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LAB 6 Play dough Resistance & Ohm’s Law

OBJECTIVES

1. Determine the functional dependence of resistance in terms of length, area and resistivity.

2. Apply Ohm’s Law to determine resistance.

EQUIPMENT DC Power Supply, DMM, Play dough, resistors

PROCEDURE Part 1: Play dough Resistance dependence on Length & Area

Length Dependence a. Roll dough out into a flat pancake shaped sheet about one to two centimeters in

thickness evenly across the pancake. Cut the pancake into four even sections that are uniform and all the same length. This is your resistor. Measure the length and area of the resistor and record in the data table.

b. Set up the resistor in a simple circuit with the ammeter and voltmeter connected in the proper places to take voltage and current readings for the resistor. Use aluminum foil to connect the play dough resistors with the power supply.

c. Turn on the power supply and turn the voltage dial up until the current reads about 100 mA. Take a voltage and current reading from the meters immediately and then turn off the power.

d. Now connect the next piece of play dough, making a longer resistor. Take data for the voltage, current, and length and record in the data table.

e. Continue adding length and taking readings until all four strips have been added.

Area Dependence f. Take all four strips apart. Set up your circuit how you originally had it with one strip of

play dough. g. Decrease the voltage in the circuit until the ammeter reads around 30 mA. Take a

voltage, current and height (thickness, cross sectional area) reading. h. 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 • Find the resistance of each resistor by using Ohm’s Law • Use Excel to plot R vs. L and R vs. A • Find the resistivity from your R vs. L (ρL) and R vs. A (ρA) data.

Answer the following questions using short concise sentences: • How does resistance depend on length L? How does resistance depend on area A? • Are the resistivities ρL and ρA equivalent? If so, what does the resistivity mean? If

not, explain the difference.

Part 2: 1kΩ Resistor a. Draw a circuit diagram of a resistor R = 1kΩ 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.

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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. Is the resistor ohmic or non-ohmic? 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.