{"id":4189,"date":"2014-12-11T02:29:24","date_gmt":"2014-12-11T02:29:24","guid":{"rendered":"https:\/\/courses.candelalearning.com\/colphysics\/?post_type=chapter&#038;p=4189"},"modified":"2016-11-03T18:35:41","modified_gmt":"2016-11-03T18:35:41","slug":"20-3-resistance-and-resistivity","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-physics\/chapter\/20-3-resistance-and-resistivity\/","title":{"raw":"Resistance and Resistivity","rendered":"Resistance and Resistivity"},"content":{"raw":"<div>\r\n<div>\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<div>\r\n<ul>\r\n\t<li>Explain the concept of resistivity.<\/li>\r\n\t<li>Use resistivity to calculate the resistance of specified configurations of material.<\/li>\r\n\t<li>Use the thermal coefficient of resistivity to calculate the change of resistance with temperature.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div title=\"Material and Shape Dependence of Resistance\">\r\n<div>\r\n<div>\r\n<div>\r\n<h2>Material and Shape Dependence of Resistance<\/h2>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nThe resistance of an object depends on its shape and the material of which it is composed. The cylindrical resistor in Figure 1\u00a0is easy to analyze, and, by so doing, we can gain insight into the resistance of more complicated shapes. As you might expect, the cylinder\u2019s electric resistance <em>R<\/em> is directly proportional to its length <em>L<\/em>, similar to the resistance of a pipe to fluid flow. The longer the cylinder, the more collisions charges will make with its atoms. The greater the diameter of the cylinder, the more current it can carry (again similar to the flow of fluid through a pipe). In fact, <em>R<\/em> is inversely proportional to the cylinder\u2019s cross-sectional area <em>A<\/em>.\r\n<div title=\"Figure 20.11.\">\r\n<div>\r\n<div>\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"225\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105943\/Figure_21_03_01a.jpg\" alt=\"A cylindrical conductor of length L and cross section A is shown. The resistivity of the cylindrical section is represented as rho. The resistance of this cross section R is equal to rho L divided by A. The section of length L of cylindrical conductor is shown equivalent to a resistor represented by symbol R.\" width=\"225\" height=\"223\" \/> Figure 1. A uniform cylinder of length L and cross-sectional area A. Its resistance to the flow of current is similar to the resistance posed by a pipe to fluid flow. The longer the cylinder, the greater its resistance. The larger its cross-sectional area A, the smaller its resistance.[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nFor a given shape, the resistance depends on the material of which the object is composed. Different materials offer different resistance to the flow of charge. We define the <em> resistivity<\/em><em>\u03c1<\/em> of a substance so that the <strong>resistance\u00a0<em>R<\/em><\/strong> of an object is directly proportional to <em>\u03c1<\/em>. Resistivity <em>\u03c1<\/em> is an <em><em>intrinsic <\/em><\/em> property of a material, independent of its shape or size. The resistance <em>R<\/em> of a uniform cylinder of length <em>L<\/em>, of cross-sectional area <em>A<\/em>, and made of a material with resistivity <em>\u03c1<\/em>, is\r\n<div style=\"text-align: center;\" title=\"Equation 20.19.\">[latex]R=\\frac{\\rho L}{A}\\\\[\/latex].<\/div>\r\nTable 1\u00a0gives representative values of <em>\u03c1<\/em>. The materials listed in the table are separated into categories of conductors, semiconductors, and insulators, based on broad groupings of resistivities. Conductors have the smallest resistivities, and insulators have the largest; semiconductors have intermediate resistivities. Conductors have varying but large free charge densities, whereas most charges in insulators are bound to atoms and are not free to move. Semiconductors are intermediate, having far fewer free charges than conductors, but having properties that make the number of free charges depend strongly on the type and amount of impurities in the semiconductor. These unique properties of semiconductors are put to use in modern electronics, as will be explored in later chapters.\r\n<div>\r\n<table summary=\"Table 21_03_01\" cellspacing=\"0\" cellpadding=\"0\"><caption><strong>Table 1. Resistivities <em>\u03c1<\/em> of Various materials at 20\u00ba C <\/strong><\/caption>\r\n<thead>\r\n<tr>\r\n<th>Material<\/th>\r\n<th>Resistivity <em>\u03c1<\/em> <strong> (<\/strong> \u03a9 \u22c5 m <strong>)<\/strong><\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td><em>Conductors<\/em><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silver<\/td>\r\n<td>1. 59 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Copper<\/td>\r\n<td>1. 72 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Gold<\/td>\r\n<td>2. 44 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Aluminum<\/td>\r\n<td>2. 65 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Tungsten<\/td>\r\n<td>5. 6 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Iron<\/td>\r\n<td>9. 71 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Platinum<\/td>\r\n<td>10. 6 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Steel<\/td>\r\n<td>20 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Lead<\/td>\r\n<td>22 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Manganin (Cu, Mn, Ni alloy)<\/td>\r\n<td>44 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Constantan (Cu, Ni alloy)<\/td>\r\n<td>49 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Mercury<\/td>\r\n<td>96 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Nichrome (Ni, Fe, Cr alloy)<\/td>\r\n<td>100 \u00d7 10<sup>\u22128 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Semiconductors[footnote]Values depend strongly on amounts and types of impurities[\/footnote]<\/em><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Carbon (pure)<\/td>\r\n<td>3.5 \u00d7 10<sup>5 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Carbon<\/td>\r\n<td>(3.5 \u2212 60) \u00d7 10<sup>5 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Germanium (pure)<\/td>\r\n<td>600 \u00d7 10<sup>\u22123<\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Germanium<\/td>\r\n<td>(1\u2212600) \u00d7 10<sup>\u22123<\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silicon (pure)<\/td>\r\n<td>2300<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silicon<\/td>\r\n<td>0.1\u20132300<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Insulators<\/em><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Amber<\/td>\r\n<td>5 \u00d7 10<sup>14 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Glass<\/td>\r\n<td>10<sup>9 <\/sup> \u2212 10<sup>14 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Lucite<\/td>\r\n<td>&gt;10<sup>13 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Mica<\/td>\r\n<td>10<sup>11 <\/sup> \u2212 10<sup>15 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Quartz (fused)<\/td>\r\n<td>75 \u00d7 10<sup>16 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Rubber (hard)<\/td>\r\n<td>10<sup>13 <\/sup> \u2212 10<sup>16 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Sulfur<\/td>\r\n<td>10<sup>15 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Teflon<\/td>\r\n<td>&gt;10<sup>13 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Wood<\/td>\r\n<td>10<sup>8 <\/sup> \u2212 10<sup>11 <\/sup><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<div title=\"Example 20.5. Calculating Resistor Diameter: A Headlight Filament\">\r\n<div class=\"textbox examples\">\r\n<h3>Example 1. Calculating Resistor Diameter: A Headlight Filament<\/h3>\r\n<div>\r\n\r\nA car headlight filament is made of tungsten and has a cold resistance of 0.350 \u03a9. If the filament is a cylinder 4.00 cm long (it may be coiled to save space), what is its diameter?