DC Fundamentals Ohm s Law Exercise 1: Ohm s Law Circuit Resistance EXERCISE OBJECTIVE When you have completed this exercise, you will be able to determine resistance by using Ohm s law. You will verify your results with a multimeter. DISCUSSION resistance in the circuit. When voltage and current values are known, apply Ohm s law to determine circuit resistance. R = E/I ( ) ( ) VOLTAGE IN VOLTS RESISTANCE ( IN OHMS ) = CURRENT IN AMPS In circuit (a), the current is 10 ma (0.01A). Notice that voltage, resistance, and current are given. In circuit (b), the current decreases to 5 ma because a resistor (R S ) is added to the circuit. The total resistance of circuit (b) equals E/I (10V/5 ma = 2 k ). FACET by Lab-Volt 97
Ohm s Law DC Fundamentals In a circuit with an applied voltage of 10V, circuit current measures 15 ma (0.015A). What is the value of circuit resistance? NOTE: R = E/I a. Circuit resistance equals 6670. b. Circuit resistance equals 667. c. Circuit resistance cannot be determined. PROCEDURE Locate the OHM S LAW circuit block. Based on the resistive color code and the bands on R1, what is the resistance value of R1? R1 = (Recall Value 1) 98 FACET by Lab-Volt
DC Fundamentals Ohm s Law Based on the resistive color code and the bands on R2, what is the resistance value of R2? R2 = (Recall Value 2) Using the ohms function of your multimeter, measure and enter the value of R1. Your measured value for R1 and its nominal color code value should be approximately equal. R1 = (Recall Value 3) Using the ohms function of your multimeter, measure and enter the value of R2. Your measured value for R2 and its nominal color code value should be approximately equal. R2 = (Recall Value 4) FACET by Lab-Volt 99
Ohm s Law DC Fundamentals Insert a two-post connector between R1 and R2. Using the ohms function of your multimeter, measure and enter the value of the total circuit resistance (R T ). R T = (Recall Value 5) Set your multimeter to read voltage. Adjust the positive variable supply for 10 Vdc. Enter your voltage reading in volts. V A = Vdc (Recall Value 6) Set your multimeter to measure current. Connect the ammeter between the voltage source 100 FACET by Lab-Volt
DC Fundamentals Ohm s Law Enter your total circuit current (I T ) in milliamps (0.001A = 1 ma). I T = ma (Recall Value 7) Based on your measured values recalled below, apply Ohm s law to calculate the total resistance (R T ) of your circuit. V A = Vdc (Step 6, Recall Value 6) I T = ma (Step 8, Recall Value 7) R T = V A /I T R T = (Recall Value 8) Compare the measured total resistance (Step 5, Recall Value 5) with your calculated total resistance (Step 9, Recall Value 8). Based on your results, which statement is correct? a. Applying voltage and current to Ohm s law does not provide an accurate total circuit resistance. b. Ohm s law, in the form of voltage/current, provides an accurate total resistance value. c. Ohm s law will not apply to total circuit resistance unless the applied voltage and circuit current are equal in value. Place CM swich 19 in the ON position. Use your multimeter to measure the total current (I T ) of your circuit. Ensure that the applied voltage is set for 10 Vdc. I T = ma (Recall Value 9) Using Ohm s law formula R = E/I, determine the new total circuit resistance. Voltage = 10 Vdc Current = ma (Step 11, Recall Value 9) Total circuit resistance = (Recall Value 10) FACET by Lab-Volt 101
Ohm s Law DC Fundamentals Remove power from your circuit by removing the ammeter between the voltage source and the top of R1. Measure the total circuit resistance (CM 19 is activated). Does your measured value approximately equal the calculated total resistance of (Step 12, Recall Value 10)? a. No. The values do not agree because the individual component resistances are not known. b. Yes. The values are about the same because the added resistor increased the circuit s total current. c. value of R2. The new value can be determined by Ohm s law: R2 = V R2 / ma (Step 11, Recall Value 9). Activate your circuit and measure the voltage drop of R2. Use R2 = (Recall Value 11) 102 FACET by Lab-Volt
DC Fundamentals Ohm s Law Based on the information given in the circuit, how can you determine the value of R1? a. Divide the voltage drop of R1 by the circuit current. b. Divide the voltage drop of R2 by the circuit current. c. Divide the applied voltage (V A ) by the circuit current. Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION Ohm s law in the form of voltage/current determines a circuit s total resistance. resistor. The resistor color code is used to determine the value of color-banded resistors. Ohm s law states that current is directly related to voltage but inversely related to resistance. REVIEW QUESTIONS 1. Resistance and current are a. measured in the same unit. b. inversely proportional. c. directly proportional. d. always smaller than the voltage. 2. If the resistance in a circuit is doubled and the voltage remains the same, the current a. remains the same. b. is doubled. c. is halved. d. may increase or decrease. 3. In an electric circuit, if the current is 20 ma and the voltage is 40V, the circuit resistance is a. 20. b. 200. c. 2000. d. 20000. FACET by Lab-Volt 103
Ohm s Law DC Fundamentals 4. Locate the OHM S LAW circuit block, and place CM switch 19 in the ON position. With your ohmmeter, determine which resistor (R1 or R2) changed and its new value. a. R2, 250 b. R2, 510 c. R1, 1000 d. R2, 3200 5. When you use Ohm s law, how many values must be known? a. one b. two c. three d. all values NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. 104 FACET by Lab-Volt