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Unformatted text preview: EXPERIMENT TEMPERATURE DEPENDENCE OF RESISTIVITY Introduction: When a battery is connected across a resistor (e.g. a light bulb or a heating element), current will flow through it. How much current is determined by a property of the resistor called (naturally) its resistance. The larger the resistance, the smaller the current. What determines the resistance of an object? Among other things, it depends on what the object is made of. For example, identical cylinders of copper and rubber will have vastly different resistances to current flow. Copper is a good conductor whereas rubber is a good insulator. They differ in a property called the resistivity. Resistivity is a microscopic parameter which depends on the interaction between the current-carrying electrons and the lattice. The microscopic origin of resistivity is easy to picture. As an electron travels through a lattice, it will interact with, and be scattered by, the lattice ions. In other words, the lattice will interfere with the flow of the electrons. In general, resistivity is a function of temperature. In normal metals, resistivity tends to increase with temperature as the increased thermal motion of the lattice further obstructs the electron flow. In some semi-conductors, the resistivity actually decreases with temperature. In such materials, the effect of temperature is to make more electrons available for conduction. In this experiment you will investigate the temperature dependence of this fundamental parameter in a variety of materials. In Exercise 1 you will practice measuring resistance using a Wheatstone bridge. Then in Exercise 2 you will use the bridge to measure the resistance of several samples while heating them. Exercise 1: R X C A a D b B R a R b Figure 1: Wheatstone Bridge The Wheatstone bridge circuit is shown in Fig.1. An unknown resistor, X , is placed in series with a variable, but known resistor R . The resistor AB is simply a wire of uniform cross- sectional area. Its resistance is divided into two parts, R a and R b , by the sliding contact D . Since the resistance of a wire is proportional to its length, the magnitude of R a and R b depend on the contact point along the wire. For example, let the total resistance of the slide wire be R ....
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This note was uploaded on 04/01/2011 for the course PHY 135 taught by Professor Wagihghobriel during the Spring '11 term at University of Toronto.

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