\r\n<h4><strong>Strategy<\/strong><\/h4>\r\nWe can rearrange the equation [latex]R=\\frac{\\rho L}{A}\\\\[\/latex]\u00a0to find the cross-sectional area <em>A<\/em> of the filament from the given information. Then its diameter can be found by assuming it has a circular cross-section.\r\n<h4><strong>Solution<\/strong><\/h4>\r\nThe cross-sectional area, found by rearranging the expression for the resistance of a cylinder given in [latex]R=\\frac{\\rho L}{A}\\\\[\/latex], is\r\n<p style=\"text-align: center;\">[latex]A=\\frac{\\rho L}{R}\\\\[\/latex].<\/p>\r\nSubstituting the given values, and taking <em>\u03c1<\/em> from Table 1, yields\r\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}A&amp; =&amp; \\frac{\\left(5.6\\times {\\text{10}}^{-8}\\Omega \\cdot \\text{m}\\right)\\left(4.00\\times {\\text{10}}^{-2}\\text{m}\\right)}{\\text{0.350}\\Omega }\\\\ &amp; =&amp; \\text{6.40}\\times {\\text{10}}^{-9}{\\text{m}}^{2}\\end{array}\\\\[\/latex].<\/p>\r\nThe area of a circle is related to its diameter <em>D<\/em> by\r\n<p style=\"text-align: center;\">[latex]A=\\frac{{\\pi D}^{2}}{4}\\\\[\/latex].<\/p>\r\nSolving for the diameter <em>D<\/em>, and substituting the value found for <em>A<\/em>, gives\r\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}D&amp; =&amp; \\text{2}{\\left(\\frac{A}{p}\\right)}^{\\frac{1}{2}}=\\text{2}{\\left(\\frac{6.40\\times {\\text{10}}^{-9}{\\text{m}}^{2}}{3.14}\\right)}^{\\frac{1}{2}}\\\\ &amp; =&amp; 9.0\\times {\\text{10}}^{-5}\\text{m}\\end{array}\\\\[\/latex].<\/p>\r\n\r\n<h4><strong>Discussion<\/strong><\/h4>\r\nThe diameter is just under a tenth of a millimeter. It is quoted to only two digits, because <em>\u03c1<\/em> is known to only two digits.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div title=\"Temperature Variation of Resistance\">\r\n<div>\r\n<div>\r\n<div>\r\n<h2>Temperature Variation of Resistance<\/h2>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nThe resistivity of all materials depends on temperature. Some even become superconductors (zero resistivity) at very low temperatures. (See Figure 2.)\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"200\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105948\/Figure_21_03_02a.jpg\" alt=\"A graph for variation of resistance R with temperature T for a mercury sample is shown. The temperature T is plotted along the x axis and is measured in Kelvin, and the resistance R is plotted along the y axis and is measured in ohms. The curve starts at x equals zero and y equals zero, and coincides with the X axis until the value of temperature is four point two Kelvin, known as the critical temperature T sub c. At temperature T sub c, the curve shows a vertical rise, represented by a dotted line, until the resistance is about zero point one one ohms. After this temperature the resistance shows a nearly linear increase with temperature T.\" width=\"200\" height=\"448\" \/> Figure 2. The resistance of a sample of mercury is zero at very low temperatures\u2014it is a superconductor up to about 4.2 K. Above that critical temperature, its resistance makes a sudden jump and then increases nearly linearly with temperature.[\/caption]\r\n\r\nConversely, the resistivity of conductors increases with increasing temperature. Since the atoms vibrate more rapidly and over larger distances at higher temperatures, the electrons moving through a metal make more collisions, effectively making the resistivity higher. Over relatively small temperature changes (about 100\u00baC or less), resistivity <em>\u03c1<\/em> varies with temperature change \u0394<em>T<\/em> as expressed in the following equation\r\n<div style=\"text-align: center;\" title=\"Equation 20.24.\"><em>\u03c1\u00a0<\/em>=\u00a0<em>\u03c1<\/em><sub>0\u00a0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>),<\/div>\r\nwhere <em>\u03c1<\/em><sub>0<\/sub> is the original resistivity and <em>\u03b1<\/em> is the <em> temperature coefficient of resistivity<\/em>. (See the values of <em>\u03b1<\/em> in Table 2\u00a0below.) For larger temperature changes, <em>\u03b1<\/em> may vary or a nonlinear equation may be needed to find <em>\u03c1<\/em>. Note that <em>\u03b1<\/em> is positive for metals, meaning their resistivity increases with temperature. Some alloys have been developed specifically to have a small temperature dependence. Manganin (which is made of copper, manganese and nickel), for example, has <em>\u03b1<\/em> close to zero (to three digits on the scale in Table 2), and so its resistivity varies only slightly with temperature. This is useful for making a temperature-independent resistance standard, for example.\r\n<div>\r\n<table summary=\"Table 21_03_02\" cellspacing=\"0\" cellpadding=\"0\"><caption><strong>Table 2. Tempature Coefficients of Resistivity <em>\u03b1<\/em><\/strong><\/caption>\r\n<thead>\r\n<tr>\r\n<th>Material<\/th>\r\n<th>Coefficient <img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105950\/autogen-svg2png-007513.png\" alt=\"image\" \/>(1\/\u00b0C)[footnote]Values at 20\u00b0C.[\/footnote]<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td><em>Conductors<\/em><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silver<\/td>\r\n<td>3.8 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Copper<\/td>\r\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Gold<\/td>\r\n<td>3.4 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Aluminum<\/td>\r\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Tungsten<\/td>\r\n<td>4.5 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Iron<\/td>\r\n<td>5.0 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Platinum<\/td>\r\n<td>3.93 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Lead<\/td>\r\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Manganin (Cu, Mn, Ni alloy)<\/td>\r\n<td>0.000 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Constantan (Cu, Ni alloy)<\/td>\r\n<td>0.002 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Mercury<\/td>\r\n<td>0.89 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Nichrome (Ni, Fe, Cr alloy)<\/td>\r\n<td>0.4 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td><em>Semiconductors<\/em><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Carbon (pure)<\/td>\r\n<td>\u22120.5 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Germanium (pure)<\/td>\r\n<td>\u221250 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silicon (pure)<\/td>\r\n<td>\u221270 \u00d7 10<sup>\u22123 <\/sup><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\nNote also that <em>\u03b1<\/em> is negative for the semiconductors listed in Table 2, meaning that their resistivity decreases with increasing temperature. They become better conductors at higher temperature, because increased thermal agitation increases the number of free charges available to carry current. This property of decreasing <em>\u03c1<\/em> with temperature is also related to the type and amount of impurities present in the semiconductors. The resistance of an object also depends on temperature, since <em>R<\/em><sub>0<\/sub> is directly proportional to <em>\u03c1<\/em>. For a cylinder we know <em>R\u00a0<\/em>=\u00a0<em>\u03c1L<\/em>\/<em>A<\/em>, and so, if <em>L<\/em> and <em>A<\/em> do not change greatly with temperature, <em>R<\/em> will have the same temperature dependence as <em>\u03c1<\/em>. (Examination of the coefficients of linear expansion shows them to be about two orders of magnitude less than typical temperature coefficients of resistivity, and so the effect of temperature on <em>L<\/em> and <em>A<\/em> is about two orders of magnitude less than on <em>\u03c1<\/em>.) Thus,\r\n<div style=\"text-align: center;\" title=\"Equation 20.25.