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LAB 3 Ohm’s Law & Play-Doh Resistance OBJECTIVES 1. Apply Ohm’s Law to determine resistance. 2. Determine the functional dependence of resistance in terms of length & area EQUIPMENT DC Power Supply, DMM, Play-Doh, resistor PROCEDURE Part 1: Ohm’s Law a. Draw a circuit diagram of a resistor R = 100 connected across a power supply, with a voltmeter to measure the resistor voltage VR and an ammeter to measure the resistor current iR. Now build the circuit on a breadboard. b. Adjust the power supply until VR = 1.0 V. Record VR and iR in a table. Continue recording data in 1.0 V increments up to VR = 5.0 V. Record all of your data into a table. c. Reverse the polarity of the voltage applied to the resistor by switching the leads at the power supply. Measure iR at 3 different values of VR and compare with your previous results. Does the polarity of the voltage across the resistor make any difference in its behavior? d. Use Excel to plot current vs. voltage. Fit a straight line to the data and determine the resistance Rthy from the slope of the line? Use a percent difference to compare Rthy to Rexpt where Rexpt is the resistance measured by a DMM. e. Answer the following questions: Does the resistor have constant resistance? Explain. Does the resistor obey Ohm’s law? Explain. Part 2: Play-Doh Resistance dependence on Length & Area Length Dependence a. Roll the Play-Doh out into a long flat rectangular shape about one to two centimeters in thickness evenly across the dough. This is your resistor. Without cutting your PlayDoh resistor, divide it (lightly mark it with a plastic knife) into four even sections that are uniform and all the same length. Measure the length and cross-sectional area of the resistor and record in the data table. Setup an Excel table that reads Length (cm), R = ∆V/i (Ω). b. Set up the Play-Doh resistor in a simple circuit with the ammeter and voltmeter (the exact same setup as Part 1). Use aluminum foil to connect the Play-Doh resistors with the power supply. c. Turn on the power supply and turn the voltage dial up until the current reads about 20 mA. Take a voltage and current reading from the meters immediately and then turn off the power. Now that you have set this voltage, DO NOT change the voltage again for the rest of this experiment. d. Now cut off one of the even sections of the of Play-Doh resistor, making a shorter resistor. Take data for the voltage, current, and length and record in the data table. e. Continue cutting length and taking readings until the Play-Doh resistor has been divided into four strips. 1 Area Dependence Set up your circuit with only one strip of the Play-Doh resistor (from Part 1e). Measure the cross-sectional area of the resistor and record in the data table. Setup an Excel table that reads Area (cm2), R = ∆V/i (Ω). h. Decrease the voltage in the circuit until the ammeter reads around 20 mA. Now that you have set this voltage, DO NOT change the voltage again for the rest of this experiment. Take a voltage, current and area reading and record them into the Excel table. g. Now add the next strip on top of the first strip. Continue adding strips on top and taking readings until all four strips have been added. Data Analysis Use Excel to plot R vs. L and R vs. A f. Answer the following questions using short concise sentences: How does resistance depend on length L? How does resistance depend on area A? Explain physically how the resistance changed with length and area. 2