on the glass rod is “positive” and the charge on the rubber rod is negative. Negatively charged electrons transfer from glass to silk, and from fur to rubber.

21.3 Conductors and Insulators Conductors are material in which electric charges move quite freely (e.g. copper, silver aluminum,..etc.). What happens if there is excess charge on a conductor? There will be redistribution of charge. What happens if you rub a copper rod that is held in your hand? What happens if you rub a copper rod that is held by an insulating handle?

When a conductor is connected to Earth through a conducting wire, we say the conductor has been grounded. Earth is like an infinite sink for charges. Charging by conduction: rubbing (contact): Charging by induction (no contact)

Insulators are material that do not readily transport charge (e.g. glass, rubber). What happens if there is excess charge in an insulator? There will be local charging. What is a semiconductor? What is a superconductor? Why does the comb attract pieces of paper? Does the pieces of paper have a net charge? No, but there is polarization!

21.4 Coulomb’s Law F is the magnitude of the electric force. The direction is along the line connecting the two charges. Repel if they are like, attract if they are opposite. k [called Coulomb’s constant] = 8.99 x 10 9 N m 2 /C 2

The permittivity constant (  o ) is related to k by: k = 1/(4   o ) Therefore,  o = 8.85 x 10 -12 C 2 /(N m 2 ) How about charged spherical conductors? How about charged non-spherical conductors?!

21.5 Charge is Quantized q = n e e is called the elementary charge. e = 1.60 x 10 -19 C What is q electron ? What is q proton ? What is q  ? How many electrons make up a charge of magnitude 1 coulomb? Are there particles with parts of an e?

21.6 Charge is Conserved You can transfer charge, but can not create them. Which charge has the largest amount of force effecting it? Analyze the forces on the points charges!

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