\"><em>R<\/em> = <em>R<\/em><sub> 0 <\/sub> ( 1 + <em>\u03b1<\/em>\u0394<em>T<\/em> )<\/div>\r\n<div style=\"text-align: center;\" title=\"Equation 20.25.\"><\/div>\r\nis the temperature dependence of the resistance of an object, where <em>R<\/em><sub>0<\/sub> is the original resistance and <em>R<\/em> is the resistance after a temperature change \u0394<em>T<\/em>. Numerous thermometers are based on the effect of temperature on resistance. (See Figure 3.) One of the most common is the thermistor, a semiconductor crystal with a strong temperature dependence, the resistance of which is measured to obtain its temperature. The device is small, so that it quickly comes into thermal equilibrium with the part of a person it touches.\r\n<div title=\"Figure 20.13.\">\r\n<div>\r\n<div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"250\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105950\/Figure_21_03_03a.jpg\" alt=\"A photograph showing two digital thermometers used for measuring body temperature.\" width=\"250\" height=\"214\" \/> Figure 3. These familiar thermometers are based on the automated measurement of a thermistor\u2019s temperature-dependent resistance. (credit: Biol, Wikimedia Commons)[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div title=\"Example 20.6. Calculating Resistance: Hot-Filament Resistance\">\r\n<div class=\"textbox examples\">\r\n<h3>Example 2. Calculating Resistance: Hot-Filament Resistance<\/h3>\r\n<div>\r\n\r\nAlthough caution must be used in applying <em>\u03c1\u00a0<\/em>=\u00a0<em>\u03c1<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>) and <em>R\u00a0<\/em>=\u00a0<em>R<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>) for temperature changes greater than 100\u00baC, for tungsten the equations work reasonably well for very large temperature changes. What, then, is the resistance of the tungsten filament in the previous example if its temperature is increased from room temperature ( 20\u00baC ) to a typical operating temperature of 2850\u00baC?\r\n<h4><strong>Strategy<\/strong><\/h4>\r\nThis is a straightforward application of <em>R\u00a0<\/em>=\u00a0<em>R<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>), since the original resistance of the filament was given to be <em>R<\/em><sub>0\u00a0<\/sub>= 0.350 \u03a9, and the temperature change is \u0394<em>T\u00a0<\/em>= 2830\u00baC.\r\n<h4><strong>Solution<\/strong><\/h4>\r\nThe hot resistance <em>R<\/em> is obtained by entering known values into the above equation:\r\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}R &amp; =&amp; {R}_{0}\\left(1+\\alpha\\Delta T\\right)\\\\ &amp; =&amp; \\left(0.350\\Omega\\right)\\left[1+\\left(4.5\\times{10}^{-3}\/\u00ba\\text{C}\\right)\\left(2830\u00ba\\text{C}\\right)\\right]\\\\ &amp; =&amp; {4.8\\Omega}\\end{array}\\\\[\/latex].<\/p>\r\n\r\n<h4><strong>Discussion<\/strong><\/h4>\r\nThis value is consistent with the headlight resistance example in <a title=\"20.2. Ohm\u2019s Law: Resistance and Simple Circuits\" href=\".\/chapter\/20-2-ohms-law-resistance-and-simple-circuits\/\" target=\"_blank\">Ohm\u2019s Law: Resistance and Simple Circuits<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div>\r\n<div class=\"textbox\">\r\n<div title=\"Temperature Variation of Resistance\">\r\n<div>\r\n<h2><strong>PhET Explorations: Resistance in a Wire<\/strong><\/h2>\r\n<div>Learn about the physics of resistance in a wire. Change its resistivity, length, and area to see how they affect the wire's resistance. The sizes of the symbols in the equation change along with the diagram of a wire.<\/div>\r\n<\/div>\r\n<\/div>\r\n<div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"300\"]<a href=\"http:\/\/phet.colorado.edu\/sims\/html\/resistance-in-a-wire\/latest\/resistance-in-a-wire_en.html\"><img src=\"http:\/\/phet.colorado.edu\/sims\/html\/resistance-in-a-wire\/latest\/resistance-in-a-wire-600.png\" alt=\"Resistance in a Wire screenshot\" width=\"300\" height=\"200\" \/><\/a> Click to run the simulation.[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<h2 data-type=\"title\">Section Summary<\/h2>\r\n<ul>\r\n\t<li>The resistance\u00a0<em>R<\/em> of a cylinder of length<em> L<\/em>\u00a0and cross-sectional area <em>A<\/em>\u00a0is [latex]R=\\frac{\\rho L}{A}\\\\[\/latex], where <em>\u03c1<\/em> is the resistivity of the material.<\/li>\r\n\t<li>Values of\u00a0<em>\u03c1<\/em> in Table 1\u00a0show that materials fall into three groups\u2014<em data-effect=\"italics\">conductors, semiconductors, and insulators<\/em>.<\/li>\r\n\t<li>Temperature affects resistivity; for relatively small temperature changes\u00a0<span id=\"MathJax-Span-42419\" class=\"mn\">\u0394<\/span><em><span id=\"MathJax-Span-42420\" class=\"mi\">T<\/span><\/em>, resistivity is [latex]\\rho ={\\rho }_{0}\\left(\\text{1}+\\alpha \\Delta T\\right)\\\\[\/latex] , where\u00a0<em>\u03c1<\/em><sub>0<\/sub>\u00a0is the original resistivity and [latex]\\text{\\alpha }[\/latex] is the temperature coefficient of resistivity.<\/li>\r\n\t<li>Table 2 gives values for <em>\u03b1<\/em>, the temperature coefficient of resistivity.<\/li>\r\n\t<li>The resistance <em>R<\/em> of an object also varies with temperature: [latex]R={R}_{0}\\left(\\text{1}+\\alpha \\Delta T\\right)\\\\[\/latex], where\u00a0<span id=\"MathJax-Span-42507\" class=\"mi\"><em>R<\/em><sub>0<\/sub><\/span>\u00a0is the original resistance, and <em>R<\/em>\u00a0is the resistance after the temperature change.<\/li>\r\n<\/ul>\r\n<section data-depth=\"1\" data-element-type=\"conceptual-questions\">\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Conceptual Questions<\/h3>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n1. In which of the three semiconducting materials listed in Table 1\u00a0do impurities supply free charges? (Hint: Examine the range of resistivity for each and determine whether the pure semiconductor has the higher or lower conductivity.)\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n2. Does the resistance of an object depend on the path current takes through it? Consider, for example, a rectangular bar\u2014is its resistance the same along its length as across its width? (See Figure 5.)\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"325\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/1322\/2015\/12\/03211409\/Figure_21_03_04a.jpg\" alt=\"Part a of the figure shows a voltage V applied along the length of a rectangular bar using a battery. The current is shown to emerge from the positive terminal, pass along the length of the rectangular bar, and enter the negative terminal of the battery. The resistance of the rectangular bar along the length is shown as R and the current is shown as I. Part b of the figure shows a voltage V applied along the width of the same rectangular bar using a battery. The current is shown to emerge from the positive terminal, pass along the width of the rectangular bar, and enter the negative terminal of the battery. The resistance of the rectangular bar along the width is shown as R prime, and the current is shown as I prime.\" width=\"325\" height=\"378\" data-media-type=\"image\/jpg\" \/> Figure 5. Does current taking two different paths through the same object encounter different resistance?[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n3. If aluminum and copper wires of the same length have the same resistance, which has the larger diameter? Why?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\r\n<div data-type=\"problem\">\r\n\r\n4. Explain why [latex]R={R}_{0}\\left(1+\\alpha\\Delta T\\right)\\\\[\/latex] for the temperature variation of the resistance <em>R<\/em>\u00a0of an object is not as accurate as [latex]\\rho ={\\rho }_{0}\\left({1}+\\alpha \\Delta T\\right)\\\\[\/latex], which gives the temperature variation of resistivity <em>\u03c1<\/em>.