lab Activity 12 Ohm's Law-1-1

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University of Alabama *

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311

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Electrical Engineering

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Dec 6, 2023

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1 SSCC EGR 101 LA 12 Objectives • Investigate the three variables involved in a mathematical relationship known as Ohm’s Law. • Determine the effective or net resistance for resistors in series, parallel and other combinations. • Derive the relations known as Kirchhoff’s Rules. Apparatus: AC/DC Experimental Circuit Board with wire leads 2 D-cell batteries 1 Multimeter Laptop Computer 7 to 10 resistors of various resistances Procedure : 1. Choose one of the resistors in your kit and decode the resistance using the color code chart on your desk top (A color code table is attached to this lab. Please detach and make it a part of your note book.). Record this value in the first column of table 1. 2. Measuring Current : Construct the circuit shown in figure 1 by pressing the leads of the resistor into two springs on your AC/DC circuit board. a. Set the Ammeter to the 200-mA range. Remember that it is necessary to connect the Ammeter in series (never in parallel across the battery). Complete the circuit and read the current that is flowing through the resistor. Record this value in the second column adjacent to the corresponding resistance value. b. Remove this first resistor and insert a second resistor. Repeat step 2 (a) and record the values for resistance (color code) and current. Repeat step 2a for all the resistors in your kit and record the resistance values and the resultant current in the appropriate columns of data table 1.

2 SSCC EGR 101 LA 12 3 . Measuring Voltage : Disconnect the multimeter (as an Ammeter) and connect the wire from positive terminal of the multimeter to the V/  terminal of the multimeter. Change the multimeter to the 20 V scale, Connect the alligator leads of the multimeter wires across or parallel to the resistor as shown in figure 2 above. a. Remove the current resistor and insert the first one you selected. Record its voltage in the 3 rd column of data table 1. b. Remove this resistor and insert the next one in the initial order and record the resultant voltage in column 3 of the data table. Data Analysis 1. Transfer the data to an EXCEL spreadsheet and insert a scatter plot. 2. Insert an equation of the least square fit of the line. 3. Record the slope of your EXCEL straight line in data table 2. 4. Complete the data table by dividing the Voltage by the resistance for each data set. Discussion 1. From your graph, what is the physical meaning of the slope? 2. Ohm’s law states that the current is given by the ratio of the voltage to the resistance. , or V I V IR R   Does your data concur with this? 3. What is true about the voltage column in data table 1? 3. What were possible sources of experimental error in this lab? 4. What can be done to minimize experimental error?

3 SSCC EGR 101 LA 12 Data Table 1 Resistance,  Current, Amp Voltage, V Voltage/Resistance Data Table 2 Slope of straight line from EXCEL Part 2: Resistors is Series and Parallel Introduction In the first part of this lab, we discovered Ohm’s Law. In the basic circuit with one resistor, the product of current (I) and resistance (R) is always equal to the voltage (V). but can also be written as V-IR=0. V IR 

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4 SSCC EGR 101 LA 12 Does this relationship hold for more than one resistor, considering that there is more than one way to connect 2 or more resistors? Resistor can be connected in series, in parallel and for more than 2, also in combinations of parallel and series. We will consider series and parallel combinations of 3 resistors first, then the various combinations. Procedure Wire the following circuit without the multimeter connected. This time you will use the multimeter to measure the voltage drop across each resistor while in circuit, so it will be used as a probe. 1. Using the multimeter as a Voltmeter, measure the voltage rise of the battery, VB, and record this value in the first column of table 3. a. Measure the voltage drops across each resistor and record in table 3 along with the decoded resistance of each. b. Calculate the current through each resistor, using Ohm’s Law and your calculator. Record these values in the 2 nd column of table 3. c. Add the values of the 3 resistors and record this value in 3 rd column for the total resistance, R E . Table 3 V I R Battery R 1 R 2 R 3 R E Calculations and Discussions for Part 2, #1 1. Are the calculated currents values the same for each resistor? R 1 R 2 R 3

5 SSCC EGR 101 LA 12 2. How can you confirm that the current is the same for resistors is series? 3. Add the voltages for the three resistors: V 1 +V 2 +V 3 = ____________. 4. Divide the battery voltage by the sum of the resistances. V/R E = _____________. 5. State the rule for combining resistors in series. 2. Disconnect the resistors and connect them in parallel with each other. Again, we will use the multimeter as a probe to determine the voltage across each resistor. Below is a parallel resistor circuit: a. Measure the voltage drop across each resistor and record the values in column 1 of table 4. b. Using Ohm’s Law and the decoded resistance values, determine the currents in each resistor and record those values in column 2 of table 4. c. Add the currents and record this value at the sixth row of column 3 of table 4. 3. Now reconnect the Ammeter in series with the battery, between the positive terminal of the battery and the first resistor. Record this value in the last row of column 3 of table 4. Table 4 V I R Battery R 1 R 2 R 3 R e Calculations and Discussions for Part2 #2. 1. What is true for the voltage drops across the various resistors? 2. Divide the voltage drop across each resistor by the resistance. V/R = I.

6 SSCC EGR 101 LA 12 3. What is true for the currents in each resistor and the current from the battery? 4. Compare the current values of the 6 th and 7 th row of column 3 of table 4. 5. Develop the rule for combining resistances in parallel. Write the formula in the box below.

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