Unit3WEBLectureNotes - UCSD Physics 2B Summer Session I...

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UCSD Physics 2B Summer Session I Unit 3 Lecture Notes Chapter 27 Electric Current Section 1 Definition – charges in motion Recall the analogy we made between pumping water into a storage tank and “pumping” charge into a capacitor? There, we concentrated on the end product of the process, namely, the resulting charge separation between the plates. Now we focus our attention on the process of moving that charge. When water flows through a pipe or river bed, we call that flow “current” and we hear of dangerous currents near the beach, etc. The flow of charges is an electric current and, in the same sense, it can flow in a particular direction, follow a conduit or even swirl around like water. We assign the letter I to represent the electric current and define it as the amount of charge per unit time flowing past a particular point in space. If that flow is uniform in time , then we define the constant current as charge time q I t Δ == Δ Electric Current or, if that current is changing in time, we define the instantaneous current as () dq It q dt =≡ ± Instantaneous Current Since we’ll most often consider current flowing through a wire, we’ll deal with it as a scalar rather than vector, keeping in mind the direction of the current as it follows the path of the wire as a stream follows the path of its bed. Further, we define the current as positive in the direction that positive charges are flowing. Note, however, that most free charges are negative . The MKS unit of current is called the Ampere, abbreviated A , and Coulomb 1 Ampere 1 second = Units of Current
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Section 2 Conduction Mechanisms In order for charges to move , they must be free to do so. The most common conductors here on earth are metals which have valence (outer) electrons shared amongst all the atoms in the otherwise fixed and solid lattice. This “sea” of electrons can conduct current almost freely but, as we shall see, they collide with the fixed atoms much as bumper cars collide with the posts. Other kinds of conductors include ionic liquids like salt water, and plasmas – gases in which some electrons have been stripped from their homes and are free to roam. We will sometimes speak of perfect conductors, in which there are no collisions. These are mostly mental constructs for our analyses (of circuit behavior) while others (superconductors) actually exist in the real world. Comments : Invoking our analogy with water once again, note that the water level of a steady state river neither rises nor falls even though water is flowing steadily. Similarly, a current-carrying wire is neutral, even though charges are flowing through it. Also, you might find it helpful to consider the analogy between gallons per minute flowing through a pipe, and Coulombs per second flowing through a conducting wire.
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This note was uploaded on 08/23/2008 for the course PHYS 2b taught by Professor Schuller during the Summer '08 term at UCSD.

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Unit3WEBLectureNotes - UCSD Physics 2B Summer Session I...

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