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/section><section data-depth=\"1\" data-element-type=\"problems-exercises\">\r\n<div class=\"textbox exercises\">\r\n<h3>Problems &amp; Exercises<\/h3>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n1. What is the resistance of a 20.0-m-long piece of 12-gauge copper wire having a 2.053-mm diameter?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n2. The diameter of 0-gauge copper wire is 8.252 mm. Find the resistance of a 1.00-km length of such wire used for power transmission.\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n3. If the 0.100-mm diameter tungsten filament in a light bulb is to have a resistance of 0.200 \u03a9 at 20\u00baC, how long should it be?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n4. Find the ratio of the diameter of aluminum to copper wire, if they have the same resistance per unit length (as they might in household wiring).\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n5. What current flows through a 2.54-cm-diameter rod of pure silicon that is 20.0 cm long, when\u00a0<span id=\"MathJax-Span-42625\" class=\"msup\"><span id=\"MathJax-Span-42626\" class=\"mn\">1.00 \u00d7 10<\/span><sup><span id=\"MathJax-Span-42627\" class=\"mtext\">3<\/span><\/sup><\/span><span id=\"MathJax-Span-42628\" class=\"mspace\"><\/span><span id=\"MathJax-Span-42629\" class=\"mtext\">V<\/span> is applied to it? (Such a rod may be used to make nuclear-particle detectors, for example.)\r\n\r\n<\/div>\r\n<div data-type=\"solution\">\r\n\r\n6. (a) To what temperature must you raise a copper wire, originally at 20.0\u00baC, to double its resistance, neglecting any changes in dimensions? (b) Does this happen in household wiring under ordinary circumstances?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n7. A resistor made of Nichrome wire is used in an application where its resistance cannot change more than 1.00% from its value at 20.0\u00baC. Over what temperature range can it be used?\r\n\r\n<\/div>\r\n<div data-type=\"solution\">\r\n\r\n8. Of what material is a resistor made if its resistance is 40.0% greater at 100\u00baC than at 20.0\u00baC?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n9. An electronic device designed to operate at any temperature in the range from\u00a0\u201310.0\u00baC to 55.0\u00baC contains pure carbon resistors. By what factor does their resistance increase over this range?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n10. (a) Of what material is a wire made, if it is 25.0 m long with a 0.100 mm diameter and has a resistance of 77.7 \u03a9 at 20.0\u00baC? (b) What is its resistance at 150\u00baC?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n11. Assuming a constant temperature coefficient of resistivity, what is the maximum percent decrease in the resistance of a constantan wire starting at\u00a020.0\u00baC?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n12. A wire is drawn through a die, stretching it to four times its original length. By what factor does its resistance increase?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n13. A copper wire has a resistance of 0.500 \u03a9 at 20.0\u00baC, and an iron wire has a resistance of 0.525 \u03a9 at the same temperature. At what temperature are their resistances equal?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n14. (a) Digital medical thermometers determine temperature by measuring the resistance of a semiconductor device called a thermistor (which has <em>\u03b1\u00a0<\/em>= \u20130.0600\/\u00baC) when it is at the same temperature as the patient. What is a patient\u2019s temperature if the thermistor\u2019s resistance at that temperature is 82.0% of its value at 37.0\u00baC (normal body temperature)? (b) The negative value for <em>\u03b1<\/em> may not be maintained for very low temperatures. Discuss why and whether this is the case here. (Hint: Resistance can\u2019t become negative.)\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n15.<strong> Integrated Concepts\u00a0<\/strong>(a) Redo Exercise 2\u00a0taking into account the thermal expansion of the tungsten filament. You may assume a thermal expansion coefficient of 12 \u00d7 10<sup>\u22126<\/sup>\/\u00baC. (b) By what percentage does your answer differ from that in the example?\r\n\r\n<\/div>\r\n<\/div>\r\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\r\n<div data-type=\"problem\">\r\n\r\n16.<strong> Unreasonable Results\u00a0<\/strong>(a) To what temperature must you raise a resistor made of constantan to double its resistance, assuming a constant temperature coefficient of resistivity? (b) To cut it in half? (c) What is unreasonable about these results? (d) Which assumptions are unreasonable, or which premises are inconsistent?\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/section>\r\n<div data-type=\"footnote-refs\">\r\n<h2 data-type=\"footnote-title\">Footnotes<\/h2>\r\n<ol>\r\n\t<li><a href=\"#footnote-ref1\" name=\"footnote1\" data-type=\"footnote-ref\">1<\/a> Values depend strongly on amounts and types of impurities<\/li>\r\n\t<li><a href=\"#footnote-ref2\" name=\"footnote2\" data-type=\"footnote-ref\">2<\/a> Values at 20\u00b0C.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<div data-type=\"glossary\">\r\n<h2 data-type=\"glossary-title\">Glossary<\/h2>\r\n<dl><dt>resistivity:<\/dt><dd>an intrinsic property of a material, independent of its shape or size, directly proportional to the resistance, denoted by <em data-effect=\"italics\">\u03c1<\/em><\/dd><\/dl><dl><dt>temperature coefficient of resistivity:<\/dt><dd>an empirical quantity, denoted by <em data-effect=\"italics\">\u03b1<\/em>, which describes the change in resistance or resistivity of a material with temperature<\/dd><\/dl>\r\n<div class=\"textbox exercises\">\r\n<h3>Selected Solutions to Problems &amp; Exercises<\/h3>\r\n1.\u00a00.104 \u03a9\r\n\r\n3.\u00a02.8 \u00d7 10<sup>\u22122<\/sup>m\r\n\r\n5.\u00a0<span id=\"MathJax-Span-42631\" class=\"mrow\"><span id=\"MathJax-Span-42632\" class=\"semantics\"><span id=\"MathJax-Span-42633\" class=\"mrow\"><span id=\"MathJax-Span-42634\" class=\"mrow\"><span id=\"MathJax-Span-42635\" class=\"mrow\"><span id=\"MathJax-Span-42636\" class=\"mrow\"><span id=\"MathJax-Span-42637\" class=\"mn\">1<\/span><span id=\"MathJax-Span-42638\" class=\"mtext\">.<\/span><span id=\"MathJax-Span-42639\" class=\"mtext\">10\u00a0<\/span><span id=\"MathJax-Span-42640\" class=\"mi\">\u00d7\u00a0<\/span><span id=\"MathJax-Span-42641\" class=\"msup\"><span id=\"MathJax-Span-42642\" class=\"mtext\">10<\/span><sup><span id=\"MathJax-Span-42643\" class=\"mrow\"><span id=\"MathJax-Span-42644\" class=\"mrow\"><span id=\"MathJax-Span-42645\" class=\"mo\">\u2212<\/span><span id=\"MathJax-Span-42646\" class=\"mn\">3<\/span><\/span><\/span><\/sup><\/span><span id=\"MathJax-Span-42647\" class=\"mspace\"><\/span><span id=\"MathJax-Span-42648\" class=\"mtext\">A<\/span><\/span><\/span><span id=\"MathJax-Span-42649\" class=\"mrow\"><\/span><\/span><\/span><\/span><\/span>\r\n\r\n7.\u00a0\u22125\u00baC to 45\u00baC\r\n\r\n9.\u00a01.03\r\n\r\n11.\u00a00.06%\r\n\r\n13.\u221217\u00baC\r\n\r\n15.\u00a0(a) 4.7 \u03a9 (total)\u00a0(b) 3.0% decrease\r\n\r\n<\/div>\r\n<\/div>","rendered":"<div>\n<div>\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<div>\n<ul>\n<li>Explain the concept of resistivity.<\/li>\n<li>Use resistivity to calculate the resistance of specified configurations of material.<\/li>\n<li>Use the thermal coefficient of resistivity to calculate the change of resistance with temperature.