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Ohm's law and resistance (Theory).html
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<title>Ohm's law and resistance (Theory) : Physics Virtual Labs : Suryansh </title>
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<div class="divContent"><h1>Aim:</h1>
<p> To determine the resistance per cm of a given wire by plotting a graph of potential difference versus current, and hence to determine its resistivity.</p>
<h1>The Theory:</h1>
<p>What does Ohm's Law state?</p>
<p><img alt="" src="./Ohm's law and resistance (Theory)_files/ohm.jpg"></p>
<p>According to the Ohm's law, "The current flowing through a conductor is directly proportional to the potential difference across its ends provided the physical conditions (temperature, dimensions, pressure) of the conductor remains the same." If I be the current flowing through a conductor and V be the potential difference across its ends, then according to Ohm's Law,</p>
<h2> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501051176_image014(3).jpg" style="width: 43px; height: 25px;"></h2>
<p> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501051201_image015(7).jpg" style="width: 127px; height: 25px;"></p>
<p>where, R is the constant of proportionality. It is known as resistance of the conductor. Or</p>
<h2> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501051257_image016(9).jpg"></h2>
<p> ..the working formula</p>
<p>R depends upon the material, temperature and dimensions of the conductor.</p>
<p>In S.I. units, the potential difference V is measured in volt and the current I in ampere, the resistance R is measured in ohm.</p>
<p>How do we establish the current-voltage relationship?</p>
<p>To establish the current-voltage relationship, it is to be shown that the ratio V / I remains constant for a given resistance, therefore a graph between the potential difference(V) and the current (I) must be a straight line.</p>
<p>How then do we find the unknown values of resistance?</p>
<p>It is the constant ratio that gives the unknown values of resistance, </p>
<h2> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501051275_image016(9).jpg"></h2>
<p>For a wire of uniform cross-section, the resistance depends on the length l and the area of cross-section A. It also depends on the temperature of the conductor. At a given temperature the resistance,</p>
<p> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501052586_Untitled-1.jpg" style="width: 84px; height: 56px;"><br>
where ρ is the specific resistance or resistivity and is characteristic of the material of wire.</p>
<p>Hence, the specific resistance or resistivity of the material of the wire,</p>
<p> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501052959_Untitled-2.jpg" style="width: 84px; height: 56px;"></p>
<p>If 'r' is the radius of the wire, then the cross sectional area, A = πr². Then the specific resistance or resistivity of the material of the wire is,</p>
<p> <img alt="" src="./Ohm's law and resistance (Theory)_files/1501058042_Untitled-3.jpg" style="width: 84px; height: 56px;"></p>
<h1>Learning Outcomes:</h1>
<ul>
<li>Students learn Ohm’s law.</li>
<li>Students know the relation between voltage, current and resistance. </li>
<li>Students learn how to find out the resistivity of the material of a give wire.</li>
</ul>
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