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Dr. Moushab BENKAHOUL

2sd semester 2012-2013

General physics

PHYS 104

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1. Introduction2. Electric fields3. Gauss’s law 4. Electric potential 5. Capacitance and dielectric 6. Current and resistance 7. Direct current circuits8. Magnetic fields 9. Sources of the magnetic field 10. Faraday’s law 11. Inductance 12. Alternating current circuits 13. Electromagnetic waves

Topics

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1. Introduction

• There is a large overlap of the world of static electricity and the everyday world that you experience. You walk across the carpeting to exit a room and receive a door knob shock.

• During the dryness of winter, you step out of your car and receive a car door shock as you try to close the door. Sparks of electricity are seen as you pull a wool blanket off the sheets of your bed.

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After rubbing a pen on your hair you will find that the attracts bits of paper

Using the convention suggested by Franklin, the electric charge on the glassrod is called positive and that on the rubber rod is called negative. Therefore, anycharged object attracted to a charged rubber rod (or repelled by a charged glassrod) must have a positive charge, and any charged object repelled by a charged rubberrod (or attracted to a charged glass rod) must have a negative charge.

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Charges of the same sign repel one another and charges with opposite signs attract one another

Another important aspect of electricity that arises from experimental observations is that electric charge is always conserved in an isolated system. That is, when one object is rubbed against another, charge is not created in the process.

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The Structure of Matter

All material objects are composed of atoms. There are different kinds of atoms known as elements. Material objects are composed of atoms and molecules of these elements and compounds, thus providing different materials with different electrical properties.An atom consists of a nucleus and a vast region of space outside the nucleus. Electrons are present in the region of space outside the nucleus. They are negatively charged and weakly bound to the atom. Electrons are often removed from and added to an atom by normal everyday occurrences.The nucleus of the atom contains positively charged protons and neutral neutrons. These protons and neutrons are not removable or perturbable by usual everyday methods. Electrostatic phenomenon can never be explained by the movement of protons.

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In 1909, Robert Millikan (1868–1953) discovered that electric charge alwaysoccurs as integral multiples of a fundamental amount of charge e

Electric charge q is said to be quantized, where q is the standardsymbol used for charge as a variable. That is, electric charge exists as

discrete “packets,” and we can write q =-±Ne, where N is some integer.

Electron has a charge -e and the proton has a charge of equal magnitude but opposite sign +e. Some particles, such as the neutron, have no charge.

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Materials such as glass, rubber, and dry wood fall into the category of electricalinsulators. When such materials are charged by rubbing, only the area rubbedbecomes charged and the charged particles are unable to move to other regions ofthe material. In contrast, materials such as copper, aluminum, and silver are good electricalconductors. When such materials are charged in some small region, the chargereadily distributes itself over the entire surface of the material. Semiconductors are a third class of materials, and their electrical properties aresomewhere between those of insulators and those of conductors. Silicon and germaniumare well-known examples of semiconductors commonly used in the fabricationof a variety of electronic chips used in computers, cellular telephones, andhome theater systems. The electrical properties of semiconductors can be changedover many orders of magnitude by the addition of controlled amounts of certainatoms to the materials.

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To understand how to charge a conductor by a process known as induction, consider a neutral (uncharged) conducting sphere

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Charging an object by induction requires no contact with the object inducingthe charge. That is in contrast to charging an object by rubbing (that is, by conduction ), which does require contact between the two objects. A process similar to induction in conductors takes place in insulators.

In mostneutral molecules, the center of positive charge coincides with the center of negative charge. In the presence of a charged object, however, these centers inside each molecule in an insulator may shift slightly, resulting in more positive charge on one side of the molecule than on the other.