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Material and Shape Dependence of Resistance\">\n<div>\n<div>\n<div>\n<h2>Material and Shape Dependence of Resistance<\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p>The resistance of an object depends on its shape and the material of which it is composed. The cylindrical resistor in Figure 1\u00a0is easy to analyze, and, by so doing, we can gain insight into the resistance of more complicated shapes. As you might expect, the cylinder\u2019s electric resistance <em>R<\/em> is directly proportional to its length <em>L<\/em>, similar to the resistance of a pipe to fluid flow. The longer the cylinder, the more collisions charges will make with its atoms. The greater the diameter of the cylinder, the more current it can carry (again similar to the flow of fluid through a pipe). In fact, <em>R<\/em> is inversely proportional to the cylinder\u2019s cross-sectional area <em>A<\/em>.<\/p>\n<div title=\"Figure 20.11.\">\n<div>\n<div>\n<div style=\"width: 235px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105943\/Figure_21_03_01a.jpg\" alt=\"A cylindrical conductor of length L and cross section A is shown. The resistivity of the cylindrical section is represented as rho. The resistance of this cross section R is equal to rho L divided by A. The section of length L of cylindrical conductor is shown equivalent to a resistor represented by symbol R.\" width=\"225\" height=\"223\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 1. A uniform cylinder of length L and cross-sectional area A. Its resistance to the flow of current is similar to the resistance posed by a pipe to fluid flow. The longer the cylinder, the greater its resistance. The larger its cross-sectional area A, the smaller its resistance.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>For a given shape, the resistance depends on the material of which the object is composed. Different materials offer different resistance to the flow of charge. We define the <em> resistivity<\/em><em>\u03c1<\/em> of a substance so that the <strong>resistance\u00a0<em>R<\/em><\/strong> of an object is directly proportional to <em>\u03c1<\/em>. Resistivity <em>\u03c1<\/em> is an <em><em>intrinsic <\/em><\/em> property of a material, independent of its shape or size. The resistance <em>R<\/em> of a uniform cylinder of length <em>L<\/em>, of cross-sectional area <em>A<\/em>, and made of a material with resistivity <em>\u03c1<\/em>, is<\/p>\n<div style=\"text-align: center;\" title=\"Equation 20.19.\">[latex]R=\\frac{\\rho L}{A}\\\\[\/latex].<\/div>\n<p>Table 1\u00a0gives representative values of <em>\u03c1<\/em>. The materials listed in the table are separated into categories of conductors, semiconductors, and insulators, based on broad groupings of resistivities. Conductors have the smallest resistivities, and insulators have the largest; semiconductors have intermediate resistivities. Conductors have varying but large free charge densities, whereas most charges in insulators are bound to atoms and are not free to move. Semiconductors are intermediate, having far fewer free charges than conductors, but having properties that make the number of free charges depend strongly on the type and amount of impurities in the semiconductor. These unique properties of semiconductors are put to use in modern electronics, as will be explored in later chapters.<\/p>\n<div>\n<table summary=\"Table 21_03_01\" cellpadding=\"0\" style=\"border-spacing: 0px;\">\n<caption><strong>Table 1. Resistivities <em>\u03c1<\/em> of Various materials at 20\u00ba C <\/strong><\/caption>\n<thead>\n<tr>\n<th>Material<\/th>\n<th>Resistivity <em>\u03c1<\/em> <strong> (<\/strong> \u03a9 \u22c5 m <strong>)<\/strong><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td><em>Conductors<\/em><\/td>\n<\/tr>\n<tr>\n<td>Silver<\/td>\n<td>1. 59 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Copper<\/td>\n<td>1. 72 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Gold<\/td>\n<td>2. 44 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Aluminum<\/td>\n<td>2. 65 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Tungsten<\/td>\n<td>5. 6 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Iron<\/td>\n<td>9. 71 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Platinum<\/td>\n<td>10. 6 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Steel<\/td>\n<td>20 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Lead<\/td>\n<td>22 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Manganin (Cu, Mn, Ni alloy)<\/td>\n<td>44 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Constantan (Cu, Ni alloy)<\/td>\n<td>49 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Mercury<\/td>\n<td>96 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Nichrome (Ni, Fe, Cr alloy)<\/td>\n<td>100 \u00d7 10<sup>\u22128 <\/sup><\/td>\n<\/tr>\n<tr>\n<td><em>Semiconductors<a class=\"footnote\" title=\"Values depend strongly on amounts and types of impurities\" id=\"return-footnote-4189-1\" href=\"#footnote-4189-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a><\/em><\/td>\n<\/tr>\n<tr>\n<td>Carbon (pure)<\/td>\n<td>3.5 \u00d7 10<sup>5 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Carbon<\/td>\n<td>(3.5 \u2212 60) \u00d7 10<sup>5 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Germanium (pure)<\/td>\n<td>600 \u00d7 10<sup>\u22123<\/sup><\/td>\n<\/tr>\n<tr>\n<td>Germanium<\/td>\n<td>(1\u2212600) \u00d7 10<sup>\u22123<\/sup><\/td>\n<\/tr>\n<tr>\n<td>Silicon (pure)<\/td>\n<td>2300<\/td>\n<\/tr>\n<tr>\n<td>Silicon<\/td>\n<td>0.1\u20132300<\/td>\n<\/tr>\n<tr>\n<td><em>Insulators<\/em><\/td>\n<\/tr>\n<tr>\n<td>Amber<\/td>\n<td>5 \u00d7 10<sup>14 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Glass<\/td>\n<td>10<sup>9 <\/sup> \u2212 10<sup>14 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Lucite<\/td>\n<td>&gt;10<sup>13 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Mica<\/td>\n<td>10<sup>11 <\/sup> \u2212 10<sup>15 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Quartz (fused)<\/td>\n<td>75 \u00d7 10<sup>16 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Rubber (hard)<\/td>\n<td>10<sup>13 <\/sup> \u2212 10<sup>16 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Sulfur<\/td>\n<td>10<sup>15 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Teflon<\/td>\n<td>&gt;10<sup>13 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Wood<\/td>\n<td>10<sup>8 <\/sup> \u2212 10<sup>11 <\/sup><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div title=\"Example 20.5. Calculating Resistor Diameter: A Headlight Filament\">\n<div class=\"textbox examples\">\n<h3>Example 1. Calculating Resistor Diameter: A Headlight Filament<\/h3>\n<div>\n<p>A car headlight filament is made of tungsten and has a cold resistance of 0.350 \u03a9. If the filament is a cylinder 4.00 cm long (it may be coiled to save space), what is its diameter?<\/p>\n<h4><strong>Strategy<\/strong><\/h4>\n<p>We can rearrange the equation [latex]R=\\frac{\\rho L}{A}\\\\[\/latex]\u00a0to find the cross-sectional area <em>A<\/em> of the filament from the given information. Then its diameter can be found by assuming it has a circular cross-section.<\/p>\n<h4><strong>Solution<\/strong><\/h4>\n<p>The cross-sectional area, found by rearranging the expression for the resistance of a cylinder given in [latex]R=\\frac{\\rho L}{A}\\\\[\/latex], is<\/p>\n<p style=\"text-align: center;\">[latex]A=\\frac{\\rho L}{R}\\\\[\/latex].<\/p>\n<p>Substituting the given values, and taking <em>\u03c1<\/em> from Table 1, yields<\/p>\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}A& =& \\frac{\\left(5.6\\times {\\text{10}}^{-8}\\Omega \\cdot \\text{m}\\right)\\left(4.00\\times {\\text{10}}^{-2}\\text{m}\\right)}{\\text{0.350}\\Omega }\\\\ & =& \\text{6.40}\\times {\\text{10}}^{-9}{\\text{m}}^{2}\\end{array}\\\\[\/latex].<\/p>\n<p>The area of a circle is related to its diameter <em>D<\/em> by<\/p>\n<p style=\"text-align: center;\">[latex]A=\\frac{{\\pi D}^{2}}{4}\\\\[\/latex].<\/p>\n<p>Solving for the diameter <em>D<\/em>, and substituting the value found for <em>A<\/em>, gives<\/p>\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}D& =& \\text{2}{\\left(\\frac{A}{p}\\right)}^{\\frac{1}{2}}=\\text{2}{\\left(\\frac{6.40\\times {\\text{10}}^{-9}{\\text{m}}^{2}}{3.14}\\right)}^{\\frac{1}{2}}\\\\ & =& 9.0\\times {\\text{10}}^{-5}\\text{m}\\end{array}\\\\[\/latex].<\/p>\n<h4><strong>Discussion<\/strong><\/h4>\n<p>The diameter is just under a tenth of a millimeter. It is quoted to only two digits, because <em>\u03c1<\/em> is known to only two digits.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Temperature Variation of Resistance\">\n<div>\n<div>\n<div>\n<h2>Temperature Variation of Resistance<\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p>The resistivity of all materials depends on temperature. Some even become superconductors (zero resistivity) at very low temperatures. (See Figure 2.)<\/p>\n<div style=\"width: 210px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105948\/Figure_21_03_02a.jpg\" alt=\"A graph for variation of resistance R with temperature T for a mercury sample is shown. The temperature T is plotted along the x axis and is measured in Kelvin, and the resistance R is plotted along the y axis and is measured in ohms. The curve starts at x equals zero and y equals zero, and coincides with the X axis until the value of temperature is four point two Kelvin, known as the critical temperature T sub c. At temperature T sub c, the curve shows a vertical rise, represented by a dotted line, until the resistance is about zero point one one ohms. After this temperature the resistance shows a nearly linear increase with temperature T.\" width=\"200\" height=\"448\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. The resistance of a sample of mercury is zero at very low temperatures\u2014it is a superconductor up to about 4.2 K. Above that critical temperature, its resistance makes a sudden jump and then increases nearly linearly with temperature.<\/p>\n<\/div>\n<p>Conversely, the resistivity of conductors increases with increasing temperature. Since the atoms vibrate more rapidly and over larger distances at higher temperatures, the electrons moving through a metal make more collisions, effectively making the resistivity higher. Over relatively small temperature changes (about 100\u00baC or less), resistivity <em>\u03c1<\/em> varies with temperature change \u0394<em>T<\/em> as expressed in the following equation<\/p>\n<div style=\"text-align: center;\" title=\"Equation 20.24.\"><em>\u03c1\u00a0<\/em>=\u00a0<em>\u03c1<\/em><sub>0\u00a0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>),<\/div>\n<p>where <em>\u03c1<\/em><sub>0<\/sub> is the original resistivity and <em>\u03b1<\/em> is the <em> temperature coefficient of resistivity<\/em>. (See the values of <em>\u03b1<\/em> in Table 2\u00a0below.) For larger temperature changes, <em>\u03b1<\/em> may vary or a nonlinear equation may be needed to find <em>\u03c1<\/em>. Note that <em>\u03b1<\/em> is positive for metals, meaning their resistivity increases with temperature. Some alloys have been developed specifically to have a small temperature dependence. Manganin (which is made of copper, manganese and nickel), for example, has <em>\u03b1<\/em> close to zero (to three digits on the scale in Table 2), and so its resistivity varies only slightly with temperature. This is useful for making a temperature-independent resistance standard, for example.<\/p>\n<div>\n<table summary=\"Table 21_03_02\" cellpadding=\"0\" style=\"border-spacing: 0px;\">\n<caption><strong>Table 2. Tempature Coefficients of Resistivity <em>\u03b1<\/em><\/strong><\/caption>\n<thead>\n<tr>\n<th>Material<\/th>\n<th>Coefficient <img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105950\/autogen-svg2png-007513.png\" alt=\"image\" \/>(1\/\u00b0C)<a class=\"footnote\" title=\"Values at 20\u00b0C.\" id=\"return-footnote-4189-2\" href=\"#footnote-4189-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td><em>Conductors<\/em><\/td>\n<\/tr>\n<tr>\n<td>Silver<\/td>\n<td>3.8 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Copper<\/td>\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Gold<\/td>\n<td>3.4 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Aluminum<\/td>\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Tungsten<\/td>\n<td>4.5 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Iron<\/td>\n<td>5.0 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Platinum<\/td>\n<td>3.93 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Lead<\/td>\n<td>3.9 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Manganin (Cu, Mn, Ni alloy)<\/td>\n<td>0.000 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Constantan (Cu, Ni alloy)<\/td>\n<td>0.002 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Mercury<\/td>\n<td>0.89 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Nichrome (Ni, Fe, Cr alloy)<\/td>\n<td>0.4 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td><em>Semiconductors<\/em><\/td>\n<\/tr>\n<tr>\n<td>Carbon (pure)<\/td>\n<td>\u22120.5 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Germanium (pure)<\/td>\n<td>\u221250 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<tr>\n<td>Silicon (pure)<\/td>\n<td>\u221270 \u00d7 10<sup>\u22123 <\/sup><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Note also that <em>\u03b1<\/em> is negative for the semiconductors listed in Table 2, meaning that their resistivity decreases with increasing temperature. They become better conductors at higher temperature, because increased thermal agitation increases the number of free charges available to carry current. This property of decreasing <em>\u03c1<\/em> with temperature is also related to the type and amount of impurities present in the semiconductors. The resistance of an object also depends on temperature, since <em>R<\/em><sub>0<\/sub> is directly proportional to <em>\u03c1<\/em>. For a cylinder we know <em>R\u00a0<\/em>=\u00a0<em>\u03c1L<\/em>\/<em>A<\/em>, and so, if <em>L<\/em> and <em>A<\/em> do not change greatly with temperature, <em>R<\/em> will have the same temperature dependence as <em>\u03c1<\/em>. (Examination of the coefficients of linear expansion shows them to be about two orders of magnitude less than typical temperature coefficients of resistivity, and so the effect of temperature on <em>L<\/em> and <em>A<\/em> is about two orders of magnitude less than on <em>\u03c1<\/em>.) Thus,<\/p>\n<div style=\"text-align: center;\" title=\"Equation 20.25.\"><em>R<\/em> = <em>R<\/em><sub> 0 <\/sub> ( 1 + <em>\u03b1<\/em>\u0394<em>T<\/em> )<\/div>\n<div style=\"text-align: center;\" title=\"Equation 20.25.\"><\/div>\n<p>is the temperature dependence of the resistance of an object, where <em>R<\/em><sub>0<\/sub> is the original resistance and <em>R<\/em> is the resistance after a temperature change \u0394<em>T<\/em>. Numerous thermometers are based on the effect of temperature on resistance. (See Figure 3.) One of the most common is the thermistor, a semiconductor crystal with a strong temperature dependence, the resistance of which is measured to obtain its temperature. The device is small, so that it quickly comes into thermal equilibrium with the part of a person it touches.<\/p>\n<div title=\"Figure 20.13.\">\n<div>\n<div>\n<div style=\"width: 260px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/222\/2014\/12\/20105950\/Figure_21_03_03a.jpg\" alt=\"A photograph showing two digital thermometers used for measuring body temperature.\" width=\"250\" height=\"214\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 3. These familiar thermometers are based on the automated measurement of a thermistor\u2019s temperature-dependent resistance. (credit: Biol, Wikimedia Commons)<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Example 20.6. Calculating Resistance: Hot-Filament Resistance\">\n<div class=\"textbox examples\">\n<h3>Example 2. Calculating Resistance: Hot-Filament Resistance<\/h3>\n<div>\n<p>Although caution must be used in applying <em>\u03c1\u00a0<\/em>=\u00a0<em>\u03c1<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>) and <em>R\u00a0<\/em>=\u00a0<em>R<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>) for temperature changes greater than 100\u00baC, for tungsten the equations work reasonably well for very large temperature changes. What, then, is the resistance of the tungsten filament in the previous example if its temperature is increased from room temperature ( 20\u00baC ) to a typical operating temperature of 2850\u00baC?<\/p>\n<h4><strong>Strategy<\/strong><\/h4>\n<p>This is a straightforward application of <em>R\u00a0<\/em>=\u00a0<em>R<\/em><sub>0<\/sub>(1 +<em>\u03b1<\/em>\u0394<em>T<\/em>), since the original resistance of the filament was given to be <em>R<\/em><sub>0\u00a0<\/sub>= 0.350 \u03a9, and the temperature change is \u0394<em>T\u00a0<\/em>= 2830\u00baC.<\/p>\n<h4><strong>Solution<\/strong><\/h4>\n<p>The hot resistance <em>R<\/em> is obtained by entering known values into the above equation:<\/p>\n<p style=\"text-align: center;\">[latex]\\begin{array}{lll}R & =& {R}_{0}\\left(1+\\alpha\\Delta T\\right)\\\\ & =& \\left(0.350\\Omega\\right)\\left[1+\\left(4.5\\times{10}^{-3}\/\u00ba\\text{C}\\right)\\left(2830\u00ba\\text{C}\\right)\\right]\\\\ & =& {4.8\\Omega}\\end{array}\\\\[\/latex].<\/p>\n<h4><strong>Discussion<\/strong><\/h4>\n<p>This value is consistent with the headlight resistance example in <a title=\"20.2. Ohm\u2019s Law: Resistance and Simple Circuits\" href=\".\/chapter\/20-2-ohms-law-resistance-and-simple-circuits\/\" target=\"_blank\">Ohm\u2019s Law: Resistance and Simple Circuits<\/a>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox\">\n<div title=\"Temperature Variation of Resistance\">\n<div>\n<h2><strong>PhET Explorations: Resistance in a Wire<\/strong><\/h2>\n<div>Learn about the physics of resistance in a wire. Change its resistivity, length, and area to see how they affect the wire&#8217;s resistance. The sizes of the symbols in the equation change along with the diagram of a wire.<\/div>\n<\/div>\n<\/div>\n<div>\n<div style=\"width: 310px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/phet.colorado.edu\/sims\/html\/resistance-in-a-wire\/latest\/resistance-in-a-wire_en.html\"><img loading=\"lazy\" decoding=\"async\" src=\"http:\/\/phet.colorado.edu\/sims\/html\/resistance-in-a-wire\/latest\/resistance-in-a-wire-600.png\" alt=\"Resistance in a Wire screenshot\" width=\"300\" height=\"200\" \/><\/a><\/p>\n<p class=\"wp-caption-text\">Click to run the simulation.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<h2 data-type=\"title\">Section Summary<\/h2>\n<ul>\n<li>The resistance\u00a0<em>R<\/em> of a cylinder of length<em> L<\/em>\u00a0and cross-sectional area <em>A<\/em>\u00a0is [latex]R=\\frac{\\rho L}{A}\\\\[\/latex], where <em>\u03c1<\/em> is the resistivity of the material.<\/li>\n<li>Values of\u00a0<em>\u03c1<\/em> in Table 1\u00a0show that materials fall into three groups\u2014<em data-effect=\"italics\">conductors, semiconductors, and insulators<\/em>.<\/li>\n<li>Temperature affects resistivity; for relatively small temperature changes\u00a0<span id=\"MathJax-Span-42419\" class=\"mn\">\u0394<\/span><em><span id=\"MathJax-Span-42420\" class=\"mi\">T<\/span><\/em>, resistivity is [latex]\\rho ={\\rho }_{0}\\left(\\text{1}+\\alpha \\Delta T\\right)\\\\[\/latex] , where\u00a0<em>\u03c1<\/em><sub>0<\/sub>\u00a0is the original resistivity and [latex]\\text{\\alpha }[\/latex] is the temperature coefficient of resistivity.<\/li>\n<li>Table 2 gives values for <em>\u03b1<\/em>, the temperature coefficient of resistivity.<\/li>\n<li>The resistance <em>R<\/em> of an object also varies with temperature: [latex]R={R}_{0}\\left(\\text{1}+\\alpha \\Delta T\\right)\\\\[\/latex], where\u00a0<span id=\"MathJax-Span-42507\" class=\"mi\"><em>R<\/em><sub>0<\/sub><\/span>\u00a0is the original resistance, and <em>R<\/em>\u00a0is the resistance after the temperature change.<\/li>\n<\/ul>\n<section data-depth=\"1\" data-element-type=\"conceptual-questions\">\n<div class=\"textbox key-takeaways\">\n<h3>Conceptual Questions<\/h3>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>1. In which of the three semiconducting materials listed in Table 1\u00a0do impurities supply free charges? (Hint: Examine the range of resistivity for each and determine whether the pure semiconductor has the higher or lower conductivity.)<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>2. Does the resistance of an object depend on the path current takes through it? Consider, for example, a rectangular bar\u2014is its resistance the same along its length as across its width? (See Figure 5.)<\/p>\n<div style=\"width: 335px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/1322\/2015\/12\/03211409\/Figure_21_03_04a.jpg\" alt=\"Part a of the figure shows a voltage V applied along the length of a rectangular bar using a battery. The current is shown to emerge from the positive terminal, pass along the length of the rectangular bar, and enter the negative terminal of the battery. The resistance of the rectangular bar along the length is shown as R and the current is shown as I. Part b of the figure shows a voltage V applied along the width of the same rectangular bar using a battery. The current is shown to emerge from the positive terminal, pass along the width of the rectangular bar, and enter the negative terminal of the battery. The resistance of the rectangular bar along the width is shown as R prime, and the current is shown as I prime.\" width=\"325\" height=\"378\" data-media-type=\"image\/jpg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 5. Does current taking two different paths through the same object encounter different resistance?<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>3. If aluminum and copper wires of the same length have the same resistance, which has the larger diameter? Why?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"conceptual-questions\">\n<div data-type=\"problem\">\n<p>4. Explain why [latex]R={R}_{0}\\left(1+\\alpha\\Delta T\\right)\\\\[\/latex] for the temperature variation of the resistance <em>R<\/em>\u00a0of an object is not as accurate as [latex]\\rho ={\\rho }_{0}\\left({1}+\\alpha \\Delta T\\right)\\\\[\/latex], which gives the temperature variation of resistivity <em>\u03c1<\/em>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<section data-depth=\"1\" data-element-type=\"problems-exercises\">\n<div class=\"textbox exercises\">\n<h3>Problems &amp; Exercises<\/h3>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>1. What is the resistance of a 20.0-m-long piece of 12-gauge copper wire having a 2.053-mm diameter?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>2. The diameter of 0-gauge copper wire is 8.252 mm. Find the resistance of a 1.00-km length of such wire used for power transmission.<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>3. If the 0.100-mm diameter tungsten filament in a light bulb is to have a resistance of 0.200 \u03a9 at 20\u00baC, how long should it be?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>4. Find the ratio of the diameter of aluminum to copper wire, if they have the same resistance per unit length (as they might in household wiring).<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>5. What current flows through a 2.54-cm-diameter rod of pure silicon that is 20.0 cm long, when\u00a0<span id=\"MathJax-Span-42625\" class=\"msup\"><span id=\"MathJax-Span-42626\" class=\"mn\">1.00 \u00d7 10<\/span><sup><span id=\"MathJax-Span-42627\" class=\"mtext\">3<\/span><\/sup><\/span><span id=\"MathJax-Span-42628\" class=\"mspace\"><\/span><span id=\"MathJax-Span-42629\" class=\"mtext\">V<\/span> is applied to it? (Such a rod may be used to make nuclear-particle detectors, for example.)<\/p>\n<\/div>\n<div data-type=\"solution\">\n<p>6. (a) To what temperature must you raise a copper wire, originally at 20.0\u00baC, to double its resistance, neglecting any changes in dimensions? (b) Does this happen in household wiring under ordinary circumstances?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>7. A resistor made of Nichrome wire is used in an application where its resistance cannot change more than 1.00% from its value at 20.0\u00baC. Over what temperature range can it be used?<\/p>\n<\/div>\n<div data-type=\"solution\">\n<p>8. Of what material is a resistor made if its resistance is 40.0% greater at 100\u00baC than at 20.0\u00baC?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>9. An electronic device designed to operate at any temperature in the range from\u00a0\u201310.0\u00baC to 55.0\u00baC contains pure carbon resistors. By what factor does their resistance increase over this range?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>10. (a) Of what material is a wire made, if it is 25.0 m long with a 0.100 mm diameter and has a resistance of 77.7 \u03a9 at 20.0\u00baC? (b) What is its resistance at 150\u00baC?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>11. Assuming a constant temperature coefficient of resistivity, what is the maximum percent decrease in the resistance of a constantan wire starting at\u00a020.0\u00baC?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>12. A wire is drawn through a die, stretching it to four times its original length. By what factor does its resistance increase?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>13. A copper wire has a resistance of 0.500 \u03a9 at 20.0\u00baC, and an iron wire has a resistance of 0.525 \u03a9 at the same temperature. At what temperature are their resistances equal?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>14. (a) Digital medical thermometers determine temperature by measuring the resistance of a semiconductor device called a thermistor (which has <em>\u03b1\u00a0<\/em>= \u20130.0600\/\u00baC) when it is at the same temperature as the patient. What is a patient\u2019s temperature if the thermistor\u2019s resistance at that temperature is 82.0% of its value at 37.0\u00baC (normal body temperature)? (b) The negative value for <em>\u03b1<\/em> may not be maintained for very low temperatures. Discuss why and whether this is the case here. (Hint: Resistance can\u2019t become negative.)<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>15.<strong> Integrated Concepts\u00a0<\/strong>(a) Redo Exercise 2\u00a0taking into account the thermal expansion of the tungsten filament. You may assume a thermal expansion coefficient of 12 \u00d7 10<sup>\u22126<\/sup>\/\u00baC. (b) By what percentage does your answer differ from that in the example?<\/p>\n<\/div>\n<\/div>\n<div data-type=\"exercise\" data-element-type=\"problems-exercises\">\n<div data-type=\"problem\">\n<p>16.<strong> Unreasonable Results\u00a0<\/strong>(a) To what temperature must you raise a resistor made of constantan to double its resistance, assuming a constant temperature coefficient of resistivity? (b) To cut it in half? (c) What is unreasonable about these results? (d) Which assumptions are unreasonable, or which premises are inconsistent?<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<div data-type=\"footnote-refs\">\n<h2 data-type=\"footnote-title\">Footnotes<\/h2>\n<ol>\n<li><a href=\"#footnote-ref1\" name=\"footnote1\" data-type=\"footnote-ref\" id=\"footnote1\">1<\/a> Values depend strongly on amounts and types of impurities<\/li>\n<li><a href=\"#footnote-ref2\" name=\"footnote2\" data-type=\"footnote-ref\" id=\"footnote2\">2<\/a> Values at 20\u00b0C.<\/li>\n<\/ol>\n<\/div>\n<div data-type=\"glossary\">\n<h2 data-type=\"glossary-title\">Glossary<\/h2>\n<dl>\n<dt>resistivity:<\/dt>\n<dd>an intrinsic property of a material, independent of its shape or size, directly proportional to the resistance, denoted by <em data-effect=\"italics\">\u03c1<\/em><\/dd>\n<\/dl>\n<dl>\n<dt>temperature coefficient of resistivity:<\/dt>\n<dd>an empirical quantity, denoted by <em data-effect=\"italics\">\u03b1<\/em>, which describes the change in resistance or resistivity of a material with temperature<\/dd>\n<\/dl>\n<div class=\"textbox exercises\">\n<h3>Selected Solutions to Problems &amp; Exercises<\/h3>\n<p>1.\u00a00.104 \u03a9<\/p>\n<p>3.\u00a02.8 \u00d7 10<sup>\u22122<\/sup>m<\/p>\n<p>5.\u00a0<span id=\"MathJax-Span-42631\" class=\"mrow\"><span id=\"MathJax-Span-42632\" class=\"semantics\"><span id=\"MathJax-Span-42633\" class=\"mrow\"><span id=\"MathJax-Span-42634\" class=\"mrow\"><span id=\"MathJax-Span-42635\" class=\"mrow\"><span id=\"MathJax-Span-42636\" class=\"mrow\"><span id=\"MathJax-Span-42637\" class=\"mn\">1<\/span><span id=\"MathJax-Span-42638\" class=\"mtext\">.<\/span><span id=\"MathJax-Span-42639\" class=\"mtext\">10\u00a0<\/span><span id=\"MathJax-Span-42640\" class=\"mi\">\u00d7\u00a0<\/span><span id=\"MathJax-Span-42641\" class=\"msup\"><span id=\"MathJax-Span-42642\" class=\"mtext\">10<\/span><sup><span id=\"MathJax-Span-42643\" class=\"mrow\"><span id=\"MathJax-Span-42644\" class=\"mrow\"><span id=\"MathJax-Span-42645\" class=\"mo\">\u2212<\/span><span id=\"MathJax-Span-42646\" class=\"mn\">3<\/span><\/span><\/span><\/sup><\/span><span id=\"MathJax-Span-42647\" class=\"mspace\"><\/span><span id=\"MathJax-Span-42648\" class=\"mtext\">A<\/span><\/span><\/span><span id=\"MathJax-Span-42649\" class=\"mrow\"><\/span><\/span><\/span><\/span><\/span><\/p>\n<p>7.\u00a0\u22125\u00baC to 45\u00baC<\/p>\n<p>9.\u00a01.03<\/p>\n<p>11.\u00a00.06%<\/p>\n<p>13.\u221217\u00baC<\/p>\n<p>15.\u00a0(a) 4.7 \u03a9 (total)\u00a0(b) 3.0% decrease<\/p>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-4189\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>College Physics. <strong>Authored by<\/strong>: OpenStax College. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics\">http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Located at License<\/li><li>PhET Interactive Simulations . <strong>Provided by<\/strong>: University of Colorado Boulder . <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/phet.colorado.edu\">http:\/\/phet.colorado.edu<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-4189-1\">Values depend strongly on amounts and types of impurities <a href=\"#return-footnote-4189-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-4189-2\">Values at 20\u00b0C. <a href=\"#return-footnote-4189-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":1,"menu_order":213,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"College Physics\",\"author\":\"OpenStax College\",\"organization\":\"\",\"url\":\"http:\/\/cnx.org\/contents\/031da8d3-b525-429c-80cf-6c8ed997733a\/College_Physics\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Located at License\"},{\"type\":\"cc\",\"description\":\"PhET Interactive Simulations \",\"author\":\"\",\"organization\":\"University of Colorado Boulder 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